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<ul>
<li><a class="reference internal" href="#">re – Regular Expressions</a><ul>
<li><a class="reference internal" href="#finding-patterns-in-text">Finding Patterns in Text</a></li>
<li><a class="reference internal" href="#compiling-expressions">Compiling Expressions</a></li>
<li><a class="reference internal" href="#multiple-matches">Multiple Matches</a></li>
<li><a class="reference internal" href="#pattern-syntax">Pattern Syntax</a><ul>
<li><a class="reference internal" href="#repetition">Repetition</a></li>
<li><a class="reference internal" href="#character-sets">Character Sets</a></li>
<li><a class="reference internal" href="#escape-codes">Escape Codes</a></li>
<li><a class="reference internal" href="#anchoring">Anchoring</a></li>
</ul>
</li>
<li><a class="reference internal" href="#constraining-the-search">Constraining the Search</a></li>
<li><a class="reference internal" href="#dissecting-matches-with-groups">Dissecting Matches with Groups</a></li>
<li><a class="reference internal" href="#search-options">Search Options</a><ul>
<li><a class="reference internal" href="#case-insensitive-matching">Case-insensitive Matching</a></li>
<li><a class="reference internal" href="#input-with-multiple-lines">Input with Multiple Lines</a></li>
<li><a class="reference internal" href="#unicode">Unicode</a></li>
<li><a class="reference internal" href="#verbose-expression-syntax">Verbose Expression Syntax</a></li>
<li><a class="reference internal" href="#embedding-flags-in-patterns">Embedding Flags in Patterns</a></li>
</ul>
</li>
<li><a class="reference internal" href="#looking-ahead-or-behind">Looking Ahead, or Behind</a></li>
<li><a class="reference internal" href="#self-referencing-expressions">Self-referencing Expressions</a></li>
<li><a class="reference internal" href="#modifying-strings-with-patterns">Modifying Strings with Patterns</a></li>
<li><a class="reference internal" href="#splitting-with-patterns">Splitting with Patterns</a></li>
</ul>
</li>
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<div class="section" id="module-re">
<span id="re-regular-expressions"></span><h1>re – Regular Expressions<a class="headerlink" href="#module-re" title="Permalink to this headline">¶</a></h1>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Purpose:</th><td class="field-body">Searching within and changing text using formal patterns.</td>
</tr>
<tr class="field"><th class="field-name">Python Version:</th><td class="field-body">1.5 and later</td>
</tr>
</tbody>
</table>
<p><em>Regular expressions</em> are text matching patterns described with a
formal syntax. The patterns are interpreted as a set of instructions,
which are then executed with a string as input to produce a matching
subset or modified version of the original. The term “regular
expressions” is frequently shortened to as “regex” or “regexp” in
conversation. Expressions can include literal text matching,
repetition, pattern-composition, branching, and other sophisticated
rules. A large number of parsing problems are easier to solve with a
regular expression than by creating a special-purpose lexer and
parser.</p>
<p>Regular expressions are typically used in applications that involve a
lot of text processing. For example, they are commonly used as search
patterns in text editing programs used by developers, including vi,
emacs, and modern IDEs. They are also an integral part of Unix
command line utilities such as sed, grep, and awk. Many programming
languages include support for regular expressions in the language
syntax (Perl, Ruby, Awk, and Tcl). Other languages, such as C, C++,
and Python supports regular expressions through extension libraries.</p>
<p>There are multiple open source implementations of regular expressions,
each sharing a common core syntax but with different extensions or
modifications to their advanced features. The syntax used in Python’s
<a class="reference internal" href="#module-re" title="re: Searching within and changing text using formal patterns."><tt class="xref py py-mod docutils literal"><span class="pre">re</span></tt></a> module is based on the syntax used for regular expressions
in Perl, with a few Python-specific enhancements.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">Although the formal definition of “regular expression” is limited
to expressions that describe regular languages, some of the
extensions supported by <a class="reference internal" href="#module-re" title="re: Searching within and changing text using formal patterns."><tt class="xref py py-mod docutils literal"><span class="pre">re</span></tt></a> go beyond describing regular
languages. The term “regular expression” is used here in a more
general sense to mean any expression that can be evaluated by
Python’s <a class="reference internal" href="#module-re" title="re: Searching within and changing text using formal patterns."><tt class="xref py py-mod docutils literal"><span class="pre">re</span></tt></a> module.</p>
</div>
<div class="section" id="finding-patterns-in-text">
<h2>Finding Patterns in Text<a class="headerlink" href="#finding-patterns-in-text" title="Permalink to this headline">¶</a></h2>
<p>The most common use for <a class="reference internal" href="#module-re" title="re: Searching within and changing text using formal patterns."><tt class="xref py py-mod docutils literal"><span class="pre">re</span></tt></a> is to search for patterns in text.
This example looks for two literal strings, <tt class="docutils literal"><span class="pre">'this'</span></tt> and <tt class="docutils literal"><span class="pre">'that'</span></tt>,
in a text string.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="n">patterns</span> <span class="o">=</span> <span class="p">[</span> <span class="s">'this'</span><span class="p">,</span> <span class="s">'that'</span> <span class="p">]</span>
<span class="n">text</span> <span class="o">=</span> <span class="s">'Does this text match the pattern?'</span>
<span class="k">for</span> <span class="n">pattern</span> <span class="ow">in</span> <span class="n">patterns</span><span class="p">:</span>
<span class="k">print</span> <span class="s">'Looking for "</span><span class="si">%s</span><span class="s">" in "</span><span class="si">%s</span><span class="s">" ->'</span> <span class="o">%</span> <span class="p">(</span><span class="n">pattern</span><span class="p">,</span> <span class="n">text</span><span class="p">),</span>
<span class="k">if</span> <span class="n">re</span><span class="o">.</span><span class="n">search</span><span class="p">(</span><span class="n">pattern</span><span class="p">,</span> <span class="n">text</span><span class="p">):</span>
<span class="k">print</span> <span class="s">'found a match!'</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">print</span> <span class="s">'no match'</span>
</pre></div>
</div>
<p><tt class="xref py py-func docutils literal"><span class="pre">search()</span></tt> takes the pattern and text to scan, and returns a
<tt class="xref py py-class docutils literal"><span class="pre">Match</span></tt> object when the pattern is found. If the pattern is
not found, <tt class="xref py py-func docutils literal"><span class="pre">search()</span></tt> returns <tt class="xref docutils literal"><span class="pre">None</span></tt>.</p>
<div class="highlight-python"><pre>$ python re_simple.py
Looking for "this" in "Does this text match the pattern?" -> found a match!
Looking for "that" in "Does this text match the pattern?" -> no match</pre>
</div>
<p>The <tt class="xref py py-class docutils literal"><span class="pre">Match</span></tt> object returned by <tt class="xref py py-func docutils literal"><span class="pre">search()</span></tt> holds information
about the nature of the match, including the original input string,
the regular expression used, and the location within the original
string where the pattern occurs.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="n">pattern</span> <span class="o">=</span> <span class="s">'this'</span>
<span class="n">text</span> <span class="o">=</span> <span class="s">'Does this text match the pattern?'</span>
<span class="n">match</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">search</span><span class="p">(</span><span class="n">pattern</span><span class="p">,</span> <span class="n">text</span><span class="p">)</span>
<span class="n">s</span> <span class="o">=</span> <span class="n">match</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">e</span> <span class="o">=</span> <span class="n">match</span><span class="o">.</span><span class="n">end</span><span class="p">()</span>
<span class="k">print</span> <span class="s">'Found "</span><span class="si">%s</span><span class="s">" in "</span><span class="si">%s</span><span class="s">" from </span><span class="si">%d</span><span class="s"> to </span><span class="si">%d</span><span class="s"> ("</span><span class="si">%s</span><span class="s">")'</span> <span class="o">%</span> \
<span class="p">(</span><span class="n">match</span><span class="o">.</span><span class="n">re</span><span class="o">.</span><span class="n">pattern</span><span class="p">,</span> <span class="n">match</span><span class="o">.</span><span class="n">string</span><span class="p">,</span> <span class="n">s</span><span class="p">,</span> <span class="n">e</span><span class="p">,</span> <span class="n">text</span><span class="p">[</span><span class="n">s</span><span class="p">:</span><span class="n">e</span><span class="p">])</span>
</pre></div>
</div>
<p>The <tt class="xref py py-func docutils literal"><span class="pre">start()</span></tt> and <tt class="xref py py-func docutils literal"><span class="pre">end()</span></tt> methods give the integer indexes
into the string showing where the text matched by the pattern occurs.</p>
<div class="highlight-python"><pre>$ python re_simple_match.py
Found "this" in "Does this text match the pattern?" from 5 to 9 ("this")</pre>
</div>
</div>
<div class="section" id="compiling-expressions">
<h2>Compiling Expressions<a class="headerlink" href="#compiling-expressions" title="Permalink to this headline">¶</a></h2>
<p><a class="reference internal" href="#module-re" title="re: Searching within and changing text using formal patterns."><tt class="xref py py-mod docutils literal"><span class="pre">re</span></tt></a> includes module-level functions for working with regular
expressions as text strings, but it is usually more efficient to
<em>compile</em> the expressions your program uses frequently. The
<tt class="xref py py-func docutils literal"><span class="pre">compile()</span></tt> function converts an expression string into a
<tt class="xref py py-class docutils literal"><span class="pre">RegexObject</span></tt>.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="c"># Pre-compile the patterns</span>
<span class="n">regexes</span> <span class="o">=</span> <span class="p">[</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span><span class="n">p</span><span class="p">)</span> <span class="k">for</span> <span class="n">p</span> <span class="ow">in</span> <span class="p">[</span> <span class="s">'this'</span><span class="p">,</span>
<span class="s">'that'</span><span class="p">,</span>
<span class="p">]</span>
<span class="p">]</span>
<span class="n">text</span> <span class="o">=</span> <span class="s">'Does this text match the pattern?'</span>
<span class="k">for</span> <span class="n">regex</span> <span class="ow">in</span> <span class="n">regexes</span><span class="p">:</span>
<span class="k">print</span> <span class="s">'Looking for "</span><span class="si">%s</span><span class="s">" in "</span><span class="si">%s</span><span class="s">" ->'</span> <span class="o">%</span> <span class="p">(</span><span class="n">regex</span><span class="o">.</span><span class="n">pattern</span><span class="p">,</span> <span class="n">text</span><span class="p">),</span>
<span class="k">if</span> <span class="n">regex</span><span class="o">.</span><span class="n">search</span><span class="p">(</span><span class="n">text</span><span class="p">):</span>
<span class="k">print</span> <span class="s">'found a match!'</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">print</span> <span class="s">'no match'</span>
</pre></div>
</div>
<p>The module-level functions maintain a cache of compiled expressions,
but the size of the cache is limited and using compiled expressions
directly means you can avoid the cache lookup overhead. By
pre-compiling any expressions your module uses when the module is
loaded you shift the compilation work to application startup time,
instead of a point where the program is responding to a user action.</p>
<div class="highlight-python"><pre>$ python re_simple_compiled.py
Looking for "this" in "Does this text match the pattern?" -> found a match!
