22

✏️ Editing: regex.html

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
 "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
 <head>
 <meta http-equiv="X-UA-Compatible" content="IE=Edge" />
 <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
 <title>Regular Expression HOWTO &#8212; Python 3.6.7 documentation</title>
 <link rel="stylesheet" href="../_static/pydoctheme.css" type="text/css" />
 <link rel="stylesheet" href="../_static/pygments.css" type="text/css" />
 <script type="text/javascript" id="documentation_options" data-url_root="../" src="../_static/documentation_options.js"></script>
 <script type="text/javascript" src="../_static/jquery.js"></script>
 <script type="text/javascript" src="../_static/underscore.js"></script>
 <script type="text/javascript" src="../_static/doctools.js"></script>
 <script type="text/javascript" src="../_static/sidebar.js"></script>
 <link rel="search" type="application/opensearchdescription+xml"
 title="Search within Python 3.6.7 documentation"
 href="../_static/opensearch.xml"/>
 <link rel="author" title="About these documents" href="../about.html" />
 <link rel="index" title="Index" href="../genindex.html" />
 <link rel="search" title="Search" href="../search.html" />
 <link rel="copyright" title="Copyright" href="../copyright.html" />
 <link rel="next" title="Socket Programming HOWTO" href="sockets.html" />
 <link rel="prev" title="Logging Cookbook" href="logging-cookbook.html" />
 <link rel="shortcut icon" type="image/png" href="../_static/py.png" />
 <link rel="canonical" href="https://docs.python.org/3/howto/regex.html" />
 
 <script type="text/javascript" src="../_static/copybutton.js"></script>

</head><body> 
 <div class="related" role="navigation" aria-label="related navigation">
 <h3>Navigation</h3>
 <ul>
 <li class="right" style="margin-right: 10px">
 <a href="../genindex.html" title="General Index"
 accesskey="I">index</a></li>
 <li class="right" >
 <a href="../py-modindex.html" title="Python Module Index"
 >modules</a> |</li>
 <li class="right" >
 <a href="sockets.html" title="Socket Programming HOWTO"
 accesskey="N">next</a> |</li>
 <li class="right" >
 <a href="logging-cookbook.html" title="Logging Cookbook"
 accesskey="P">previous</a> |</li>
 <li><img src="../_static/py.png" alt=""
 style="vertical-align: middle; margin-top: -1px"/></li>
 <li><a href="https://www.python.org/">Python</a> &#187;</li>
 <li>
 <a href="../index.html">3.6.7 Documentation</a> &#187;
 </li>

<li class="nav-item nav-item-1"><a href="index.html" accesskey="U">Python HOWTOs</a> &#187;</li>
 <li class="right">

<div class="inline-search" style="display: none" role="search">
 <form class="inline-search" action="../search.html" method="get">
 <input placeholder="Quick search" type="text" name="q" />
 <input type="submit" value="Go" />
 <input type="hidden" name="check_keywords" value="yes" />
 <input type="hidden" name="area" value="default" />
 </form>
 </div>
 <script type="text/javascript">$('.inline-search').show(0);</script>
 |
 </li>

</ul>
 </div>

<div class="document">
 <div class="documentwrapper">
 <div class="bodywrapper">
 <div class="body" role="main">
 
 <div class="section" id="regular-expression-howto">
<h1>Regular Expression HOWTO<a class="headerlink" href="#regular-expression-howto" 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-odd field"><th class="field-name">Author:</th><td class="field-body">A.M. Kuchling &lt;<a class="reference external" href="mailto:amk&#37;&#52;&#48;amk&#46;ca">amk&#64;amk&#46;ca</a>&gt;</td>
</tr>
</tbody>
</table>
<div class="topic">
Abstract
This document is an introductory tutorial to using regular expressions in Python
with the <a class="reference internal" href="../library/re.html#module-re" title="re: Regular expression operations."><code class="xref py py-mod docutils literal notranslate">re</code></a> module. It provides a gentler introduction than the
corresponding section in the Library Reference.