Looking for "that" in "Does this text match the pattern?" -> no match</pre>
</div>
</div>
<div class="section" id="multiple-matches">
<h2>Multiple Matches<a class="headerlink" href="#multiple-matches" title="Permalink to this headline">¶</a></h2>
<p>So far the example patterns have all used <tt class="xref py py-func docutils literal"><span class="pre">search()</span></tt> to look for
single instances of literal text strings. The <tt class="xref py py-func docutils literal"><span class="pre">findall()</span></tt>
function returns all of the substrings of the input that match the
pattern without overlapping.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="n">text</span> <span class="o">=</span> <span class="s">'abbaaabbbbaaaaa'</span>
<span class="n">pattern</span> <span class="o">=</span> <span class="s">'ab'</span>
<span class="k">for</span> <span class="n">match</span> <span class="ow">in</span> <span class="n">re</span><span class="o">.</span><span class="n">findall</span><span class="p">(</span><span class="n">pattern</span><span class="p">,</span> <span class="n">text</span><span class="p">):</span>
<span class="k">print</span> <span class="s">'Found "</span><span class="si">%s</span><span class="s">"'</span> <span class="o">%</span> <span class="n">match</span>
</pre></div>
</div>
<p>There are two instances of <tt class="docutils literal"><span class="pre">ab</span></tt> in the input string.</p>
<div class="highlight-python"><pre>$ python re_findall.py
Found "ab"
Found "ab"</pre>
</div>
<p><tt class="xref py py-func docutils literal"><span class="pre">finditer()</span></tt> returns an iterator that produces <tt class="xref py py-class docutils literal"><span class="pre">Match</span></tt>
instances instead of the strings returned by <tt class="xref py py-func docutils literal"><span class="pre">findall()</span></tt>.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="n">text</span> <span class="o">=</span> <span class="s">'abbaaabbbbaaaaa'</span>
<span class="n">pattern</span> <span class="o">=</span> <span class="s">'ab'</span>
<span class="k">for</span> <span class="n">match</span> <span class="ow">in</span> <span class="n">re</span><span class="o">.</span><span class="n">finditer</span><span class="p">(</span><span class="n">pattern</span><span class="p">,</span> <span class="n">text</span><span class="p">):</span>
<span class="n">s</span> <span class="o">=</span> <span class="n">match</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">e</span> <span class="o">=</span> <span class="n">match</span><span class="o">.</span><span class="n">end</span><span class="p">()</span>
<span class="k">print</span> <span class="s">'Found "</span><span class="si">%s</span><span class="s">" at </span><span class="si">%d</span><span class="s">:</span><span class="si">%d</span><span class="s">'</span> <span class="o">%</span> <span class="p">(</span><span class="n">text</span><span class="p">[</span><span class="n">s</span><span class="p">:</span><span class="n">e</span><span class="p">],</span> <span class="n">s</span><span class="p">,</span> <span class="n">e</span><span class="p">)</span>
</pre></div>
</div>
<p>This example finds the same two occurrences of <tt class="docutils literal"><span class="pre">ab</span></tt>, and the
<tt class="xref py py-class docutils literal"><span class="pre">Match</span></tt> instance shows where they are in the original input.</p>
<div class="highlight-python"><pre>$ python re_finditer.py
Found "ab" at 0:2
Found "ab" at 5:7</pre>
</div>
</div>
<div class="section" id="pattern-syntax">
<h2>Pattern Syntax<a class="headerlink" href="#pattern-syntax" title="Permalink to this headline">¶</a></h2>
<p>Regular expressions support more powerful patterns than simple literal
text strings. Patterns can repeat, can be anchored to different
logical locations within the input, and can be expressed in compact
forms that don’t require every literal character be present in the
pattern. All of these features are used by combining literal text
values with <em>metacharacters</em> that are part of the regular expression
pattern syntax implemented by <a class="reference internal" href="#module-re" title="re: Searching within and changing text using formal patterns."><tt class="xref py py-mod docutils literal"><span class="pre">re</span></tt></a>. The following examples will
use this test program to explore variations in patterns.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="k">def</span> <span class="nf">test_patterns</span><span class="p">(</span><span class="n">text</span><span class="p">,</span> <span class="n">patterns</span><span class="o">=</span><span class="p">[]):</span>
<span class="sd">"""Given source text and a list of patterns, look for</span>
<span class="sd"> matches for each pattern within the text and print</span>
<span class="sd"> them to stdout.</span>
<span class="sd"> """</span>
<span class="c"># Show the character positions and input text</span>
<span class="k">print</span>
<span class="k">print</span> <span class="s">''</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="nb">str</span><span class="p">(</span><span class="n">i</span><span class="o">/</span><span class="mi">10</span> <span class="ow">or</span> <span class="s">' '</span><span class="p">)</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="nb">len</span><span class="p">(</span><span class="n">text</span><span class="p">)))</span>
<span class="k">print</span> <span class="s">''</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="nb">str</span><span class="p">(</span><span class="n">i</span><span class="o">%</span><span class="mi">10</span><span class="p">)</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="nb">len</span><span class="p">(</span><span class="n">text</span><span class="p">)))</span>
<span class="k">print</span> <span class="n">text</span>
<span class="c"># Look for each pattern in the text and print the results</span>
<span class="k">for</span> <span class="n">pattern</span> <span class="ow">in</span> <span class="n">patterns</span><span class="p">:</span>
<span class="k">print</span>
<span class="k">print</span> <span class="s">'Matching "</span><span class="si">%s</span><span class="s">"'</span> <span class="o">%</span> <span class="n">pattern</span>
<span class="k">for</span> <span class="n">match</span> <span class="ow">in</span> <span class="n">re</span><span class="o">.</span><span class="n">finditer</span><span class="p">(</span><span class="n">pattern</span><span class="p">,</span> <span class="n">text</span><span class="p">):</span>
<span class="n">s</span> <span class="o">=</span> <span class="n">match</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">e</span> <span class="o">=</span> <span class="n">match</span><span class="o">.</span><span class="n">end</span><span class="p">()</span>
<span class="k">print</span> <span class="s">' </span><span class="si">%2d</span><span class="s"> : </span><span class="si">%2d</span><span class="s"> = "</span><span class="si">%s</span><span class="s">"'</span> <span class="o">%</span> \
<span class="p">(</span><span class="n">s</span><span class="p">,</span> <span class="n">e</span><span class="o">-</span><span class="mi">1</span><span class="p">,</span> <span class="n">text</span><span class="p">[</span><span class="n">s</span><span class="p">:</span><span class="n">e</span><span class="p">])</span>
<span class="k">return</span>
<span class="k">if</span> <span class="n">__name__</span> <span class="o">==</span> <span class="s">'__main__'</span><span class="p">:</span>
<span class="n">test_patterns</span><span class="p">(</span><span class="s">'abbaaabbbbaaaaa'</span><span class="p">,</span> <span class="p">[</span><span class="s">'ab'</span><span class="p">])</span>
</pre></div>
</div>
<p>The output of <tt class="xref py py-func docutils literal"><span class="pre">test_patterns()</span></tt> shows the input text, including
the character positions, as well as the substring range from each
portion of the input that matches the pattern.</p>
<div class="highlight-python"><pre>$ python re_test_patterns.py
11111
012345678901234
abbaaabbbbaaaaa
Matching "ab"
0 : 1 = "ab"
5 : 6 = "ab"</pre>
</div>
<div class="section" id="repetition">
<h3>Repetition<a class="headerlink" href="#repetition" title="Permalink to this headline">¶</a></h3>
<p>There are five ways to express repetition in a pattern. A pattern
followed by the metacharacter <tt class="docutils literal"><span class="pre">*</span></tt> is repeated zero or more times
(allowing a pattern to repeat zero times means it does not need to
appear at all to match). Replace the <tt class="docutils literal"><span class="pre">*</span></tt> with <tt class="docutils literal"><span class="pre">+</span></tt> and the pattern
must appear at least once. Using <tt class="docutils literal"><span class="pre">?</span></tt> means the pattern appears zero
or one time. For a specific number of occurrences, use <tt class="docutils literal"><span class="pre">{m}</span></tt> after
the pattern, where <em>m</em> is replaced with the number of times the
pattern should repeat. And finally, to allow a variable but limited
number of repetitions, use <tt class="docutils literal"><span class="pre">{m,n}</span></tt> where <em>m</em> is the minimum number
of repetitions and <em>n</em> is the maximum. Leaving out <em>n</em> (<tt class="docutils literal"><span class="pre">{m,}</span></tt>)
means the value appears at least <em>m</em> times, with no maximum.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">re_test_patterns</span> <span class="kn">import</span> <span class="n">test_patterns</span>
<span class="n">test_patterns</span><span class="p">(</span><span class="s">'abbaaabbbbaaaaa'</span><span class="p">,</span>
<span class="p">[</span> <span class="s">'ab*'</span><span class="p">,</span> <span class="c"># a followed by zero or more b</span>
<span class="s">'ab+'</span><span class="p">,</span> <span class="c"># a followed by one or more b</span>
<span class="s">'ab?'</span><span class="p">,</span> <span class="c"># a followed by zero or one b</span>
<span class="s">'ab{3}'</span><span class="p">,</span> <span class="c"># a followed by three b</span>
<span class="s">'ab{2,3}'</span><span class="p">,</span> <span class="c"># a followed by two to three b</span>
<span class="p">])</span>
</pre></div>
</div>
<p>Notice how many more matches there are for <tt class="docutils literal"><span class="pre">ab*</span></tt> and <tt class="docutils literal"><span class="pre">ab?</span></tt> than
<tt class="docutils literal"><span class="pre">ab+</span></tt>.</p>
<div class="highlight-python"><pre>$ python re_repetition.py
11111
012345678901234
abbaaabbbbaaaaa
Matching "ab*"
0 : 2 = "abb"
3 : 3 = "a"
4 : 4 = "a"
5 : 9 = "abbbb"
10 : 10 = "a"
11 : 11 = "a"
12 : 12 = "a"
13 : 13 = "a"
14 : 14 = "a"
Matching "ab+"
0 : 2 = "abb"
5 : 9 = "abbbb"
Matching "ab?"
0 : 1 = "ab"
3 : 3 = "a"
4 : 4 = "a"
5 : 6 = "ab"
10 : 10 = "a"
11 : 11 = "a"
12 : 12 = "a"
13 : 13 = "a"
14 : 14 = "a"
Matching "ab{3}"
5 : 8 = "abbb"
Matching "ab{2,3}"
0 : 2 = "abb"
5 : 8 = "abbb"</pre>
</div>
<p>The normal processing for a repetition instruction is to consume as
much of the input as possible while matching the pattern. This
so-called <em>greedy</em> behavior may result in fewer individual matches, or
the matches may include more of the input text than intended.
Greediness can be turned off by following the repetition instruction
with <tt class="docutils literal"><span class="pre">?</span></tt>.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">re_test_patterns</span> <span class="kn">import</span> <span class="n">test_patterns</span>
<span class="n">test_patterns</span><span class="p">(</span><span class="s">'abbaaabbbbaaaaa'</span><span class="p">,</span>
<span class="p">[</span> <span class="s">'ab*?'</span><span class="p">,</span> <span class="c"># a followed by zero or more b</span>
<span class="s">'ab+?'</span><span class="p">,</span> <span class="c"># a followed by one or more b</span>
<span class="s">'ab??'</span><span class="p">,</span> <span class="c"># a followed by zero or one b</span>
<span class="s">'ab{3}?'</span><span class="p">,</span> <span class="c"># a followed by three b</span>
<span class="s">'ab{2,3}?'</span><span class="p">,</span> <span class="c"># a followed by two to three b</span>
<span class="p">])</span>
</pre></div>
</div>
<p>Disabling greedy consumption of the input for any of the patterns
where zero occurences of <tt class="docutils literal"><span class="pre">b</span></tt> are allowed means the matched substring
does not include any <tt class="docutils literal"><span class="pre">b</span></tt> characters.</p>
<div class="highlight-python"><pre>$ python re_repetition_non_greedy.py
11111
012345678901234
abbaaabbbbaaaaa
Matching "ab*?"
0 : 0 = "a"
3 : 3 = "a"
4 : 4 = "a"
5 : 5 = "a"
10 : 10 = "a"
11 : 11 = "a"
12 : 12 = "a"
13 : 13 = "a"
14 : 14 = "a"
Matching "ab+?"
0 : 1 = "ab"
5 : 6 = "ab"
Matching "ab??"
0 : 0 = "a"
3 : 3 = "a"
4 : 4 = "a"
5 : 5 = "a"
10 : 10 = "a"
11 : 11 = "a"
12 : 12 = "a"
13 : 13 = "a"
14 : 14 = "a"
Matching "ab{3}?"
5 : 8 = "abbb"
Matching "ab{2,3}?"
0 : 2 = "abb"
5 : 7 = "abb"</pre>
</div>
</div>
<div class="section" id="character-sets">
<h3>Character Sets<a class="headerlink" href="#character-sets" title="Permalink to this headline">¶</a></h3>
<p>A <em>character set</em> is a group of characters, any one of which can match
at that point in the pattern. For example, <tt class="docutils literal"><span class="pre">[ab]</span></tt> would match
either <tt class="docutils literal"><span class="pre">a</span></tt> or <tt class="docutils literal"><span class="pre">b</span></tt>.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">re_test_patterns</span> <span class="kn">import</span> <span class="n">test_patterns</span>
<span class="n">test_patterns</span><span class="p">(</span><span class="s">'abbaaabbbbaaaaa'</span><span class="p">,</span>
<span class="p">[</span> <span class="s">'[ab]'</span><span class="p">,</span> <span class="c"># either a or b</span>
<span class="s">'a[ab]+'</span><span class="p">,</span> <span class="c"># a followed by one or more a or b</span>
<span class="s">'a[ab]+?'</span><span class="p">,</span> <span class="c"># a followed by one or more a or b, not greedy</span>
<span class="p">])</span>
</pre></div>
</div>
<p>The greedy form of the expression, <tt class="docutils literal"><span class="pre">a[ab]+</span></tt>, consumes the entire
string because the first letter is <tt class="docutils literal"><span class="pre">a</span></tt> and every subsequent
character is either <tt class="docutils literal"><span class="pre">a</span></tt> or <tt class="docutils literal"><span class="pre">b</span></tt>.</p>
<div class="highlight-python"><pre>$ python re_charset.py
11111
012345678901234
abbaaabbbbaaaaa
Matching "[ab]"
0 : 0 = "a"
1 : 1 = "b"
2 : 2 = "b"
3 : 3 = "a"
4 : 4 = "a"
5 : 5 = "a"
6 : 6 = "b"
7 : 7 = "b"
8 : 8 = "b"
9 : 9 = "b"
10 : 10 = "a"
11 : 11 = "a"
12 : 12 = "a"
13 : 13 = "a"
14 : 14 = "a"
Matching "a[ab]+"
0 : 14 = "abbaaabbbbaaaaa"
Matching "a[ab]+?"
0 : 1 = "ab"
3 : 4 = "aa"
5 : 6 = "ab"
10 : 11 = "aa"
12 : 13 = "aa"</pre>
</div>
<p>A character set can also be used to exclude specific characters. The
special marker <tt class="docutils literal"><span class="pre">^</span></tt> means to look for characters not in the set
following.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">re_test_patterns</span> <span class="kn">import</span> <span class="n">test_patterns</span>
<span class="n">test_patterns</span><span class="p">(</span><span class="s">'This is some text -- with punctuation.'</span><span class="p">,</span>
<span class="p">[</span> <span class="s">'[^-. ]+'</span><span class="p">,</span> <span class="c"># sequences without -, ., or space</span>
<span class="p">])</span>
</pre></div>
</div>
<p>This pattern finds all of the substrings that do not contain the
characters <tt class="docutils literal"><span class="pre">-</span></tt>, <tt class="docutils literal"><span class="pre">.</span></tt>, or a space.</p>
<div class="highlight-python"><pre>$ python re_charset_exclude.py
1111111111222222222233333333
01234567890123456789012345678901234567
This is some text -- with punctuation.
Matching "[^-. ]+"
0 : 3 = "This"
5 : 6 = "is"
8 : 11 = "some"
13 : 16 = "text"
21 : 24 = "with"
26 : 36 = "punctuation"</pre>
</div>
<p>As character sets grow larger, typing every character that should (or
should not) match becomes tedious. A more compact format using
<em>character ranges</em> lets you define a character set to include all of
the contiguous characters between a start and stop point.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">re_test_patterns</span> <span class="kn">import</span> <span class="n">test_patterns</span>
<span class="n">test_patterns</span><span class="p">(</span><span class="s">'This is some text -- with punctuation.'</span><span class="p">,</span>
<span class="p">[</span> <span class="s">'[a-z]+'</span><span class="p">,</span> <span class="c"># sequences of lower case letters</span>
<span class="s">'[A-Z]+'</span><span class="p">,</span> <span class="c"># sequences of upper case letters</span>
<span class="s">'[a-zA-Z]+'</span><span class="p">,</span> <span class="c"># sequences of lower or upper case letters</span>
<span class="s">'[A-Z][a-z]+'</span><span class="p">,</span> <span class="c"># one upper case letter followed by lower case letters</span>
<span class="p">])</span>
</pre></div>
</div>
<p>Here the range <tt class="docutils literal"><span class="pre">a-z</span></tt> includes the lower case ASCII letters, and the
range <tt class="docutils literal"><span class="pre">A-Z</span></tt> includes the upper case ASCII letters. The ranges can
also be combined into a single character set.</p>
<div class="highlight-python"><pre>$ python re_charset_ranges.py
1111111111222222222233333333
01234567890123456789012345678901234567
This is some text -- with punctuation.
Matching "[a-z]+"
1 : 3 = "his"
5 : 6 = "is"
8 : 11 = "some"
13 : 16 = "text"
21 : 24 = "with"
26 : 36 = "punctuation"
Matching "[A-Z]+"
0 : 0 = "T"
Matching "[a-zA-Z]+"
0 : 3 = "This"
5 : 6 = "is"
8 : 11 = "some"
13 : 16 = "text"
21 : 24 = "with"
26 : 36 = "punctuation"
Matching "[A-Z][a-z]+"
0 : 3 = "This"</pre>
</div>
<p>As a special case of a character set the metacharacter dot, or period
(<tt class="docutils literal"><span class="pre">.</span></tt>), indicates that the pattern should match any single character
in that position.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">re_test_patterns</span> <span class="kn">import</span> <span class="n">test_patterns</span>
<span class="n">test_patterns</span><span class="p">(</span><span class="s">'abbaaabbbbaaaaa'</span><span class="p">,</span>
<span class="p">[</span> <span class="s">'a.'</span><span class="p">,</span> <span class="c"># a followed by any one character</span>
<span class="s">'b.'</span><span class="p">,</span> <span class="c"># b followed by any one character</span>
<span class="s">'a.*b'</span><span class="p">,</span> <span class="c"># a followed by anything, ending in b</span>
<span class="s">'a.*?b'</span><span class="p">,</span> <span class="c"># a followed by anything, ending in b</span>
<span class="p">])</span>
</pre></div>
</div>
<p>Combining dot with repetition can result in very long matches, unless
the non-greedy form is used.</p>
<div class="highlight-python"><pre>$ python re_charset_dot.py
11111
012345678901234
abbaaabbbbaaaaa
Matching "a."