</div>
<div class="section" id="introduction">
<h2>Introduction<a class="headerlink" href="#introduction" title="Permalink to this headline">¶</a></h2>
Regular expressions (called REs, or regexes, or regex patterns) are essentially
a tiny, highly specialized programming language embedded inside Python and made
available through the <a class="reference internal" href="../library/re.html#module-re" title="re: Regular expression operations."><code class="xref py py-mod docutils literal notranslate">re</code></a> module. Using this little language, you specify
the rules for the set of possible strings that you want to match; this set might
contain English sentences, or e-mail addresses, or TeX commands, or anything you
like. You can then ask questions such as “Does this string match the pattern?”,
or “Is there a match for the pattern anywhere in this string?”. You can also
use REs to modify a string or to split it apart in various ways.
Regular expression patterns are compiled into a series of bytecodes which are
then executed by a matching engine written in C. For advanced use, it may be
necessary to pay careful attention to how the engine will execute a given RE,
and write the RE in a certain way in order to produce bytecode that runs faster.
Optimization isn’t covered in this document, because it requires that you have a
good understanding of the matching engine’s internals.
The regular expression language is relatively small and restricted, so not all
possible string processing tasks can be done using regular expressions. There
are also tasks that can be done with regular expressions, but the expressions
turn out to be very complicated. In these cases, you may be better off writing
Python code to do the processing; while Python code will be slower than an
elaborate regular expression, it will also probably be more understandable.
</div>
<div class="section" id="simple-patterns">
<h2>Simple Patterns<a class="headerlink" href="#simple-patterns" title="Permalink to this headline">¶</a></h2>
We’ll start by learning about the simplest possible regular expressions. Since
regular expressions are used to operate on strings, we’ll begin with the most
common task: matching characters.
For a detailed explanation of the computer science underlying regular
expressions (deterministic and non-deterministic finite automata), you can refer
to almost any textbook on writing compilers.
<div class="section" id="matching-characters">
<h3>Matching Characters<a class="headerlink" href="#matching-characters" title="Permalink to this headline">¶</a></h3>
Most letters and characters will simply match themselves. For example, the
regular expression <code class="docutils literal notranslate">test</code> will match the string <code class="docutils literal notranslate">test</code> exactly. (You can
enable a case-insensitive mode that would let this RE match <code class="docutils literal notranslate">Test</code> or <code class="docutils literal notranslate">TEST</code>
as well; more about this later.)
There are exceptions to this rule; some characters are special
metacharacters, and don’t match themselves. Instead, they signal that
some out-of-the-ordinary thing should be matched, or they affect other portions
of the RE by repeating them or changing their meaning. Much of this document is
devoted to discussing various metacharacters and what they do.
Here’s a complete list of the metacharacters; their meanings will be discussed
in the rest of this HOWTO.
<div class="highlight-none notranslate"><div class="highlight"><pre>. ^ $ * + ? { } [ ] \ | ( )
</pre></div>
</div>
The first metacharacters we’ll look at are <code class="docutils literal notranslate">[</code> and <code class="docutils literal notranslate">]</code>. They’re used for
specifying a character class, which is a set of characters that you wish to
match. Characters can be listed individually, or a range of characters can be
indicated by giving two characters and separating them by a <code class="docutils literal notranslate">'-'</code>. For
example, <code class="docutils literal notranslate">[abc]</code> will match any of the characters <code class="docutils literal notranslate">a</code>, <code class="docutils literal notranslate">b</code>, or <code class="docutils literal notranslate">c</code>; this
is the same as <code class="docutils literal notranslate">[a-c]</code>, which uses a range to express the same set of
characters. If you wanted to match only lowercase letters, your RE would be
<code class="docutils literal notranslate">[a-z]</code>.
Metacharacters are not active inside classes. For example, <code class="docutils literal notranslate">[akm$]</code> will
match any of the characters <code class="docutils literal notranslate">'a'</code>, <code class="docutils literal notranslate">'k'</code>, <code class="docutils literal notranslate">'m'</code>, or <code class="docutils literal notranslate">'$'</code>; <code class="docutils literal notranslate">'$'</code> is
usually a metacharacter, but inside a character class it’s stripped of its
special nature.
You can match the characters not listed within the class by complementing
the set. This is indicated by including a <code class="docutils literal notranslate">'^'</code> as the first character of the
class; <code class="docutils literal notranslate">'^'</code> outside a character class will simply match the <code class="docutils literal notranslate">'^'</code>
character. For example, <code class="docutils literal notranslate">[^5]</code> will match any character except <code class="docutils literal notranslate">'5'</code>.
Perhaps the most important metacharacter is the backslash, <code class="docutils literal notranslate">\</code>. As in Python
string literals, the backslash can be followed by various characters to signal
various special sequences. It’s also used to escape all the metacharacters so
you can still match them in patterns; for example, if you need to match a <code class="docutils literal notranslate">[</code>
or <code class="docutils literal notranslate">\</code>, you can precede them with a backslash to remove their special
meaning: <code class="docutils literal notranslate">\[</code> or <code class="docutils literal notranslate">\\</code>.
Some of the special sequences beginning with <code class="docutils literal notranslate">'\'</code> represent
predefined sets of characters that are often useful, such as the set
of digits, the set of letters, or the set of anything that isn’t
whitespace.
Let’s take an example: <code class="docutils literal notranslate">\w</code> matches any alphanumeric character. If
the regex pattern is expressed in bytes, this is equivalent to the
class <code class="docutils literal notranslate">[a-zA-Z0-9_]</code>. If the regex pattern is a string, <code class="docutils literal notranslate">\w</code> will
match all the characters marked as letters in the Unicode database
provided by the <a class="reference internal" href="../library/unicodedata.html#module-unicodedata" title="unicodedata: Access the Unicode Database."><code class="xref py py-mod docutils literal notranslate">unicodedata</code></a> module. You can use the more
restricted definition of <code class="docutils literal notranslate">\w</code> in a string pattern by supplying the
<a class="reference internal" href="../library/re.html#re.ASCII" title="re.ASCII"><code class="xref py py-const docutils literal notranslate">re.ASCII</code></a> flag when compiling the regular expression.
The following list of special sequences isn’t complete. For a complete
list of sequences and expanded class definitions for Unicode string
patterns, see the last part of <a class="reference internal" href="../library/re.html#re-syntax">Regular Expression Syntax</a> in the Standard Library reference. In general, the
Unicode versions match any character that’s in the appropriate
category in the Unicode database.
<dl class="docutils">
<dt><code class="docutils literal notranslate">\d</code></dt>
<dd>Matches any decimal digit; this is equivalent to the class <code class="docutils literal notranslate">[0-9]</code>.</dd>
<dt><code class="docutils literal notranslate">\D</code></dt>
<dd>Matches any non-digit character; this is equivalent to the class <code class="docutils literal notranslate">[^0-9]</code>.</dd>
<dt><code class="docutils literal notranslate">\s</code></dt>
<dd>Matches any whitespace character; this is equivalent to the class <code class="docutils literal notranslate">[
\t\n\r\f\v]</code>.</dd>
<dt><code class="docutils literal notranslate">\S</code></dt>
<dd>Matches any non-whitespace character; this is equivalent to the class <code class="docutils literal notranslate">[^
\t\n\r\f\v]</code>.</dd>
<dt><code class="docutils literal notranslate">\w</code></dt>
<dd>Matches any alphanumeric character; this is equivalent to the class
<code class="docutils literal notranslate">[a-zA-Z0-9_]</code>.</dd>
<dt><code class="docutils literal notranslate">\W</code></dt>
<dd>Matches any non-alphanumeric character; this is equivalent to the class
<code class="docutils literal notranslate">[^a-zA-Z0-9_]</code>.</dd>
</dl>
These sequences can be included inside a character class. For example,
<code class="docutils literal notranslate">[\s,.]</code> is a character class that will match any whitespace character, or
<code class="docutils literal notranslate">','</code> or <code class="docutils literal notranslate">'.'</code>.