0 : 1 = "ab"
3 : 4 = "aa"
5 : 6 = "ab"
10 : 11 = "aa"
12 : 13 = "aa"
Matching "b."
1 : 2 = "bb"
6 : 7 = "bb"
8 : 9 = "bb"
Matching "a.*b"
0 : 9 = "abbaaabbbb"
Matching "a.*?b"
0 : 1 = "ab"
3 : 6 = "aaab"</pre>
</div>
</div>
<div class="section" id="escape-codes">
<h3>Escape Codes<a class="headerlink" href="#escape-codes" title="Permalink to this headline">¶</a></h3>
<p>An even more compact representation uses escape codes for several
pre-defined character sets. The escape codes recognized by <a class="reference internal" href="#module-re" title="re: Searching within and changing text using formal patterns."><tt class="xref py py-mod docutils literal"><span class="pre">re</span></tt></a>
are:</p>
<table border="1" class="docutils">
<colgroup>
<col width="14%" />
<col width="86%" />
</colgroup>
<thead valign="bottom">
<tr><th class="head">Code</th>
<th class="head">Meaning</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><tt class="docutils literal"><span class="pre">\d</span></tt></td>
<td>a digit</td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">\D</span></tt></td>
<td>a non-digit</td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">\s</span></tt></td>
<td>whitespace (tab, space, newline, etc.)</td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">\S</span></tt></td>
<td>non-whitespace</td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">\w</span></tt></td>
<td>alphanumeric</td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">\W</span></tt></td>
<td>non-alphanumeric</td>
</tr>
</tbody>
</table>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">Escapes are indicated by prefixing the character with a backslash
(<tt class="docutils literal"><span class="pre">\</span></tt>). Unfortunately, a backslash must itself be escaped in
normal Python strings, and that results in expressions that are
difficult to read. Using <em>raw</em> strings, created by prefixing the
literal value with <tt class="docutils literal"><span class="pre">r</span></tt>, for creating regular expressions
eliminates this problem and maintains readability.</p>
</div>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">re_test_patterns</span> <span class="kn">import</span> <span class="n">test_patterns</span>
<span class="n">test_patterns</span><span class="p">(</span><span class="s">'This is a prime #1 example!'</span><span class="p">,</span>
<span class="p">[</span> <span class="s">r'\d+'</span><span class="p">,</span> <span class="c"># sequence of digits</span>
<span class="s">r'\D+'</span><span class="p">,</span> <span class="c"># sequence of non-digits</span>
<span class="s">r'\s+'</span><span class="p">,</span> <span class="c"># sequence of whitespace</span>
<span class="s">r'\S+'</span><span class="p">,</span> <span class="c"># sequence of non-whitespace</span>
<span class="s">r'\w+'</span><span class="p">,</span> <span class="c"># alphanumeric characters</span>
<span class="s">r'\W+'</span><span class="p">,</span> <span class="c"># non-alphanumeric</span>
<span class="p">])</span>
</pre></div>
</div>
<p>These sample expressions combine escape codes with repetition to find
sequences of like characters in the input string.</p>
<div class="highlight-python"><pre>$ python re_escape_codes.py
11111111112222222
012345678901234567890123456
This is a prime #1 example!
Matching "\d+"
17 : 17 = "1"
Matching "\D+"
0 : 16 = "This is a prime #"
18 : 26 = " example!"
Matching "\s+"
4 : 4 = " "
7 : 7 = " "
9 : 9 = " "
15 : 15 = " "
18 : 18 = " "
Matching "\S+"
0 : 3 = "This"
5 : 6 = "is"
8 : 8 = "a"
10 : 14 = "prime"
16 : 17 = "#1"
19 : 26 = "example!"
Matching "\w+"
0 : 3 = "This"
5 : 6 = "is"
8 : 8 = "a"
10 : 14 = "prime"
17 : 17 = "1"
19 : 25 = "example"
Matching "\W+"
4 : 4 = " "
7 : 7 = " "
9 : 9 = " "
15 : 16 = " #"
18 : 18 = " "
26 : 26 = "!"</pre>
</div>
<p>To match the characters that are part of the regular expression
syntax, escape the characters in the search pattern.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">re_test_patterns</span> <span class="kn">import</span> <span class="n">test_patterns</span>
<span class="n">test_patterns</span><span class="p">(</span><span class="s">r'\d+ \D+ \s+ \S+ \w+ \W+'</span><span class="p">,</span>
<span class="p">[</span> <span class="s">r'</span><span class="se">\\</span><span class="s">d\+'</span><span class="p">,</span>
<span class="s">r'</span><span class="se">\\</span><span class="s">D\+'</span><span class="p">,</span>
<span class="s">r'</span><span class="se">\\</span><span class="s">s\+'</span><span class="p">,</span>
<span class="s">r'</span><span class="se">\\</span><span class="s">S\+'</span><span class="p">,</span>
<span class="s">r'</span><span class="se">\\</span><span class="s">w\+'</span><span class="p">,</span>
<span class="s">r'</span><span class="se">\\</span><span class="s">W\+'</span><span class="p">,</span>
<span class="p">])</span>
</pre></div>
</div>
<p>These patterns escape the backslash and plus characters, since as
metacharacters both have special meaning in a regular expression.</p>
<div class="highlight-python"><pre>$ python re_escape_escapes.py
1111111111222
01234567890123456789012
\d+ \D+ \s+ \S+ \w+ \W+
Matching "\\d\+"
0 : 2 = "\d+"
Matching "\\D\+"
4 : 6 = "\D+"
Matching "\\s\+"
8 : 10 = "\s+"
Matching "\\S\+"
12 : 14 = "\S+"
Matching "\\w\+"
16 : 18 = "\w+"
Matching "\\W\+"
20 : 22 = "\W+"</pre>
</div>
</div>
<div class="section" id="anchoring">
<h3>Anchoring<a class="headerlink" href="#anchoring" title="Permalink to this headline">¶</a></h3>
<p>In addition to describing the content of a pattern to match, you can
also specify the relative location in the input text where the pattern
should appear using <em>anchoring</em> instructions.</p>
<table border="1" class="docutils">
<colgroup>
<col width="11%" />
<col width="89%" />
</colgroup>
<thead valign="bottom">
<tr><th class="head">Code</th>
<th class="head">Meaning</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><tt class="docutils literal"><span class="pre">^</span></tt></td>
<td>start of string, or line</td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">$</span></tt></td>
<td>end of string, or line</td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">\A</span></tt></td>
<td>start of string</td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">\Z</span></tt></td>
<td>end of string</td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">\b</span></tt></td>
<td>empty string at the beginning or end of a word</td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">\B</span></tt></td>
<td>empty string not at the beginning or end of a word</td>
</tr>
</tbody>
</table>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">re_test_patterns</span> <span class="kn">import</span> <span class="n">test_patterns</span>
<span class="n">test_patterns</span><span class="p">(</span><span class="s">'This is some text -- with punctuation.'</span><span class="p">,</span>
<span class="p">[</span> <span class="s">r'^\w+'</span><span class="p">,</span> <span class="c"># word at start of string</span>
<span class="s">r'\A\w+'</span><span class="p">,</span> <span class="c"># word at start of string</span>
<span class="s">r'\w+\S*$'</span><span class="p">,</span> <span class="c"># word at end of string, with optional punctuation</span>
<span class="s">r'\w+\S*\Z'</span><span class="p">,</span> <span class="c"># word at end of string, with optional punctuation</span>
<span class="s">r'\w*t\w*'</span><span class="p">,</span> <span class="c"># word containing 't'</span>
<span class="s">r'\bt\w+'</span><span class="p">,</span> <span class="c"># 't' at start of word</span>
<span class="s">r'\w+t\b'</span><span class="p">,</span> <span class="c"># 't' at end of word</span>
<span class="s">r'\Bt\B'</span><span class="p">,</span> <span class="c"># 't', not start or end of word</span>
<span class="p">])</span>
</pre></div>
</div>
<p>The patterns in the example for matching words at the beginning and
end of the string are different because the word at the end of the
string is followed by punctuation to terminate the sentence. The
pattern <tt class="docutils literal"><span class="pre">\w+$</span></tt> would not match, since <tt class="docutils literal"><span class="pre">.</span></tt> is not considered an
alphanumeric character.</p>
<div class="highlight-python"><pre>$ python re_anchoring.py
1111111111222222222233333333
01234567890123456789012345678901234567
This is some text -- with punctuation.
Matching "^\w+"
0 : 3 = "This"
Matching "\A\w+"
0 : 3 = "This"
Matching "\w+\S*$"
26 : 37 = "punctuation."
Matching "\w+\S*\Z"
26 : 37 = "punctuation."
Matching "\w*t\w*"
13 : 16 = "text"
21 : 24 = "with"
26 : 36 = "punctuation"
Matching "\bt\w+"
13 : 16 = "text"
Matching "\w+t\b"
13 : 16 = "text"
Matching "\Bt\B"
23 : 23 = "t"
30 : 30 = "t"
33 : 33 = "t"</pre>
</div>
</div>
</div>
<div class="section" id="constraining-the-search">
<h2>Constraining the Search<a class="headerlink" href="#constraining-the-search" title="Permalink to this headline">¶</a></h2>
<p>In situations where you know in advance that only a subset of the full
input should be searched, you can further constrain the regular
expression match by telling <a class="reference internal" href="#module-re" title="re: Searching within and changing text using formal patterns."><tt class="xref py py-mod docutils literal"><span class="pre">re</span></tt></a> to limit the search range. For
example, if your pattern must appear at the front of the input, then
using <tt class="xref py py-func docutils literal"><span class="pre">match()</span></tt> instead of <tt class="xref py py-func docutils literal"><span class="pre">search()</span></tt> will anchor the search
without having to explicitly include an anchor in the search pattern.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="n">text</span> <span class="o">=</span> <span class="s">'This is some text -- with punctuation.'</span>
<span class="n">pattern</span> <span class="o">=</span> <span class="s">'is'</span>
<span class="k">print</span> <span class="s">'Text :'</span><span class="p">,</span> <span class="n">text</span>
<span class="k">print</span> <span class="s">'Pattern:'</span><span class="p">,</span> <span class="n">pattern</span>
<span class="n">m</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">match</span><span class="p">(</span><span class="n">pattern</span><span class="p">,</span> <span class="n">text</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Match :'</span><span class="p">,</span> <span class="n">m</span>
<span class="n">s</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">search</span><span class="p">(</span><span class="n">pattern</span><span class="p">,</span> <span class="n">text</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Search :'</span><span class="p">,</span> <span class="n">s</span>
</pre></div>
</div>
<p>Since the literal text <tt class="docutils literal"><span class="pre">is</span></tt> does not appear at the start of the
input text, it is not found using <tt class="xref py py-func docutils literal"><span class="pre">match()</span></tt>. The sequence appears
two other times in the text, though, so <tt class="xref py py-func docutils literal"><span class="pre">search()</span></tt> finds it.</p>
<div class="highlight-python"><pre>$ python re_match.py
Text : This is some text -- with punctuation.
Pattern: is
Match : None
Search : <_sre.SRE_Match object at 0x1002d2ed0></pre>
</div>
<p>The <tt class="xref py py-func docutils literal"><span class="pre">search()</span></tt> method of a compiled regular expression accepts
optional <em>start</em> and <em>end</em> position parameters to limit the search to
a substring of the input.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="n">text</span> <span class="o">=</span> <span class="s">'This is some text -- with punctuation.'</span>
<span class="n">pattern</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span><span class="s">r'\b\w*is\w*\b'</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Text:'</span><span class="p">,</span> <span class="n">text</span>
<span class="k">print</span>
<span class="n">pos</span> <span class="o">=</span> <span class="mi">0</span>
<span class="k">while</span> <span class="bp">True</span><span class="p">:</span>
<span class="n">match</span> <span class="o">=</span> <span class="n">pattern</span><span class="o">.</span><span class="n">search</span><span class="p">(</span><span class="n">text</span><span class="p">,</span> <span class="n">pos</span><span class="p">)</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">match</span><span class="p">:</span>
<span class="k">break</span>
<span class="n">s</span> <span class="o">=</span> <span class="n">match</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">e</span> <span class="o">=</span> <span class="n">match</span><span class="o">.</span><span class="n">end</span><span class="p">()</span>
<span class="k">print</span> <span class="s">' </span><span class="si">%2d</span><span class="s"> : </span><span class="si">%2d</span><span class="s"> = "</span><span class="si">%s</span><span class="s">"'</span> <span class="o">%</span> \
<span class="p">(</span><span class="n">s</span><span class="p">,</span> <span class="n">e</span><span class="o">-</span><span class="mi">1</span><span class="p">,</span> <span class="n">text</span><span class="p">[</span><span class="n">s</span><span class="p">:</span><span class="n">e</span><span class="p">])</span>
<span class="c"># Move forward in text for the next search</span>
<span class="n">pos</span> <span class="o">=</span> <span class="n">e</span>
</pre></div>
</div>
<p>This example implements a less efficient form of <tt class="xref py py-func docutils literal"><span class="pre">iterall()</span></tt>.
Each time a match is found, the end position of that match is used for
the next search.</p>
<div class="highlight-python"><pre>$ python re_search_substring.py
Text: This is some text -- with punctuation.