The final metacharacter in this section is <code class="docutils literal notranslate">.</code>. It matches anything except a
newline character, and there’s an alternate mode (<a class="reference internal" href="../library/re.html#re.DOTALL" title="re.DOTALL"><code class="xref py py-const docutils literal notranslate">re.DOTALL</code></a>) where it will
match even a newline. <code class="docutils literal notranslate">.</code> is often used where you want to match “any
character”.
</div>
<div class="section" id="repeating-things">
<h3>Repeating Things<a class="headerlink" href="#repeating-things" title="Permalink to this headline">¶</a></h3>
Being able to match varying sets of characters is the first thing regular
expressions can do that isn’t already possible with the methods available on
strings. However, if that was the only additional capability of regexes, they
wouldn’t be much of an advance. Another capability is that you can specify that
portions of the RE must be repeated a certain number of times.
The first metacharacter for repeating things that we’ll look at is <code class="docutils literal notranslate">*</code>. <code class="docutils literal notranslate">*</code>
doesn’t match the literal character <code class="docutils literal notranslate">'*'</code>; instead, it specifies that the
previous character can be matched zero or more times, instead of exactly once.
For example, <code class="docutils literal notranslate">ca*t</code> will match <code class="docutils literal notranslate">'ct'</code> (0 <code class="docutils literal notranslate">'a'</code> characters), <code class="docutils literal notranslate">'cat'</code> (1 <code class="docutils literal notranslate">'a'</code>),
<code class="docutils literal notranslate">'caaat'</code> (3 <code class="docutils literal notranslate">'a'</code> characters), and so forth.
Repetitions such as <code class="docutils literal notranslate">*</code> are greedy; when repeating a RE, the matching
engine will try to repeat it as many times as possible. If later portions of the
pattern don’t match, the matching engine will then back up and try again with
fewer repetitions.
A step-by-step example will make this more obvious. Let’s consider the
expression <code class="docutils literal notranslate">a[bcd]*b</code>. This matches the letter <code class="docutils literal notranslate">'a'</code>, zero or more letters
from the class <code class="docutils literal notranslate">[bcd]</code>, and finally ends with a <code class="docutils literal notranslate">'b'</code>. Now imagine matching
this RE against the string <code class="docutils literal notranslate">'abcbd'</code>.
<table border="1" class="docutils">
<colgroup>
<col width="12%" />
<col width="22%" />
<col width="66%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Step</th>
<th class="head">Matched</th>
<th class="head">Explanation</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td>1</td>
<td><code class="docutils literal notranslate">a</code></td>
<td>The <code class="docutils literal notranslate">a</code> in the RE matches.</td>
</tr>
<tr class="row-odd"><td>2</td>
<td><code class="docutils literal notranslate">abcbd</code></td>
<td>The engine matches <code class="docutils literal notranslate">[bcd]*</code>,
going as far as it can, which
is to the end of the string.</td>
</tr>
<tr class="row-even"><td>3</td>
<td>Failure</td>
<td>The engine tries to match
<code class="docutils literal notranslate">b</code>, but the current position
is at the end of the string, so
it fails.</td>
</tr>
<tr class="row-odd"><td>4</td>
<td><code class="docutils literal notranslate">abcb</code></td>
<td>Back up, so that <code class="docutils literal notranslate">[bcd]*</code>
matches one less character.</td>
</tr>
<tr class="row-even"><td>5</td>
<td>Failure</td>
<td>Try <code class="docutils literal notranslate">b</code> again, but the
current position is at the last
character, which is a <code class="docutils literal notranslate">'d'</code>.</td>
</tr>
<tr class="row-odd"><td>6</td>
<td><code class="docutils literal notranslate">abc</code></td>
<td>Back up again, so that
<code class="docutils literal notranslate">[bcd]*</code> is only matching
<code class="docutils literal notranslate">bc</code>.</td>
</tr>
<tr class="row-even"><td>6</td>
<td><code class="docutils literal notranslate">abcb</code></td>
<td>Try <code class="docutils literal notranslate">b</code> again. This time
the character at the
current position is <code class="docutils literal notranslate">'b'</code>, so
it succeeds.</td>
</tr>
</tbody>
</table>
The end of the RE has now been reached, and it has matched <code class="docutils literal notranslate">'abcb'</code>. This
demonstrates how the matching engine goes as far as it can at first, and if no
match is found it will then progressively back up and retry the rest of the RE
again and again. It will back up until it has tried zero matches for
<code class="docutils literal notranslate">[bcd]*</code>, and if that subsequently fails, the engine will conclude that the
string doesn’t match the RE at all.