0 : 3 = "This"
5 : 6 = "is"</pre>
</div>
</div>
<div class="section" id="dissecting-matches-with-groups">
<h2>Dissecting Matches with Groups<a class="headerlink" href="#dissecting-matches-with-groups" title="Permalink to this headline">¶</a></h2>
<p>Searching for pattern matches is the basis of the powerful
capabilities provided by regular expressions. Adding <em>groups</em> to a
pattern lets you isolate parts of the matching text, expanding those
capabilities to create a parser. Groups are defined by enclosing
patterns in parentheses (<tt class="docutils literal"><span class="pre">(</span></tt> and <tt class="docutils literal"><span class="pre">)</span></tt>).</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">re_test_patterns</span> <span class="kn">import</span> <span class="n">test_patterns</span>
<span class="n">test_patterns</span><span class="p">(</span><span class="s">'abbaaabbbbaaaaa'</span><span class="p">,</span>
<span class="p">[</span> <span class="s">'a(ab)'</span><span class="p">,</span> <span class="c"># 'a' followed by literal 'ab'</span>
<span class="s">'a(a*b*)'</span><span class="p">,</span> <span class="c"># 'a' followed by 0-n 'a' and 0-n 'b'</span>
<span class="s">'a(ab)*'</span><span class="p">,</span> <span class="c"># 'a' followed by 0-n 'ab'</span>
<span class="s">'a(ab)+'</span><span class="p">,</span> <span class="c"># 'a' followed by 1-n 'ab'</span>
<span class="p">])</span>
</pre></div>
</div>
<p>Any complete regular expression can be converted to a group and nested
within a larger expression. All of the repetition modifiers can be
applied to a group as a whole, requiring the entire group pattern to
repeat.</p>
<div class="highlight-python"><pre>$ python re_groups.py
11111
012345678901234
abbaaabbbbaaaaa
Matching "a(ab)"
4 : 6 = "aab"
Matching "a(a*b*)"
0 : 2 = "abb"
3 : 9 = "aaabbbb"
10 : 14 = "aaaaa"
Matching "a(ab)*"
0 : 0 = "a"
3 : 3 = "a"
4 : 6 = "aab"
10 : 10 = "a"
11 : 11 = "a"
12 : 12 = "a"
13 : 13 = "a"
14 : 14 = "a"
Matching "a(ab)+"
4 : 6 = "aab"</pre>
</div>
<p>To access the substrings matched by the individual groups within a
pattern, use the <tt class="xref py py-func docutils literal"><span class="pre">groups()</span></tt> method of the <tt class="xref py py-class docutils literal"><span class="pre">Match</span></tt> object.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="n">text</span> <span class="o">=</span> <span class="s">'This is some text -- with punctuation.'</span>
<span class="k">print</span> <span class="n">text</span>
<span class="k">print</span>
<span class="k">for</span> <span class="n">pattern</span> <span class="ow">in</span> <span class="p">[</span> <span class="s">r'^(\w+)'</span><span class="p">,</span> <span class="c"># word at start of string</span>
<span class="s">r'(\w+)\S*$'</span><span class="p">,</span> <span class="c"># word at end of string, with optional punctuation</span>
<span class="s">r'(\bt\w+)\W+(\w+)'</span><span class="p">,</span> <span class="c"># word starting with 't' then another word</span>
<span class="s">r'(\w+t)\b'</span><span class="p">,</span> <span class="c"># word ending with 't'</span>
<span class="p">]:</span>
<span class="n">regex</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span><span class="n">pattern</span><span class="p">)</span>
<span class="n">match</span> <span class="o">=</span> <span class="n">regex</span><span class="o">.</span><span class="n">search</span><span class="p">(</span><span class="n">text</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Matching "</span><span class="si">%s</span><span class="s">"'</span> <span class="o">%</span> <span class="n">pattern</span>
<span class="k">print</span> <span class="s">' '</span><span class="p">,</span> <span class="n">match</span><span class="o">.</span><span class="n">groups</span><span class="p">()</span>
<span class="k">print</span>
</pre></div>
</div>
<p><tt class="xref py py-func docutils literal"><span class="pre">Match.groups()</span></tt> returns a sequence of strings in the order of the
group within the expression that matches the string.</p>
<div class="highlight-python"><pre>$ python re_groups_match.py
This is some text -- with punctuation.
Matching "^(\w+)"
('This',)
Matching "(\w+)\S*$"
('punctuation',)
Matching "(\bt\w+)\W+(\w+)"
('text', 'with')
Matching "(\w+t)\b"
('text',)</pre>
</div>
<p>If you are using grouping to find parts of the string, but you don’t
need all of the parts matched by groups, you can ask for the match of
only a single group with <tt class="xref py py-func docutils literal"><span class="pre">group()</span></tt>.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="n">text</span> <span class="o">=</span> <span class="s">'This is some text -- with punctuation.'</span>
<span class="k">print</span> <span class="s">'Input text :'</span><span class="p">,</span> <span class="n">text</span>
<span class="c"># word starting with 't' then another word</span>
<span class="n">regex</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span><span class="s">r'(\bt\w+)\W+(\w+)'</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Pattern :'</span><span class="p">,</span> <span class="n">regex</span><span class="o">.</span><span class="n">pattern</span>
<span class="n">match</span> <span class="o">=</span> <span class="n">regex</span><span class="o">.</span><span class="n">search</span><span class="p">(</span><span class="n">text</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Entire match :'</span><span class="p">,</span> <span class="n">match</span><span class="o">.</span><span class="n">group</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Word starting with "t":'</span><span class="p">,</span> <span class="n">match</span><span class="o">.</span><span class="n">group</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Word after "t" word :'</span><span class="p">,</span> <span class="n">match</span><span class="o">.</span><span class="n">group</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span>
</pre></div>
</div>
<p>Group <tt class="docutils literal"><span class="pre">0</span></tt> represents the string matched by the entire expression,
and sub-groups are numbered starting with <tt class="docutils literal"><span class="pre">1</span></tt> in the order their
left parenthesis appears in the expression.</p>
<div class="highlight-python"><pre>$ python re_groups_individual.py
Input text : This is some text -- with punctuation.
Pattern : (\bt\w+)\W+(\w+)
Entire match : text -- with
Word starting with "t": text
Word after "t" word : with</pre>
</div>
<p>Python extends the basic grouping syntax to add <em>named groups</em>. Using
names to refer to groups makes it easier to modify the pattern over
time, without having to also modify the code using the match results.
To set the name of a group, use the syntax <tt class="docutils literal"><span class="pre">(P?<name>pattern)</span></tt>.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="n">text</span> <span class="o">=</span> <span class="s">'This is some text -- with punctuation.'</span>
<span class="k">print</span> <span class="n">text</span>
<span class="k">print</span>
<span class="k">for</span> <span class="n">pattern</span> <span class="ow">in</span> <span class="p">[</span> <span class="s">r'^(?P<first_word>\w+)'</span><span class="p">,</span>
<span class="s">r'(?P<last_word>\w+)\S*$'</span><span class="p">,</span>
<span class="s">r'(?P<t_word>\bt\w+)\W+(?P<other_word>\w+)'</span><span class="p">,</span>
<span class="s">r'(?P<ends_with_t>\w+t)\b'</span><span class="p">,</span>
<span class="p">]:</span>
<span class="n">regex</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span><span class="n">pattern</span><span class="p">)</span>
<span class="n">match</span> <span class="o">=</span> <span class="n">regex</span><span class="o">.</span><span class="n">search</span><span class="p">(</span><span class="n">text</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Matching "</span><span class="si">%s</span><span class="s">"'</span> <span class="o">%</span> <span class="n">pattern</span>
<span class="k">print</span> <span class="s">' '</span><span class="p">,</span> <span class="n">match</span><span class="o">.</span><span class="n">groups</span><span class="p">()</span>
<span class="k">print</span> <span class="s">' '</span><span class="p">,</span> <span class="n">match</span><span class="o">.</span><span class="n">groupdict</span><span class="p">()</span>
<span class="k">print</span>
</pre></div>
</div>
<p>Use <tt class="xref py py-func docutils literal"><span class="pre">groupdict()</span></tt> to retrieve the dictionary mapping group names
to substrings from the match. Named patterns are included in the
ordered sequence returned by <tt class="xref py py-func docutils literal"><span class="pre">groups()</span></tt>, as well.</p>
<div class="highlight-python"><pre>$ python re_groups_named.py
This is some text -- with punctuation.
Matching "^(?P<first_word>\w+)"
('This',)
{'first_word': 'This'}
Matching "(?P<last_word>\w+)\S*$"
('punctuation',)
{'last_word': 'punctuation'}
Matching "(?P<t_word>\bt\w+)\W+(?P<other_word>\w+)"
('text', 'with')
{'other_word': 'with', 't_word': 'text'}
Matching "(?P<ends_with_t>\w+t)\b"
('text',)
{'ends_with_t': 'text'}</pre>
</div>
<p>An updated version of <tt class="xref py py-func docutils literal"><span class="pre">test_patterns()</span></tt> that shows the numbered
and named groups matched by a pattern will make the following examples
easier to follow.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="k">def</span> <span class="nf">test_patterns</span><span class="p">(</span><span class="n">text</span><span class="p">,</span> <span class="n">patterns</span><span class="o">=</span><span class="p">[]):</span>
<span class="sd">"""Given source text and a list of patterns, look for</span>
<span class="sd"> matches for each pattern within the text and print</span>
<span class="sd"> them to stdout.</span>
<span class="sd"> """</span>
<span class="c"># Show the character positions and input text</span>
<span class="k">print</span>
<span class="k">print</span> <span class="s">''</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="nb">str</span><span class="p">(</span><span class="n">i</span><span class="o">/</span><span class="mi">10</span> <span class="ow">or</span> <span class="s">' '</span><span class="p">)</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="nb">len</span><span class="p">(</span><span class="n">text</span><span class="p">)))</span>
<span class="k">print</span> <span class="s">''</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="nb">str</span><span class="p">(</span><span class="n">i</span><span class="o">%</span><span class="mi">10</span><span class="p">)</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="nb">len</span><span class="p">(</span><span class="n">text</span><span class="p">)))</span>
<span class="k">print</span> <span class="n">text</span>
<span class="c"># Look for each pattern in the text and print the results</span>
<span class="k">for</span> <span class="n">pattern</span> <span class="ow">in</span> <span class="n">patterns</span><span class="p">:</span>
<span class="k">print</span>
<span class="k">print</span> <span class="s">'Matching "</span><span class="si">%s</span><span class="s">"'</span> <span class="o">%</span> <span class="n">pattern</span>
<span class="k">for</span> <span class="n">match</span> <span class="ow">in</span> <span class="n">re</span><span class="o">.</span><span class="n">finditer</span><span class="p">(</span><span class="n">pattern</span><span class="p">,</span> <span class="n">text</span><span class="p">):</span>
<span class="n">s</span> <span class="o">=</span> <span class="n">match</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">e</span> <span class="o">=</span> <span class="n">match</span><span class="o">.</span><span class="n">end</span><span class="p">()</span>
<span class="k">print</span> <span class="s">' </span><span class="si">%2d</span><span class="s"> : </span><span class="si">%2d</span><span class="s"> = "</span><span class="si">%s</span><span class="s">"'</span> <span class="o">%</span> \
<span class="p">(</span><span class="n">s</span><span class="p">,</span> <span class="n">e</span><span class="o">-</span><span class="mi">1</span><span class="p">,</span> <span class="n">text</span><span class="p">[</span><span class="n">s</span><span class="p">:</span><span class="n">e</span><span class="p">])</span>
<span class="k">print</span> <span class="s">' Groups:'</span><span class="p">,</span> <span class="n">match</span><span class="o">.</span><span class="n">groups</span><span class="p">()</span>
<span class="k">if</span> <span class="n">match</span><span class="o">.</span><span class="n">groupdict</span><span class="p">():</span>
<span class="k">print</span> <span class="s">' Named groups:'</span><span class="p">,</span> <span class="n">match</span><span class="o">.</span><span class="n">groupdict</span><span class="p">()</span>
<span class="k">print</span>
<span class="k">return</span>
</pre></div>
</div>
<p>Since a group is itself a complete regular expression, groups can be
nested within other groups to build even more complicated expressions.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">re_test_patterns_groups</span> <span class="kn">import</span> <span class="n">test_patterns</span>
<span class="n">test_patterns</span><span class="p">(</span><span class="s">'abbaaabbbbaaaaa'</span><span class="p">,</span>
<span class="p">[</span><span class="s">r'a((a*)(b*))'</span><span class="p">,</span> <span class="c"># 'a' followed by 0-n 'a' and 0-n 'b'</span>
<span class="p">])</span>
</pre></div>
</div>
<p>In this case, the group <tt class="docutils literal"><span class="pre">(a*)</span></tt> matches an empty string, so the
return value from <tt class="xref py py-func docutils literal"><span class="pre">groups()</span></tt> includes that empty string as the
matched value.</p>
<div class="highlight-python"><pre>$ python re_groups_nested.py
11111
012345678901234
abbaaabbbbaaaaa
Matching "a((a*)(b*))"
0 : 2 = "abb"
Groups: ('bb', '', 'bb')
3 : 9 = "aaabbbb"
Groups: ('aabbbb', 'aa', 'bbbb')
10 : 14 = "aaaaa"
Groups: ('aaaa', 'aaaa', '')</pre>
</div>
<p>Groups are also useful for specifying alternative patterns. Use <tt class="docutils literal"><span class="pre">|</span></tt>
to indicate that one pattern or another should match. Consider the
placement of the <tt class="docutils literal"><span class="pre">|</span></tt> carefully, though. The first expression in
this example matches a sequence of <tt class="docutils literal"><span class="pre">a</span></tt> followed by a sequence
consisting entirely of a single letter, <tt class="docutils literal"><span class="pre">a</span></tt> or <tt class="docutils literal"><span class="pre">b</span></tt>. The second
pattern matches <tt class="docutils literal"><span class="pre">a</span></tt> followed by a sequence that may include <em>either</em>
<tt class="docutils literal"><span class="pre">a</span></tt> or <tt class="docutils literal"><span class="pre">b</span></tt>. The patterns are similar, but the resulting matches
are completely different.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">re_test_patterns_groups</span> <span class="kn">import</span> <span class="n">test_patterns</span>
<span class="n">test_patterns</span><span class="p">(</span><span class="s">'abbaaabbbbaaaaa'</span><span class="p">,</span>
<span class="p">[</span><span class="s">r'a((a+)|(b+))'</span><span class="p">,</span> <span class="c"># 'a' followed by a sequence of 'a' or sequence of 'b'</span>
<span class="s">r'a((a|b)+)'</span><span class="p">,</span> <span class="c"># 'a' followed by a sequence of 'a' or 'b'</span>
<span class="p">])</span>
</pre></div>
</div>
<p>When an alternative group is not matched, but the entire pattern does
match, the return value of <tt class="xref py py-func docutils literal"><span class="pre">groups()</span></tt> includes a <tt class="xref docutils literal"><span class="pre">None</span></tt> value at
the point in the sequence where the alternative group should appear.</p>
<div class="highlight-python"><pre>$ python re_groups_alternative.py
11111
012345678901234
abbaaabbbbaaaaa
Matching "a((a+)|(b+))"
0 : 2 = "abb"
Groups: ('bb', None, 'bb')
3 : 5 = "aaa"
Groups: ('aa', 'aa', None)
10 : 14 = "aaaaa"
Groups: ('aaaa', 'aaaa', None)
Matching "a((a|b)+)"
0 : 14 = "abbaaabbbbaaaaa"
Groups: ('bbaaabbbbaaaaa', 'a')</pre>
</div>
<p>Defining a group containing a sub-pattern is also useful in cases
where the string matching the sub-pattern is not part of what you want
to extract from the full text. These groups are called
<em>non-capturing</em>. To create a non-capturing group, use the syntax
<tt class="docutils literal"><span class="pre">(?:pattern)</span></tt>.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">re_test_patterns_groups</span> <span class="kn">import</span> <span class="n">test_patterns</span>
<span class="n">test_patterns</span><span class="p">(</span><span class="s">'abbaaabbbbaaaaa'</span><span class="p">,</span>
<span class="p">[</span><span class="s">r'a((a+)|(b+))'</span><span class="p">,</span> <span class="c"># capturing form</span>
<span class="s">r'a((?:a+)|(?:b+))'</span><span class="p">,</span> <span class="c"># non-capturing</span>
<span class="p">])</span>
</pre></div>
</div>
<p>Compare the groups returned for the capturing and non-capturing forms
of a pattern that matches the same results.</p>
<div class="highlight-python"><pre>$ python re_groups_non_capturing.py
11111
012345678901234
abbaaabbbbaaaaa
Matching "a((a+)|(b+))"
0 : 2 = "abb"
Groups: ('bb', None, 'bb')
3 : 5 = "aaa"
Groups: ('aa', 'aa', None)
10 : 14 = "aaaaa"
Groups: ('aaaa', 'aaaa', None)
Matching "a((?:a+)|(?:b+))"
0 : 2 = "abb"
Groups: ('bb',)
3 : 5 = "aaa"
Groups: ('aa',)
10 : 14 = "aaaaa"
Groups: ('aaaa',)</pre>
</div>
</div>
<div class="section" id="search-options">
<h2>Search Options<a class="headerlink" href="#search-options" title="Permalink to this headline">¶</a></h2>
<p>You can change the way the matching engine processes an expression
using option flags. The flags can be combined using a bitwise or
operation, and passed to <tt class="xref py py-func docutils literal"><span class="pre">compile()</span></tt>, <tt class="xref py py-func docutils literal"><span class="pre">search()</span></tt>,
<tt class="xref py py-func docutils literal"><span class="pre">match()</span></tt>, and other functions that accept a pattern for
searching.</p>
<div class="section" id="case-insensitive-matching">
<h3>Case-insensitive Matching<a class="headerlink" href="#case-insensitive-matching" title="Permalink to this headline">¶</a></h3>
<p><tt class="xref py py-const docutils literal"><span class="pre">IGNORECASE</span></tt> causes literal characters and character ranges in
the pattern to match both upper and lower case characters.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="n">text</span> <span class="o">=</span> <span class="s">'This is some text -- with punctuation.'</span>
<span class="n">pattern</span> <span class="o">=</span> <span class="s">r'\bT\w+'</span>
<span class="n">with_case</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span><span class="n">pattern</span><span class="p">)</span>
<span class="n">without_case</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span><span class="n">pattern</span><span class="p">,</span> <span class="n">re</span><span class="o">.</span><span class="n">IGNORECASE</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Text :'</span><span class="p">,</span> <span class="n">text</span>
<span class="k">print</span> <span class="s">'Pattern :'</span><span class="p">,</span> <span class="n">pattern</span>
<span class="k">print</span> <span class="s">'Case-sensitive :'</span><span class="p">,</span> <span class="n">with_case</span><span class="o">.</span><span class="n">findall</span><span class="p">(</span><span class="n">text</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Case-insensitive:'</span><span class="p">,</span> <span class="n">without_case</span><span class="o">.</span><span class="n">findall</span><span class="p">(</span><span class="n">text</span><span class="p">)</span>
</pre></div>
</div>
<p>Since the pattern includes the literal <tt class="docutils literal"><span class="pre">T</span></tt>, without setting
<tt class="xref py py-const docutils literal"><span class="pre">IGNORECASE</span></tt> the only match is the word <tt class="docutils literal"><span class="pre">This</span></tt>. When case is
ignored, <tt class="docutils literal"><span class="pre">text</span></tt> also matches.</p>
<div class="highlight-python"><pre>$ python re_flags_ignorecase.py
Text : This is some text -- with punctuation.
Pattern : \bT\w+
Case-sensitive : ['This']
Case-insensitive: ['This', 'text']</pre>
</div>
</div>
<div class="section" id="input-with-multiple-lines">
<h3>Input with Multiple Lines<a class="headerlink" href="#input-with-multiple-lines" title="Permalink to this headline">¶</a></h3>
<p>There are two flags that effect how searching in multi-line input
works. The <tt class="xref py py-const docutils literal"><span class="pre">MULTILINE</span></tt> flag controls how the pattern matching
code processes anchoring instructions for text containing newline
characters. When multiline mode is turned on, the anchor rules for
<tt class="docutils literal"><span class="pre">^</span></tt> and <tt class="docutils literal"><span class="pre">$</span></tt> apply at the beginning and end of each line, in
addition to the entire string.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="n">text</span> <span class="o">=</span> <span class="s">'This is some text -- with punctuation.</span><span class="se">\n</span><span class="s">And a second line.'</span>
<span class="n">pattern</span> <span class="o">=</span> <span class="s">r'(^\w+)|(\w+\S*$)'</span>
<span class="n">single_line</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span><span class="n">pattern</span><span class="p">)</span>
<span class="n">multiline</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span><span class="n">pattern</span><span class="p">,</span> <span class="n">re</span><span class="o">.</span><span class="n">MULTILINE</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Text :'</span><span class="p">,</span> <span class="nb">repr</span><span class="p">(</span><span class="n">text</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Pattern :'</span><span class="p">,</span> <span class="n">pattern</span>
<span class="k">print</span> <span class="s">'Single Line :'</span><span class="p">,</span> <span class="n">single_line</span><span class="o">.</span><span class="n">findall</span><span class="p">(</span><span class="n">text</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Multline :'</span><span class="p">,</span> <span class="n">multiline</span><span class="o">.</span><span class="n">findall</span><span class="p">(</span><span class="n">text</span><span class="p">)</span>
</pre></div>
</div>
<p>The pattern in the example matches the first or last word of the
input. It matches <tt class="docutils literal"><span class="pre">line.</span></tt> at the end of the string, even though
there is no newline.</p>
<div class="highlight-python"><pre>$ python re_flags_multiline.py
Text : 'This is some text -- with punctuation.\nAnd a second line.'
Pattern : (^\w+)|(\w+\S*$)
Single Line : [('This', ''), ('', 'line.')]
Multline : [('This', ''), ('', 'punctuation.'), ('And', ''), ('', 'line.')]</pre>
</div>
<p><tt class="xref py py-const docutils literal"><span class="pre">DOTALL</span></tt> is the other flag related to multiline text. Normally
the dot character <tt class="docutils literal"><span class="pre">.</span></tt> matches everything in the input text except a
newline character. The flag allows dot to match newlines as well.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="n">text</span> <span class="o">=</span> <span class="s">'This is some text -- with punctuation.</span><span class="se">\n</span><span class="s">And a second line.'</span>
<span class="n">pattern</span> <span class="o">=</span> <span class="s">r'.+'</span>
<span class="n">no_newlines</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span><span class="n">pattern</span><span class="p">)</span>
<span class="n">dotall</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span><span class="n">pattern</span><span class="p">,</span> <span class="n">re</span><span class="o">.</span><span class="n">DOTALL</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Text :'</span><span class="p">,</span> <span class="nb">repr</span><span class="p">(</span><span class="n">text</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Pattern :'</span><span class="p">,</span> <span class="n">pattern</span>
<span class="k">print</span> <span class="s">'No newlines :'</span><span class="p">,</span> <span class="n">no_newlines</span><span class="o">.</span><span class="n">findall</span><span class="p">(</span><span class="n">text</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Dotall :'</span><span class="p">,</span> <span class="n">dotall</span><span class="o">.</span><span class="n">findall</span><span class="p">(</span><span class="n">text</span><span class="p">)</span>
</pre></div>
</div>
<p>Without the flag, each line of the input text matches the pattern
separately. Adding the flag causes the entire string to be consumed.</p>
<div class="highlight-python"><pre>$ python re_flags_dotall.py
Text : 'This is some text -- with punctuation.\nAnd a second line.'
Pattern : .+
No newlines : ['This is some text -- with punctuation.', 'And a second line.']
Dotall : ['This is some text -- with punctuation.\nAnd a second line.']</pre>
</div>
</div>
<div class="section" id="unicode">
<h3>Unicode<a class="headerlink" href="#unicode" title="Permalink to this headline">¶</a></h3>
<p>Under Python 2, <tt class="xref py py-class docutils literal"><span class="pre">str</span></tt> objects use the ASCII character set, and
regular expression processing assumes that the pattern and input text
are both ASCII. The escape codes described earlier are defined in
terms of ASCII by default. Those assumptions mean that the pattern
<tt class="docutils literal"><span class="pre">\w+</span></tt> will match the word “French” but not “Français”, since the
<tt class="docutils literal"><span class="pre">ç</span></tt> is not part of the ASCII character set. To enable Unicode
matching in Python 2, add the <tt class="xref py py-const docutils literal"><span class="pre">UNICODE</span></tt> flag when compiling the
pattern.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="kn">import</span> <span class="nn">codecs</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="c"># Set standard output encoding to UTF-8.</span>
<span class="n">sys</span><span class="o">.</span><span class="n">stdout</span> <span class="o">=</span> <span class="n">codecs</span><span class="o">.</span><span class="n">getwriter</span><span class="p">(</span><span class="s">'UTF-8'</span><span class="p">)(</span><span class="n">sys</span><span class="o">.</span><span class="n">stdout</span><span class="p">)</span>
<span class="n">text</span> <span class="o">=</span> <span class="s">u'Français złoty Österreich'</span>
<span class="n">pattern</span> <span class="o">=</span> <span class="s">ur'\w+'</span>
<span class="n">ascii_pattern</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span><span class="n">pattern</span><span class="p">)</span>
<span class="n">unicode_pattern</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span><span class="n">pattern</span><span class="p">,</span> <span class="n">re</span><span class="o">.</span><span class="n">UNICODE</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Text :'</span><span class="p">,</span> <span class="n">text</span>
<span class="k">print</span> <span class="s">'Pattern :'</span><span class="p">,</span> <span class="n">pattern</span>
<span class="k">print</span> <span class="s">'ASCII :'</span><span class="p">,</span> <span class="s">u', '</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">ascii_pattern</span><span class="o">.</span><span class="n">findall</span><span class="p">(</span><span class="n">text</span><span class="p">))</span>
<span class="k">print</span> <span class="s">'Unicode :'</span><span class="p">,</span> <span class="s">u', '</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">unicode_pattern</span><span class="o">.</span><span class="n">findall</span><span class="p">(</span><span class="n">text</span><span class="p">))</span>
</pre></div>
</div>
<p>The other escape sequences (<tt class="docutils literal"><span class="pre">\W</span></tt>, <tt class="docutils literal"><span class="pre">\b</span></tt>, <tt class="docutils literal"><span class="pre">\B</span></tt>, <tt class="docutils literal"><span class="pre">\d</span></tt>, <tt class="docutils literal"><span class="pre">\D</span></tt>,
<tt class="docutils literal"><span class="pre">\s</span></tt>, and <tt class="docutils literal"><span class="pre">\S</span></tt>) are also processed differently for Unicode text.
Instead of assuming the members of the character set identified by the
escape sequence, the regular expression engine consults the Unicode
database to find the properties of each character.</p>
<div class="highlight-python"><pre>$ python re_flags_unicode.py
Text : Français złoty Österreich
Pattern : \w+
ASCII : Fran, ais, z, oty, sterreich
Unicode : Français, złoty, Österreich</pre>
</div>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">Python 3 uses Unicode for all strings by default, so the flag is not
necessary.</p>
</div>
</div>
<div class="section" id="verbose-expression-syntax">
<h3>Verbose Expression Syntax<a class="headerlink" href="#verbose-expression-syntax" title="Permalink to this headline">¶</a></h3>
<p>The compact format of regular expression syntax can become a hindrance
as expressions grow more complicated. As the number of groups in your
expression increases, you will have trouble keeping track of why each
element is needed and how exactly the parts of the expression
interact. Using named groups helps mitigate these issues, but a
better solution is to use <em>verbose mode</em> expressions, which allow you
to add comments and extra whitespace.</p>
<p>A pattern to validate email addresses will illustrate how verbose mode
makes working with regular expressions easier. The first version
recognizes addresses that end in one of three top-level domains,
<tt class="docutils literal"><span class="pre">.com</span></tt>, <tt class="docutils literal"><span class="pre">.org</span></tt>, and <tt class="docutils literal"><span class="pre">.edu</span></tt>.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="n">address</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span><span class="s">'[\w\d.+-]+@([\w\d.]+\.)+(com|org|edu)'</span><span class="p">,</span> <span class="n">re</span><span class="o">.</span><span class="n">UNICODE</span><span class="p">)</span>
<span class="n">candidates</span> <span class="o">=</span> <span class="p">[</span>
<span class="s">u'first.last@example.com'</span><span class="p">,</span>
<span class="s">u'first.last+category@gmail.com'</span><span class="p">,</span>
<span class="s">u'valid-address@mail.example.com'</span><span class="p">,</span>
<span class="s">u'not-valid@example.foo'</span><span class="p">,</span>
<span class="p">]</span>
<span class="k">for</span> <span class="n">candidate</span> <span class="ow">in</span> <span class="n">candidates</span><span class="p">:</span>
<span class="k">print</span>
<span class="k">print</span> <span class="s">'Candidate:'</span><span class="p">,</span> <span class="n">candidate</span>
<span class="n">match</span> <span class="o">=</span> <span class="n">address</span><span class="o">.</span><span class="n">search</span><span class="p">(</span><span class="n">candidate</span><span class="p">)</span>
<span class="k">if</span> <span class="n">match</span><span class="p">:</span>
<span class="k">print</span> <span class="s">' Matches'</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">print</span> <span class="s">' No match'</span>
</pre></div>
</div>
<p>This expression is already complex. There are several character
classes, groups, and repetition expressions.</p>
<div class="highlight-python"><pre>$ python re_email_compact.py
Candidate: first.last@example.com
Matches
Candidate: first.last+category@gmail.com
Matches
Candidate: valid-address@mail.example.com
Matches
Candidate: not-valid@example.foo
No match</pre>
</div>
<p>Converting the expression to a more verbose format will make it easier
to extend.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="n">address</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span>
<span class="sd">'''</span>
<span class="sd"> [\w\d.+-]+ # username</span>
<span class="sd"> @</span>
<span class="sd"> ([\w\d.]+\.)+ # domain name prefix</span>
<span class="sd"> (com|org|edu) # we should support more top-level domains</span>
<span class="sd"> '''</span><span class="p">,</span>
<span class="n">re</span><span class="o">.</span><span class="n">UNICODE</span> <span class="o">|</span> <span class="n">re</span><span class="o">.</span><span class="n">VERBOSE</span><span class="p">)</span>
<span class="n">candidates</span> <span class="o">=</span> <span class="p">[</span>
<span class="s">u'first.last@example.com'</span><span class="p">,</span>
<span class="s">u'first.last+category@gmail.com'</span><span class="p">,</span>
<span class="s">u'valid-address@mail.example.com'</span><span class="p">,</span>
<span class="s">u'not-valid@example.foo'</span><span class="p">,</span>
<span class="p">]</span>
<span class="k">for</span> <span class="n">candidate</span> <span class="ow">in</span> <span class="n">candidates</span><span class="p">:</span>
<span class="k">print</span>
<span class="k">print</span> <span class="s">'Candidate:'</span><span class="p">,</span> <span class="n">candidate</span>
<span class="n">match</span> <span class="o">=</span> <span class="n">address</span><span class="o">.</span><span class="n">search</span><span class="p">(</span><span class="n">candidate</span><span class="p">)</span>
<span class="k">if</span> <span class="n">match</span><span class="p">:</span>
<span class="k">print</span> <span class="s">' Matches'</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">print</span> <span class="s">' No match'</span>
</pre></div>
</div>
<p>The expression matches the same inputs, but in this extended format it
is easier to read. The comments also help identify different parts of
the pattern so that it can be expanded to match more inputs.</p>
<div class="highlight-python"><pre>$ python re_email_verbose.py
Candidate: first.last@example.com
Matches
Candidate: first.last+category@gmail.com
Matches
Candidate: valid-address@mail.example.com
Matches
Candidate: not-valid@example.foo
No match</pre>
</div>
<p>This expanded version parses inputs that include a person’s name and
email address, as might appear in an email header. The name comes
first and stands on its own, and the email address follows surrounded
by angle brackets (<tt class="docutils literal"><span class="pre"><</span></tt> and <tt class="docutils literal"><span class="pre">></span></tt>).</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="n">address</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span>
<span class="sd">'''</span>
<span class="sd"> # A name is made up of letters, and may include "." for title</span>
<span class="sd"> # abbreviations and middle initials.</span>
<span class="sd"> ((?P<name></span>
<span class="sd"> ([\w.,]+\s+)*[\w.,]+)</span>
<span class="sd"> \s*</span>
<span class="sd"> # Email addresses are wrapped in angle brackets: < ></span>
<span class="sd"> # but we only want one if we found a name, so keep</span>
<span class="sd"> # the start bracket in this group.</span>
<span class="sd"> <</span>
<span class="sd"> )? # the entire name is optional</span>
<span class="sd"> # The address itself: username@domain.tld</span>
<span class="sd"> (?P<email></span>
<span class="sd"> [\w\d.+-]+ # username</span>
<span class="sd"> @</span>
<span class="sd"> ([\w\d.]+\.)+ # domain name prefix</span>
<span class="sd"> (com|org|edu) # limit the allowed top-level domains</span>
<span class="sd"> )</span>
<span class="sd"> >? # optional closing angle bracket</span>
<span class="sd"> '''</span><span class="p">,</span>
<span class="n">re</span><span class="o">.</span><span class="n">UNICODE</span> <span class="o">|</span> <span class="n">re</span><span class="o">.</span><span class="n">VERBOSE</span><span class="p">)</span>
<span class="n">candidates</span> <span class="o">=</span> <span class="p">[</span>
<span class="s">u'first.last@example.com'</span><span class="p">,</span>
<span class="s">u'first.last+category@gmail.com'</span><span class="p">,</span>
<span class="s">u'valid-address@mail.example.com'</span><span class="p">,</span>
<span class="s">u'not-valid@example.foo'</span><span class="p">,</span>
<span class="s">u'First Last <first.last@example.com>'</span><span class="p">,</span>
<span class="s">u'No Brackets first.last@example.com'</span><span class="p">,</span>
<span class="s">u'First Last'</span><span class="p">,</span>
<span class="s">u'First Middle Last <first.last@example.com>'</span><span class="p">,</span>
<span class="s">u'First M. Last <first.last@example.com>'</span><span class="p">,</span>
<span class="s">u'<first.last@example.com>'</span><span class="p">,</span>
<span class="p">]</span>
<span class="k">for</span> <span class="n">candidate</span> <span class="ow">in</span> <span class="n">candidates</span><span class="p">:</span>
<span class="k">print</span>
<span class="k">print</span> <span class="s">'Candidate:'</span><span class="p">,</span> <span class="n">candidate</span>
<span class="n">match</span> <span class="o">=</span> <span class="n">address</span><span class="o">.</span><span class="n">search</span><span class="p">(</span><span class="n">candidate</span><span class="p">)</span>
<span class="k">if</span> <span class="n">match</span><span class="p">:</span>
<span class="k">print</span> <span class="s">' Match name :'</span><span class="p">,</span> <span class="n">match</span><span class="o">.</span><span class="n">groupdict</span><span class="p">()[</span><span class="s">'name'</span><span class="p">]</span>
<span class="k">print</span> <span class="s">' Match email:'</span><span class="p">,</span> <span class="n">match</span><span class="o">.</span><span class="n">groupdict</span><span class="p">()[</span><span class="s">'email'</span><span class="p">]</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">print</span> <span class="s">' No match'</span>
</pre></div>
</div>
<p>As with other programming languages, the ability to insert comments
into verbose regular expressions helps with their maintainability.