Another repeating metacharacter is <code class="docutils literal notranslate">+</code>, which matches one or more times. Pay
careful attention to the difference between <code class="docutils literal notranslate">*</code> and <code class="docutils literal notranslate">+</code>; <code class="docutils literal notranslate">*</code> matches
zero or more times, so whatever’s being repeated may not be present at all,
while <code class="docutils literal notranslate">+</code> requires at least one occurrence. To use a similar example,
<code class="docutils literal notranslate">ca+t</code> will match <code class="docutils literal notranslate">'cat'</code> (1 <code class="docutils literal notranslate">'a'</code>), <code class="docutils literal notranslate">'caaat'</code> (3 <code class="docutils literal notranslate">'a'</code>s), but won’t
match <code class="docutils literal notranslate">'ct'</code>.
There are two more repeating qualifiers. The question mark character, <code class="docutils literal notranslate">?</code>,
matches either once or zero times; you can think of it as marking something as
being optional. For example, <code class="docutils literal notranslate">home-?brew</code> matches either <code class="docutils literal notranslate">'homebrew'</code> or
<code class="docutils literal notranslate">'home-brew'</code>.
The most complicated repeated qualifier is <code class="docutils literal notranslate">{m,n}</code>, where m and n are
decimal integers. This qualifier means there must be at least m repetitions,
and at most n. For example, <code class="docutils literal notranslate">a/{1,3}b</code> will match <code class="docutils literal notranslate">'a/b'</code>, <code class="docutils literal notranslate">'a//b'</code>, and
<code class="docutils literal notranslate">'a///b'</code>. It won’t match <code class="docutils literal notranslate">'ab'</code>, which has no slashes, or <code class="docutils literal notranslate">'a////b'</code>, which
has four.
You can omit either m or n; in that case, a reasonable value is assumed for
the missing value. Omitting m is interpreted as a lower limit of 0, while
omitting n results in an upper bound of infinity.
Readers of a reductionist bent may notice that the three other qualifiers can
all be expressed using this notation. <code class="docutils literal notranslate">{0,}</code> is the same as <code class="docutils literal notranslate">*</code>, <code class="docutils literal notranslate">{1,}</code>
is equivalent to <code class="docutils literal notranslate">+</code>, and <code class="docutils literal notranslate">{0,1}</code> is the same as <code class="docutils literal notranslate">?</code>. It’s better to use
<code class="docutils literal notranslate">*</code>, <code class="docutils literal notranslate">+</code>, or <code class="docutils literal notranslate">?</code> when you can, simply because they’re shorter and easier
to read.