This final version includes implementation notes to future maintainers
and whitespace to separate the groups from each other and highlight
their nesting level.</p>
<div class="highlight-python"><pre>$ python re_email_with_name.py
Candidate: first.last@example.com
Match name : None
Match email: first.last@example.com
Candidate: first.last+category@gmail.com
Match name : None
Match email: first.last+category@gmail.com
Candidate: valid-address@mail.example.com
Match name : None
Match email: valid-address@mail.example.com
Candidate: not-valid@example.foo
No match
Candidate: First Last <first.last@example.com>
Match name : First Last
Match email: first.last@example.com
Candidate: No Brackets first.last@example.com
Match name : None
Match email: first.last@example.com
Candidate: First Last
No match
Candidate: First Middle Last <first.last@example.com>
Match name : First Middle Last
Match email: first.last@example.com
Candidate: First M. Last <first.last@example.com>
Match name : First M. Last
Match email: first.last@example.com
Candidate: <first.last@example.com>
Match name : None
Match email: first.last@example.com</pre>
</div>
</div>
<div class="section" id="embedding-flags-in-patterns">
<h3>Embedding Flags in Patterns<a class="headerlink" href="#embedding-flags-in-patterns" title="Permalink to this headline">¶</a></h3>
<p>In situations where you cannot add flags when compiling an expression,
such as when you are passing a pattern to a library function that will
compile it later, you can embed the flags inside the expression string
itself. For example, to turn case-insensitive matching on, add
<tt class="docutils literal"><span class="pre">(?i)</span></tt> to the beginning of the expression.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="n">text</span> <span class="o">=</span> <span class="s">'This is some text -- with punctuation.'</span>
<span class="n">pattern</span> <span class="o">=</span> <span class="s">r'(?i)\bT\w+'</span>
<span class="n">regex</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span><span class="n">pattern</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Text :'</span><span class="p">,</span> <span class="n">text</span>
<span class="k">print</span> <span class="s">'Pattern :'</span><span class="p">,</span> <span class="n">pattern</span>
<span class="k">print</span> <span class="s">'Matches :'</span><span class="p">,</span> <span class="n">regex</span><span class="o">.</span><span class="n">findall</span><span class="p">(</span><span class="n">text</span><span class="p">)</span>
</pre></div>
</div>
<p>Because the options control the way the entire expression is evaluated
or parsed, they should always come at the beginning of the expression.</p>
<div class="highlight-python"><pre>$ python re_flags_embedded.py
Text : This is some text -- with punctuation.
Pattern : (?i)\bT\w+
Matches : ['This', 'text']</pre>
</div>
<p>The abbreviations for all of the flags are:</p>
<table border="1" class="docutils">
<colgroup>
<col width="63%" />
<col width="38%" />
</colgroup>
<thead valign="bottom">
<tr><th class="head">Flag</th>
<th class="head">Abbreviation</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><tt class="xref py py-const docutils literal"><span class="pre">IGNORECASE</span></tt></td>
<td><tt class="docutils literal"><span class="pre">i</span></tt></td>
</tr>
<tr><td><tt class="xref py py-const docutils literal"><span class="pre">MULTILINE</span></tt></td>
<td><tt class="docutils literal"><span class="pre">m</span></tt></td>
</tr>
<tr><td><tt class="xref py py-const docutils literal"><span class="pre">DOTALL</span></tt></td>
<td><tt class="docutils literal"><span class="pre">s</span></tt></td>
</tr>
<tr><td><tt class="xref py py-const docutils literal"><span class="pre">UNICODE</span></tt></td>
<td><tt class="docutils literal"><span class="pre">u</span></tt></td>
</tr>
<tr><td><tt class="xref py py-const docutils literal"><span class="pre">VERBOSE</span></tt></td>
<td><tt class="docutils literal"><span class="pre">x</span></tt></td>
</tr>
</tbody>
</table>
<p>Embedded flags can be combined by placing them within the same group.
For example, <tt class="docutils literal"><span class="pre">(?imu)</span></tt> turns on case-insensitive matching for
multiline Unicode strings.</p>
</div>
</div>
<div class="section" id="looking-ahead-or-behind">
<h2>Looking Ahead, or Behind<a class="headerlink" href="#looking-ahead-or-behind" title="Permalink to this headline">¶</a></h2>
<p>There are many cases where it is useful to match a part of a pattern
only if some other part will also match. For example, in the email
parsing expression the angle brackets were each marked as optional.
Really, though, the brackets should be paired, and the expression
should only match if both are present, or neither are. This modified
version of the expression uses a <em>positive look ahead</em> assertion to
match the pair. The look ahead assertion syntax is <tt class="docutils literal"><span class="pre">(?=pattern)</span></tt>.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="n">address</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span>
<span class="sd">'''</span>
<span class="sd"> # A name is made up of letters, and may include "." for title</span>
<span class="sd"> # abbreviations and middle initials.</span>
<span class="sd"> ((?P<name></span>
<span class="sd"> ([\w.,]+\s+)*[\w.,]+</span>
<span class="sd"> )</span>
<span class="sd"> \s+</span>
<span class="sd"> ) # name is no longer optional</span>
<span class="sd"> # LOOKAHEAD</span>
<span class="sd"> # Email addresses are wrapped in angle brackets, but we only want</span>
<span class="sd"> # the brackets if they are both there, or neither are.</span>
<span class="sd"> (?= (<.*>$) # remainder wrapped in angle brackets</span>
<span class="sd"> |</span>
<span class="sd"> ([^<].*[^>]$) # remainder *not* wrapped in angle brackets</span>
<span class="sd"> )</span>
<span class="sd"> <? # optional opening angle bracket</span>
<span class="sd"> # The address itself: username@domain.tld</span>
<span class="sd"> (?P<email></span>
<span class="sd"> [\w\d.+-]+ # username</span>
<span class="sd"> @</span>
<span class="sd"> ([\w\d.]+\.)+ # domain name prefix</span>
<span class="sd"> (com|org|edu) # limit the allowed top-level domains</span>
<span class="sd"> )</span>
<span class="sd"> >? # optional closing angle bracket</span>
<span class="sd"> '''</span><span class="p">,</span>
<span class="n">re</span><span class="o">.</span><span class="n">UNICODE</span> <span class="o">|</span> <span class="n">re</span><span class="o">.</span><span class="n">VERBOSE</span><span class="p">)</span>
<span class="n">candidates</span> <span class="o">=</span> <span class="p">[</span>
<span class="s">u'First Last <first.last@example.com>'</span><span class="p">,</span>
<span class="s">u'No Brackets first.last@example.com'</span><span class="p">,</span>
<span class="s">u'Open Bracket <first.last@example.com'</span><span class="p">,</span>
<span class="s">u'Close Bracket first.last@example.com>'</span><span class="p">,</span>
<span class="p">]</span>
<span class="k">for</span> <span class="n">candidate</span> <span class="ow">in</span> <span class="n">candidates</span><span class="p">:</span>
<span class="k">print</span>
<span class="k">print</span> <span class="s">'Candidate:'</span><span class="p">,</span> <span class="n">candidate</span>
<span class="n">match</span> <span class="o">=</span> <span class="n">address</span><span class="o">.</span><span class="n">search</span><span class="p">(</span><span class="n">candidate</span><span class="p">)</span>
<span class="k">if</span> <span class="n">match</span><span class="p">:</span>
<span class="k">print</span> <span class="s">' Match name :'</span><span class="p">,</span> <span class="n">match</span><span class="o">.</span><span class="n">groupdict</span><span class="p">()[</span><span class="s">'name'</span><span class="p">]</span>
<span class="k">print</span> <span class="s">' Match email:'</span><span class="p">,</span> <span class="n">match</span><span class="o">.</span><span class="n">groupdict</span><span class="p">()[</span><span class="s">'email'</span><span class="p">]</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">print</span> <span class="s">' No match'</span>
</pre></div>
</div>
<p>There are several important changes in this version of the expression.
First, the name portion is no longer optional. That means stand-alone
addresses do not match, but it also prevents improperly formatted
name/address combinations from matching. The positive look ahead rule
after the “name” group asserts that the remainder of the string is
either wrapped with a pair of angle brackets, or there is not a
mismatched bracket; the brackets are either both present or neither
is. The look ahead is expressed as a group, but the match for a look
ahead group does not consume any of the input text, so the rest of the
pattern picks up from the same spot after the look ahead matches.</p>
<div class="highlight-python"><pre>$ python re_look_ahead.py
Candidate: First Last <first.last@example.com>
Match name : First Last
Match email: first.last@example.com
Candidate: No Brackets first.last@example.com
Match name : No Brackets
Match email: first.last@example.com
Candidate: Open Bracket <first.last@example.com
No match
Candidate: Close Bracket first.last@example.com>
No match</pre>
</div>
<p>A <em>negative look ahead</em> assertion (<tt class="docutils literal"><span class="pre">(?!pattern)</span></tt>) says that the
pattern does not match the text following the current point. For
example, the email recognition pattern could be modified to ignore
<tt class="docutils literal"><span class="pre">noreply</span></tt> mailing addresses commonly used by automated systems.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="n">address</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span>
<span class="sd">'''</span>
<span class="sd"> ^</span>
<span class="sd"> # An address: username@domain.tld</span>
<span class="sd"> # Ignore noreply addresses</span>
<span class="sd"> (?!noreply@.*$)</span>
<span class="sd"> [\w\d.+-]+ # username</span>
<span class="sd"> @</span>
<span class="sd"> ([\w\d.]+\.)+ # domain name prefix</span>
<span class="sd"> (com|org|edu) # limit the allowed top-level domains</span>
<span class="sd"> $</span>
<span class="sd"> '''</span><span class="p">,</span>
<span class="n">re</span><span class="o">.</span><span class="n">UNICODE</span> <span class="o">|</span> <span class="n">re</span><span class="o">.</span><span class="n">VERBOSE</span><span class="p">)</span>
<span class="n">candidates</span> <span class="o">=</span> <span class="p">[</span>
<span class="s">u'first.last@example.com'</span><span class="p">,</span>
<span class="s">u'noreply@example.com'</span><span class="p">,</span>
<span class="p">]</span>
<span class="k">for</span> <span class="n">candidate</span> <span class="ow">in</span> <span class="n">candidates</span><span class="p">:</span>
<span class="k">print</span>
<span class="k">print</span> <span class="s">'Candidate:'</span><span class="p">,</span> <span class="n">candidate</span>
<span class="n">match</span> <span class="o">=</span> <span class="n">address</span><span class="o">.</span><span class="n">search</span><span class="p">(</span><span class="n">candidate</span><span class="p">)</span>
<span class="k">if</span> <span class="n">match</span><span class="p">:</span>
<span class="k">print</span> <span class="s">' Match:'</span><span class="p">,</span> <span class="n">candidate</span><span class="p">[</span><span class="n">match</span><span class="o">.</span><span class="n">start</span><span class="p">():</span><span class="n">match</span><span class="o">.</span><span class="n">end</span><span class="p">()]</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">print</span> <span class="s">' No match'</span>
</pre></div>
</div>
<p>The address starting <tt class="docutils literal"><span class="pre">noreply</span></tt> does not match the pattern, since the
look ahead assertion fails.</p>
<div class="highlight-python"><pre>$ python re_negative_look_ahead.py
Candidate: first.last@example.com
Match: first.last@example.com
Candidate: noreply@example.com
No match</pre>
</div>
<p>Instead of looking ahead for <tt class="docutils literal"><span class="pre">noreply</span></tt> in the username portion of
the email address, the pattern can also be written using a <em>negative
look behind</em> assertion after the username is matched using the syntax
<tt class="docutils literal"><span class="pre">(?<!pattern)</span></tt>.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="n">address</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span>
<span class="sd">'''</span>
<span class="sd"> ^</span>
<span class="sd"> # An address: username@domain.tld</span>
<span class="sd"> [\w\d.+-]+ # username</span>
<span class="sd"> # Ignore noreply addresses</span>
<span class="sd"> (?<!noreply)</span>
<span class="sd"> @</span>
<span class="sd"> ([\w\d.]+\.)+ # domain name prefix</span>
<span class="sd"> (com|org|edu) # limit the allowed top-level domains</span>
<span class="sd"> $</span>
<span class="sd"> '''</span><span class="p">,</span>
<span class="n">re</span><span class="o">.</span><span class="n">UNICODE</span> <span class="o">|</span> <span class="n">re</span><span class="o">.</span><span class="n">VERBOSE</span><span class="p">)</span>
<span class="n">candidates</span> <span class="o">=</span> <span class="p">[</span>
<span class="s">u'first.last@example.com'</span><span class="p">,</span>
<span class="s">u'noreply@example.com'</span><span class="p">,</span>
<span class="p">]</span>
<span class="k">for</span> <span class="n">candidate</span> <span class="ow">in</span> <span class="n">candidates</span><span class="p">:</span>
<span class="k">print</span>
<span class="k">print</span> <span class="s">'Candidate:'</span><span class="p">,</span> <span class="n">candidate</span>
<span class="n">match</span> <span class="o">=</span> <span class="n">address</span><span class="o">.</span><span class="n">search</span><span class="p">(</span><span class="n">candidate</span><span class="p">)</span>
<span class="k">if</span> <span class="n">match</span><span class="p">:</span>
<span class="k">print</span> <span class="s">' Match:'</span><span class="p">,</span> <span class="n">candidate</span><span class="p">[</span><span class="n">match</span><span class="o">.</span><span class="n">start</span><span class="p">():</span><span class="n">match</span><span class="o">.</span><span class="n">end</span><span class="p">()]</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">print</span> <span class="s">' No match'</span>
</pre></div>
</div>
<p>Looking backwards works a little differently than looking ahead, in
that the expression must use a fixed length pattern. Repetitions are
allowed, as long as there is a fixed number (no wildcards or ranges).</p>
<div class="highlight-python"><pre>$ python re_negative_look_behind.py
Candidate: first.last@example.com
Match: first.last@example.com
Candidate: noreply@example.com
No match</pre>
</div>
<p>A <em>positive look behind</em> assertion can be used to find text following
a pattern using the syntax <tt class="docutils literal"><span class="pre">(?<=pattern)</span></tt>. For example, this
expression finds Twitter handles.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="n">twitter</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span>
<span class="sd">'''</span>
<span class="sd"> # A twitter handle: @username</span>
<span class="sd"> (?<=@)</span>
<span class="sd"> ([\w\d_]+) # username</span>
<span class="sd"> '''</span><span class="p">,</span>
<span class="n">re</span><span class="o">.</span><span class="n">UNICODE</span> <span class="o">|</span> <span class="n">re</span><span class="o">.</span><span class="n">VERBOSE</span><span class="p">)</span>
<span class="n">text</span> <span class="o">=</span> <span class="s">'''This text includes two Twitter handles.</span>
<span class="s">One for @ThePSF, and one for the author, @doughellmann.</span>
<span class="s">'''</span>
<span class="k">print</span> <span class="n">text</span>
<span class="k">for</span> <span class="n">match</span> <span class="ow">in</span> <span class="n">twitter</span><span class="o">.</span><span class="n">findall</span><span class="p">(</span><span class="n">text</span><span class="p">):</span>
<span class="k">print</span> <span class="s">'Handle:'</span><span class="p">,</span> <span class="n">match</span>
</pre></div>
</div>
<p>The pattern matches sequences of characters that can make up a Twitter
handle, as long as they are preceded by an <tt class="docutils literal"><span class="pre">@</span></tt>.</p>
<div class="highlight-python"><pre>$ python re_look_behind.py
This text includes two Twitter handles.
One for @ThePSF, and one for the author, @doughellmann.
Handle: ThePSF
Handle: doughellmann</pre>
</div>
</div>
<div class="section" id="self-referencing-expressions">
<h2>Self-referencing Expressions<a class="headerlink" href="#self-referencing-expressions" title="Permalink to this headline">¶</a></h2>
<p>Matched values can be used in later parts of an expression. For
example, the email example can be updated to match only addresses
composed of the first and last name of the person by including
back-references to those groups. The easiest way to achieve this is
by referring to the previously matched group by id number, using
<tt class="docutils literal"><span class="pre">\num</span></tt>.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="n">address</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span>
<span class="s">r'''</span>
<span class="s"> # The regular name</span>
<span class="s"> (\w+) # first name</span>
<span class="s"> \s+</span>
<span class="s"> (([\w.]+)\s+)? # optional middle name or initial</span>
<span class="s"> (\w+) # last name</span>
<span class="s"> \s+</span>
<span class="s"> <</span>
<span class="s"> # The address: first_name.last_name@domain.tld</span>
<span class="s"> (?P<email></span>
<span class="s"> \1 # first name</span>
<span class="s"> \.</span>
<span class="s"> \4 # last name</span>
<span class="s"> @</span>
<span class="s"> ([\w\d.]+\.)+ # domain name prefix</span>
<span class="s"> (com|org|edu) # limit the allowed top-level domains</span>
<span class="s"> )</span>
<span class="s"> ></span>
<span class="s"> '''</span><span class="p">,</span>
<span class="n">re</span><span class="o">.</span><span class="n">UNICODE</span> <span class="o">|</span> <span class="n">re</span><span class="o">.</span><span class="n">VERBOSE</span> <span class="o">|</span> <span class="n">re</span><span class="o">.</span><span class="n">IGNORECASE</span><span class="p">)</span>
<span class="n">candidates</span> <span class="o">=</span> <span class="p">[</span>
<span class="s">u'First Last <first.last@example.com>'</span><span class="p">,</span>
<span class="s">u'Different Name <first.last@example.com>'</span><span class="p">,</span>
<span class="s">u'First Middle Last <first.last@example.com>'</span><span class="p">,</span>
<span class="s">u'First M. Last <first.last@example.com>'</span><span class="p">,</span>
<span class="p">]</span>
<span class="k">for</span> <span class="n">candidate</span> <span class="ow">in</span> <span class="n">candidates</span><span class="p">:</span>
<span class="k">print</span>
<span class="k">print</span> <span class="s">'Candidate:'</span><span class="p">,</span> <span class="n">candidate</span>
<span class="n">match</span> <span class="o">=</span> <span class="n">address</span><span class="o">.</span><span class="n">search</span><span class="p">(</span><span class="n">candidate</span><span class="p">)</span>
<span class="k">if</span> <span class="n">match</span><span class="p">:</span>
<span class="k">print</span> <span class="s">' Match name :'</span><span class="p">,</span> <span class="n">match</span><span class="o">.</span><span class="n">group</span><span class="p">(</span><span class="mi">1</span><span class="p">),</span> <span class="n">match</span><span class="o">.</span><span class="n">group</span><span class="p">(</span><span class="mi">4</span><span class="p">)</span>
<span class="k">print</span> <span class="s">' Match email:'</span><span class="p">,</span> <span class="n">match</span><span class="o">.</span><span class="n">group</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">print</span> <span class="s">' No match'</span>
</pre></div>
</div>
<p>Although the syntax is simple, creating back-references by numerical
id has a couple of disadvantages. From a practical standpoint, as the
expression changes, you must count the groups again and possibly
update every reference. The other disadvantage is that only 99
references can be made this way, because if the id number is three
digits long it will be interpreted as an octal character value instead
of a group reference. On the other hand, if you have more than 99
groups in your expression you will have more serious maintenance
challenges than not being able to refer to some of the groups in the
expression.</p>
<div class="highlight-python"><pre>$ python re_refer_to_group.py
Candidate: First Last <first.last@example.com>
Match name : First Last
Match email: first.last@example.com
Candidate: Different Name <first.last@example.com>
No match
Candidate: First Middle Last <first.last@example.com>
Match name : First Last
Match email: first.last@example.com
Candidate: First M. Last <first.last@example.com>
Match name : First Last
Match email: first.last@example.com</pre>
</div>
<p>Python’s expression parser includes an extension that uses
<tt class="docutils literal"><span class="pre">(?P=name)</span></tt> to refer to the value of a named group matched earlier
in the expression.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="n">address</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span>
<span class="sd">'''</span>
<span class="sd"> # The regular name</span>
<span class="sd"> (?P<first_name>\w+)</span>
<span class="sd"> \s+</span>
<span class="sd"> (([\w.]+)\s+)? # optional middle name or initial</span>
<span class="sd"> (?P<last_name>\w+)</span>
<span class="sd"> \s+</span>
<span class="sd"> <</span>
<span class="sd"> # The address: first_name.last_name@domain.tld</span>
<span class="sd"> (?P<email></span>
<span class="sd"> (?P=first_name)</span>
<span class="sd"> \.</span>
<span class="sd"> (?P=last_name)</span>
<span class="sd"> @</span>
<span class="sd"> ([\w\d.]+\.)+ # domain name prefix</span>
<span class="sd"> (com|org|edu) # limit the allowed top-level domains</span>
<span class="sd"> )</span>
<span class="sd"> ></span>
<span class="sd"> '''</span><span class="p">,</span>
<span class="n">re</span><span class="o">.</span><span class="n">UNICODE</span> <span class="o">|</span> <span class="n">re</span><span class="o">.</span><span class="n">VERBOSE</span> <span class="o">|</span> <span class="n">re</span><span class="o">.</span><span class="n">IGNORECASE</span><span class="p">)</span>
<span class="n">candidates</span> <span class="o">=</span> <span class="p">[</span>
<span class="s">u'First Last <first.last@example.com>'</span><span class="p">,</span>
<span class="s">u'Different Name <first.last@example.com>'</span><span class="p">,</span>
<span class="s">u'First Middle Last <first.last@example.com>'</span><span class="p">,</span>
<span class="s">u'First M. Last <first.last@example.com>'</span><span class="p">,</span>
<span class="p">]</span>
<span class="k">for</span> <span class="n">candidate</span> <span class="ow">in</span> <span class="n">candidates</span><span class="p">:</span>
<span class="k">print</span>
<span class="k">print</span> <span class="s">'Candidate:'</span><span class="p">,</span> <span class="n">candidate</span>
<span class="n">match</span> <span class="o">=</span> <span class="n">address</span><span class="o">.</span><span class="n">search</span><span class="p">(</span><span class="n">candidate</span><span class="p">)</span>
<span class="k">if</span> <span class="n">match</span><span class="p">:</span>
<span class="k">print</span> <span class="s">' Match name :'</span><span class="p">,</span> <span class="n">match</span><span class="o">.</span><span class="n">groupdict</span><span class="p">()[</span><span class="s">'first_name'</span><span class="p">],</span> <span class="n">match</span><span class="o">.</span><span class="n">groupdict</span><span class="p">()[</span><span class="s">'last_name'</span><span class="p">]</span>
<span class="k">print</span> <span class="s">' Match email:'</span><span class="p">,</span> <span class="n">match</span><span class="o">.</span><span class="n">groupdict</span><span class="p">()[</span><span class="s">'email'</span><span class="p">]</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">print</span> <span class="s">' No match'</span>
</pre></div>
</div>
<p>The address expression is compiled with the <tt class="xref py py-const docutils literal"><span class="pre">IGNORECASE</span></tt> flag
on, since proper names are normally capitalized but email addresses
are not.</p>
<div class="highlight-python"><pre>$ python re_refer_to_named_group.py
Candidate: First Last <first.last@example.com>
Match name : First Last
Match email: first.last@example.com
Candidate: Different Name <first.last@example.com>
No match
Candidate: First Middle Last <first.last@example.com>
Match name : First Last
Match email: first.last@example.com
Candidate: First M. Last <first.last@example.com>
Match name : First Last
Match email: first.last@example.com</pre>
</div>
<p>The other mechanism for using back-references in expressions lets you
choose a different pattern based on whether or not a previous group
matched. The email pattern can be corrected so that the angle
brackets are required if a name is present, and not if the email
address is by itself. The syntax for testing to see if a group has
matched is <tt class="docutils literal"><span class="pre">(?(id)yes-expression|no-expression)</span></tt>, where <em>id</em> is the
group name or number, <em>yes-expression</em> is the pattern to use if the
group has a value and <em>no-expression</em> is the pattern to use otherwise.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="n">address</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span>
<span class="sd">'''</span>
<span class="sd"> ^</span>
<span class="sd"> # A name is made up of letters, and may include "." for title</span>
<span class="sd"> # abbreviations and middle initials.</span>
<span class="sd"> (?P<name></span>
<span class="sd"> ([\w.]+\s+)*[\w.]+</span>
<span class="sd"> )?</span>
<span class="sd"> \s*</span>
<span class="sd"> # Email addresses are wrapped in angle brackets, but we only want</span>
<span class="sd"> # the brackets if we found a name.</span>
<span class="sd"> (?(name)</span>
<span class="sd"> # remainder wrapped in angle brackets because we have a name</span>
<span class="sd"> (?P<brackets>(?=(<.*>$)))</span>
<span class="sd"> |</span>
<span class="sd"> # remainder does not include angle brackets without name</span>
<span class="sd"> (?=([^<].*[^>]$))</span>
<span class="sd"> )</span>
<span class="sd"> # Only look for a bracket if our look ahead assertion found both</span>
<span class="sd"> # of them.</span>
<span class="sd"> (?(brackets)<|\s*)</span>
<span class="sd"> # The address itself: username@domain.tld</span>
<span class="sd"> (?P<email></span>
<span class="sd"> [\w\d.+-]+ # username</span>
<span class="sd"> @</span>
<span class="sd"> ([\w\d.]+\.)+ # domain name prefix</span>
<span class="sd"> (com|org|edu) # limit the allowed top-level domains</span>
<span class="sd"> )</span>
<span class="sd"> # Only look for a bracket if our look ahead assertion found both</span>
<span class="sd"> # of them.</span>
<span class="sd"> (?(brackets)>|\s*)</span>
<span class="sd"> $</span>
<span class="sd"> '''</span><span class="p">,</span>
<span class="n">re</span><span class="o">.</span><span class="n">UNICODE</span> <span class="o">|</span> <span class="n">re</span><span class="o">.</span><span class="n">VERBOSE</span><span class="p">)</span>
<span class="n">candidates</span> <span class="o">=</span> <span class="p">[</span>
<span class="s">u'First Last <first.last@example.com>'</span><span class="p">,</span>
<span class="s">u'No Brackets first.last@example.com'</span><span class="p">,</span>
<span class="s">u'Open Bracket <first.last@example.com'</span><span class="p">,</span>
<span class="s">u'Close Bracket first.last@example.com>'</span><span class="p">,</span>
<span class="s">u'no.brackets@example.com'</span><span class="p">,</span>
<span class="p">]</span>
<span class="k">for</span> <span class="n">candidate</span> <span class="ow">in</span> <span class="n">candidates</span><span class="p">:</span>
<span class="k">print</span>
<span class="k">print</span> <span class="s">'Candidate:'</span><span class="p">,</span> <span class="n">candidate</span>
<span class="n">match</span> <span class="o">=</span> <span class="n">address</span><span class="o">.</span><span class="n">search</span><span class="p">(</span><span class="n">candidate</span><span class="p">)</span>
<span class="k">if</span> <span class="n">match</span><span class="p">:</span>
<span class="k">print</span> <span class="s">' Match name :'</span><span class="p">,</span> <span class="n">match</span><span class="o">.</span><span class="n">groupdict</span><span class="p">()[</span><span class="s">'name'</span><span class="p">]</span>
<span class="k">print</span> <span class="s">' Match email:'</span><span class="p">,</span> <span class="n">match</span><span class="o">.</span><span class="n">groupdict</span><span class="p">()[</span><span class="s">'email'</span><span class="p">]</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">print</span> <span class="s">' No match'</span>
</pre></div>
</div>
<p>This version of the email address parser uses two tests. If the
<tt class="docutils literal"><span class="pre">name</span></tt> group matches, then the look ahead assertion requires both
angle brackets and sets up the <tt class="docutils literal"><span class="pre">brackets</span></tt> group. If <tt class="docutils literal"><span class="pre">name</span></tt> is not
matched, the assertion requires the rest of the text not have angle
brackets around it. Later, if the <tt class="docutils literal"><span class="pre">brackets</span></tt> group is set, the
actual pattern matching code consumes the brackets in the input using
literal patterns, otherwise it consumes any blank space.</p>
<div class="highlight-python"><pre>$ python re_id.py
Candidate: First Last <first.last@example.com>
Match name : First Last
Match email: first.last@example.com
Candidate: No Brackets first.last@example.com
No match
Candidate: Open Bracket <first.last@example.com
No match
Candidate: Close Bracket first.last@example.com>
No match
Candidate: no.brackets@example.com
Match name : None
Match email: no.brackets@example.com</pre>
</div>
</div>
<div class="section" id="modifying-strings-with-patterns">
<h2>Modifying Strings with Patterns<a class="headerlink" href="#modifying-strings-with-patterns" title="Permalink to this headline">¶</a></h2>
<p>In addition to searching through text, <a class="reference internal" href="#module-re" title="re: Searching within and changing text using formal patterns."><tt class="xref py py-mod docutils literal"><span class="pre">re</span></tt></a> also supports
modifying text using regular expressions as the search mechanism, and
the replacements can reference groups matched in the regex as part of
the substitution text. Use <tt class="xref py py-func docutils literal"><span class="pre">sub()</span></tt> to replace all occurances of a
pattern with another string.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="n">bold</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span><span class="s">r'\*{2}(.*?)\*{2}'</span><span class="p">,</span> <span class="n">re</span><span class="o">.</span><span class="n">UNICODE</span><span class="p">)</span>
<span class="n">text</span> <span class="o">=</span> <span class="s">'Make this **bold**. This **too**.'</span>
<span class="k">print</span> <span class="s">'Text:'</span><span class="p">,</span> <span class="n">text</span>
<span class="k">print</span> <span class="s">'Bold:'</span><span class="p">,</span> <span class="n">bold</span><span class="o">.</span><span class="n">sub</span><span class="p">(</span><span class="s">r'<b>\1</b>'</span><span class="p">,</span> <span class="n">text</span><span class="p">)</span>
</pre></div>
</div>
<p>References to the text matched by the pattern can be inserted using
the <tt class="docutils literal"><span class="pre">\num</span></tt> syntax used for back-references above.</p>
<div class="highlight-python"><pre>$ python re_sub.py
Text: Make this **bold**. This **too**.