</div>
</div>
<div class="section" id="using-regular-expressions">
<h2>Using Regular Expressions<a class="headerlink" href="#using-regular-expressions" title="Permalink to this headline">¶</a></h2>
Now that we’ve looked at some simple regular expressions, how do we actually use
them in Python? The <a class="reference internal" href="../library/re.html#module-re" title="re: Regular expression operations."><code class="xref py py-mod docutils literal notranslate">re</code></a> module provides an interface to the regular
expression engine, allowing you to compile REs into objects and then perform
matches with them.
<div class="section" id="compiling-regular-expressions">
<h3>Compiling Regular Expressions<a class="headerlink" href="#compiling-regular-expressions" title="Permalink to this headline">¶</a></h3>
Regular expressions are compiled into pattern objects, which have
methods for various operations such as searching for pattern matches or
performing string substitutions.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; import re
&gt;&gt;&gt; p = re.compile(&#39;ab*&#39;)
&gt;&gt;&gt; p
re.compile(&#39;ab*&#39;)
</pre></div>
</div>
<a class="reference internal" href="../library/re.html#re.compile" title="re.compile"><code class="xref py py-func docutils literal notranslate">re.compile()</code></a> also accepts an optional flags argument, used to enable
various special features and syntax variations. We’ll go over the available
settings later, but for now a single example will do:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; p = re.compile(&#39;ab*&#39;, re.IGNORECASE)
</pre></div>
</div>
The RE is passed to <a class="reference internal" href="../library/re.html#re.compile" title="re.compile"><code class="xref py py-func docutils literal notranslate">re.compile()</code></a> as a string. REs are handled as strings
because regular expressions aren’t part of the core Python language, and no
special syntax was created for expressing them. (There are applications that
don’t need REs at all, so there’s no need to bloat the language specification by
including them.) Instead, the <a class="reference internal" href="../library/re.html#module-re" title="re: Regular expression operations."><code class="xref py py-mod docutils literal notranslate">re</code></a> module is simply a C extension module
included with Python, just like the <a class="reference internal" href="../library/socket.html#module-socket" title="socket: Low-level networking interface."><code class="xref py py-mod docutils literal notranslate">socket</code></a> or <a class="reference internal" href="../library/zlib.html#module-zlib" title="zlib: Low-level interface to compression and decompression routines compatible with gzip."><code class="xref py py-mod docutils literal notranslate">zlib</code></a> modules.
Putting REs in strings keeps the Python language simpler, but has one
disadvantage which is the topic of the next section.
</div>
<div class="section" id="the-backslash-plague">
<h3>The Backslash Plague<a class="headerlink" href="#the-backslash-plague" title="Permalink to this headline">¶</a></h3>
As stated earlier, regular expressions use the backslash character (<code class="docutils literal notranslate">'\'</code>) to
indicate special forms or to allow special characters to be used without
invoking their special meaning. This conflicts with Python’s usage of the same
character for the same purpose in string literals.
Let’s say you want to write a RE that matches the string <code class="docutils literal notranslate">\section</code>, which
might be found in a LaTeX file. To figure out what to write in the program
code, start with the desired string to be matched. Next, you must escape any
backslashes and other metacharacters by preceding them with a backslash,
resulting in the string <code class="docutils literal notranslate">\\section</code>. The resulting string that must be passed
to <a class="reference internal" href="../library/re.html#re.compile" title="re.compile"><code class="xref py py-func docutils literal notranslate">re.compile()</code></a> must be <code class="docutils literal notranslate">\\section</code>. However, to express this as a
Python string literal, both backslashes must be escaped again.
<table border="1" class="docutils">
<colgroup>
<col width="31%" />
<col width="69%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Characters</th>
<th class="head">Stage</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><code class="docutils literal notranslate">\section</code></td>
<td>Text string to be matched</td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate">\\section</code></td>
<td>Escaped backslash for <a class="reference internal" href="../library/re.html#re.compile" title="re.compile"><code class="xref py py-func docutils literal notranslate">re.compile()</code></a></td>
</tr>
<tr class="row-even"><td><code class="docutils literal notranslate">&quot;\\\\section&quot;</code></td>
<td>Escaped backslashes for a string literal</td>
</tr>
</tbody>
</table>
In short, to match a literal backslash, one has to write <code class="docutils literal notranslate">'\\\\'</code> as the RE
string, because the regular expression must be <code class="docutils literal notranslate">\\</code>, and each backslash must
be expressed as <code class="docutils literal notranslate">\\</code> inside a regular Python string literal. In REs that
feature backslashes repeatedly, this leads to lots of repeated backslashes and
makes the resulting strings difficult to understand.