Bold: Make this <b>bold</b>. This <b>too</b>.</pre>
</div>
<p>To use named groups in the substitution, use the syntax <tt class="docutils literal"><span class="pre">\g<name></span></tt>.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="n">bold</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span><span class="s">r'\*{2}(?P<bold_text>.*?)\*{2}'</span><span class="p">,</span> <span class="n">re</span><span class="o">.</span><span class="n">UNICODE</span><span class="p">)</span>
<span class="n">text</span> <span class="o">=</span> <span class="s">'Make this **bold**. This **too**.'</span>
<span class="k">print</span> <span class="s">'Text:'</span><span class="p">,</span> <span class="n">text</span>
<span class="k">print</span> <span class="s">'Bold:'</span><span class="p">,</span> <span class="n">bold</span><span class="o">.</span><span class="n">sub</span><span class="p">(</span><span class="s">r'<b>\g<bold_text></b>'</span><span class="p">,</span> <span class="n">text</span><span class="p">)</span>
</pre></div>
</div>
<p>The <tt class="docutils literal"><span class="pre">\g<name></span></tt> syntax also works with numbered references, and using
it eliminates any ambiguity between group numbers and surrounding
literal digits.</p>
<div class="highlight-python"><pre>$ python re_sub_named_groups.py
Text: Make this **bold**. This **too**.
Bold: Make this <b>bold</b>. This <b>too</b>.</pre>
</div>
<p>Pass a value to <em>count</em> to limit the number of substitutions
performed.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="n">bold</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span><span class="s">r'\*{2}(.*?)\*{2}'</span><span class="p">,</span> <span class="n">re</span><span class="o">.</span><span class="n">UNICODE</span><span class="p">)</span>
<span class="n">text</span> <span class="o">=</span> <span class="s">'Make this **bold**. This **too**.'</span>
<span class="k">print</span> <span class="s">'Text:'</span><span class="p">,</span> <span class="n">text</span>
<span class="k">print</span> <span class="s">'Bold:'</span><span class="p">,</span> <span class="n">bold</span><span class="o">.</span><span class="n">sub</span><span class="p">(</span><span class="s">r'<b>\1</b>'</span><span class="p">,</span> <span class="n">text</span><span class="p">,</span> <span class="n">count</span><span class="o">=</span><span class="mi">1</span><span class="p">)</span>
</pre></div>
</div>
<p>Only the first substitution is made because <em>count</em> is <tt class="docutils literal"><span class="pre">1</span></tt>.</p>
<div class="highlight-python"><pre>$ python re_sub_count.py
Text: Make this **bold**. This **too**.
Bold: Make this <b>bold</b>. This **too**.</pre>
</div>
<p><tt class="xref py py-func docutils literal"><span class="pre">subn()</span></tt> works just like <tt class="xref py py-func docutils literal"><span class="pre">sub()</span></tt> except that it returns both
the modified string and the count of substitutions made.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="n">bold</span> <span class="o">=</span> <span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span><span class="s">r'\*{2}(.*?)\*{2}'</span><span class="p">,</span> <span class="n">re</span><span class="o">.</span><span class="n">UNICODE</span><span class="p">)</span>
<span class="n">text</span> <span class="o">=</span> <span class="s">'Make this **bold**. This **too**.'</span>
<span class="k">print</span> <span class="s">'Text:'</span><span class="p">,</span> <span class="n">text</span>
<span class="k">print</span> <span class="s">'Bold:'</span><span class="p">,</span> <span class="n">bold</span><span class="o">.</span><span class="n">subn</span><span class="p">(</span><span class="s">r'<b>\1</b>'</span><span class="p">,</span> <span class="n">text</span><span class="p">)</span>
</pre></div>
</div>
<p>The search pattern matches twice in the example.</p>
<div class="highlight-python"><pre>$ python re_subn.py
Text: Make this **bold**. This **too**.
Bold: ('Make this <b>bold</b>. This <b>too</b>.', 2)</pre>
</div>
</div>
<div class="section" id="splitting-with-patterns">
<h2>Splitting with Patterns<a class="headerlink" href="#splitting-with-patterns" title="Permalink to this headline">¶</a></h2>
<p><tt class="xref py py-func docutils literal"><span class="pre">str.split()</span></tt> is one of the most frequently used methods for
breaking apart strings to parse them. It only supports using literal
values as separators, though, and sometimes a regular expression is
necessary if the input is not consistently formatted. For example,
many plain text markup languages define paragraph separators as two or
more newline (<tt class="docutils literal"><span class="pre">\n</span></tt>) characters. In this case, <tt class="xref py py-func docutils literal"><span class="pre">str.split()</span></tt>
cannot be used because of the “or more” part of the definition.</p>
<p>A strategy for identifying paragraphs using <tt class="xref py py-func docutils literal"><span class="pre">findall()</span></tt> would use
a pattern like <tt class="docutils literal"><span class="pre">(.+?)\n{2,}</span></tt>.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="n">text</span> <span class="o">=</span> <span class="s">'Paragraph one</span><span class="se">\n</span><span class="s">on two lines.</span><span class="se">\n\n</span><span class="s">Paragraph two.</span><span class="se">\n\n\n</span><span class="s">Paragraph three.'</span>
<span class="k">for</span> <span class="n">num</span><span class="p">,</span> <span class="n">para</span> <span class="ow">in</span> <span class="nb">enumerate</span><span class="p">(</span><span class="n">re</span><span class="o">.</span><span class="n">findall</span><span class="p">(</span><span class="s">r'(.+?)\n{2,}'</span><span class="p">,</span> <span class="n">text</span><span class="p">,</span> <span class="n">flags</span><span class="o">=</span><span class="n">re</span><span class="o">.</span><span class="n">DOTALL</span><span class="p">)):</span>
<span class="k">print</span> <span class="n">num</span><span class="p">,</span> <span class="nb">repr</span><span class="p">(</span><span class="n">para</span><span class="p">)</span>
<span class="k">print</span>
</pre></div>
</div>
<p>That pattern fails for paragraphs at the end of the input text, as
illustrated by the fact that “Paragraph three.” is not part of the
output.</p>
<div class="highlight-python"><pre>$ python re_paragraphs_findall.py
0 'Paragraph one\non two lines.'
1 'Paragraph two.'</pre>
</div>
<p>Extending the pattern to say that a paragraph ends with two or more
newlines, or the end of input, fixes the problem but makes the pattern
more complicated. Converting to <tt class="xref py py-func docutils literal"><span class="pre">re.split()</span></tt> instead of
<tt class="xref py py-func docutils literal"><span class="pre">re.findall()</span></tt> handles the boundary condition automatically and
keeps the pattern simple.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="n">text</span> <span class="o">=</span> <span class="s">'Paragraph one</span><span class="se">\n</span><span class="s">on two lines.</span><span class="se">\n\n</span><span class="s">Paragraph two.</span><span class="se">\n\n\n</span><span class="s">Paragraph three.'</span>
<span class="k">print</span> <span class="s">'With findall:'</span>
<span class="k">for</span> <span class="n">num</span><span class="p">,</span> <span class="n">para</span> <span class="ow">in</span> <span class="nb">enumerate</span><span class="p">(</span><span class="n">re</span><span class="o">.</span><span class="n">findall</span><span class="p">(</span><span class="s">r'(.+?)(\n{2,}|$)'</span><span class="p">,</span> <span class="n">text</span><span class="p">,</span> <span class="n">flags</span><span class="o">=</span><span class="n">re</span><span class="o">.</span><span class="n">DOTALL</span><span class="p">)):</span>
<span class="k">print</span> <span class="n">num</span><span class="p">,</span> <span class="nb">repr</span><span class="p">(</span><span class="n">para</span><span class="p">)</span>
<span class="k">print</span>
<span class="k">print</span>
<span class="k">print</span> <span class="s">'With split:'</span>
<span class="k">for</span> <span class="n">num</span><span class="p">,</span> <span class="n">para</span> <span class="ow">in</span> <span class="nb">enumerate</span><span class="p">(</span><span class="n">re</span><span class="o">.</span><span class="n">split</span><span class="p">(</span><span class="s">r'\n{2,}'</span><span class="p">,</span> <span class="n">text</span><span class="p">)):</span>
<span class="k">print</span> <span class="n">num</span><span class="p">,</span> <span class="nb">repr</span><span class="p">(</span><span class="n">para</span><span class="p">)</span>
<span class="k">print</span>
</pre></div>
</div>
<p>The pattern argument to <tt class="xref py py-func docutils literal"><span class="pre">split()</span></tt> expresses the markup
specification more precisely: Two or more newline characters mark a
separator point between paragraphs in the input string.</p>
<div class="highlight-python"><pre>$ python re_split.py
With findall:
0 ('Paragraph one\non two lines.', '\n\n')
1 ('Paragraph two.', '\n\n\n')
2 ('Paragraph three.', '')
With split:
0 'Paragraph one\non two lines.'
1 'Paragraph two.'
2 'Paragraph three.'</pre>
</div>
<p>Enclosing the expression in parentheses to define a group causes
<tt class="xref py py-func docutils literal"><span class="pre">split()</span></tt> to work more like <tt class="xref py py-func docutils literal"><span class="pre">str.partition()</span></tt>, so it returns
the separator values as well as the other parts of the string.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">re</span>
<span class="n">text</span> <span class="o">=</span> <span class="s">'Paragraph one</span><span class="se">\n</span><span class="s">on two lines.</span><span class="se">\n\n</span><span class="s">Paragraph two.</span><span class="se">\n\n\n</span><span class="s">Paragraph three.'</span>
<span class="k">print</span>
<span class="k">print</span> <span class="s">'With split:'</span>
<span class="k">for</span> <span class="n">num</span><span class="p">,</span> <span class="n">para</span> <span class="ow">in</span> <span class="nb">enumerate</span><span class="p">(</span><span class="n">re</span><span class="o">.</span><span class="n">split</span><span class="p">(</span><span class="s">r'(\n{2,})'</span><span class="p">,</span> <span class="n">text</span><span class="p">)):</span>
<span class="k">print</span> <span class="n">num</span><span class="p">,</span> <span class="nb">repr</span><span class="p">(</span><span class="n">para</span><span class="p">)</span>
<span class="k">print</span>
</pre></div>
</div>
<p>The output now includes each paragraph, as well as the sequence of
newlines separating them.</p>
<div class="highlight-python"><pre>$ python re_split_groups.py
With split:
0 'Paragraph one\non two lines.'
1 '\n\n'
2 'Paragraph two.'
3 '\n\n\n'
4 'Paragraph three.'</pre>
</div>
<div class="admonition-see-also admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="last docutils">
<dt><a class="reference external" href="http://docs.python.org/library/re.html">re</a></dt>
<dd>The standard library documentation for this module.</dd>
<dt><a class="reference external" href="http://docs.python.org/howto/regex.html">Regular Expression HOWTO</a></dt>
<dd>Andrew Kuchling’s introduction to regular expressions for Python developers.</dd>
<dt><a class="reference external" href="http://kodos.sourceforge.net/">Kodos</a></dt>
<dd>An interactive regular expression testing tool by Phil Schwartz.</dd>
<dt><a class="reference external" href="http://www.pythonregex.com/">Python Regular Expression Testing Tool</a></dt>
<dd>A web-based tool for testing regular expressions created by
David Naffziger at BrandVerity.com. Inspired by Kodos.</dd>
<dt><a class="reference external" href="http://en.wikipedia.org/wiki/Regular_expressions">Wikipedia: Regular expression</a></dt>
<dd>General introduction to regular expression concepts and techniques.</dd>
<dt><a class="reference internal" href="../locale/index.html#module-locale" title="locale: POSIX cultural localization API"><tt class="xref py py-mod docutils literal"><span class="pre">locale</span></tt></a></dt>
<dd>Use the <a class="reference internal" href="../locale/index.html#module-locale" title="locale: POSIX cultural localization API"><tt class="xref py py-mod docutils literal"><span class="pre">locale</span></tt></a> module to set your language
configuration when working with Unicode text.</dd>
<dt><tt class="xref py py-mod docutils literal"><span class="pre">unicodedata</span></tt></dt>
<dd>Programmatic access to the Unicode character property database.</dd>
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