The solution is to use Python’s raw string notation for regular expressions;
backslashes are not handled in any special way in a string literal prefixed with
<code class="docutils literal notranslate">'r'</code>, so <code class="docutils literal notranslate">r&quot;\n&quot;</code> is a two-character string containing <code class="docutils literal notranslate">'\'</code> and <code class="docutils literal notranslate">'n'</code>,
while <code class="docutils literal notranslate">&quot;\n&quot;</code> is a one-character string containing a newline. Regular
expressions will often be written in Python code using this raw string notation.
In addition, special escape sequences that are valid in regular expressions,
but not valid as Python string literals, now result in a
<a class="reference internal" href="../library/exceptions.html#DeprecationWarning" title="DeprecationWarning"><code class="xref py py-exc docutils literal notranslate">DeprecationWarning</code></a> and will eventually become a <a class="reference internal" href="../library/exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal notranslate">SyntaxError</code></a>,
which means the sequences will be invalid if raw string notation or escaping
the backslashes isn’t used.
<table border="1" class="docutils">
<colgroup>
<col width="51%" />
<col width="49%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Regular String</th>
<th class="head">Raw string</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><code class="docutils literal notranslate">&quot;ab*&quot;</code></td>
<td><code class="docutils literal notranslate">r&quot;ab*&quot;</code></td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate">&quot;\\\\section&quot;</code></td>
<td><code class="docutils literal notranslate">r&quot;\\section&quot;</code></td>
</tr>
<tr class="row-even"><td><code class="docutils literal notranslate">&quot;\\w+\\s+\\1&quot;</code></td>
<td><code class="docutils literal notranslate">r&quot;\w+\s+\1&quot;</code></td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="performing-matches">
<h3>Performing Matches<a class="headerlink" href="#performing-matches" title="Permalink to this headline">¶</a></h3>
Once you have an object representing a compiled regular expression, what do you
do with it? Pattern objects have several methods and attributes.
Only the most significant ones will be covered here; consult the <a class="reference internal" href="../library/re.html#module-re" title="re: Regular expression operations."><code class="xref py py-mod docutils literal notranslate">re</code></a> docs
for a complete listing.
<table border="1" class="docutils">
<colgroup>
<col width="28%" />
<col width="72%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Method/Attribute</th>
<th class="head">Purpose</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><code class="docutils literal notranslate">match()</code></td>
<td>Determine if the RE matches at the beginning
of the string.</td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate">search()</code></td>
<td>Scan through a string, looking for any
location where this RE matches.</td>
</tr>
<tr class="row-even"><td><code class="docutils literal notranslate">findall()</code></td>
<td>Find all substrings where the RE matches, and
returns them as a list.</td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate">finditer()</code></td>
<td>Find all substrings where the RE matches, and
returns them as an <a class="reference internal" href="../glossary.html#term-iterator">iterator</a>.</td>
</tr>
</tbody>
</table>
<code class="xref py py-meth docutils literal notranslate">match()</code> and <code class="xref py py-meth docutils literal notranslate">search()</code> return <code class="docutils literal notranslate">None</code> if no match can be found. If
they’re successful, a <a class="reference internal" href="../library/re.html#match-objects">match object</a> instance is returned,
containing information about the match: where it starts and ends, the substring
it matched, and more.
You can learn about this by interactively experimenting with the <a class="reference internal" href="../library/re.html#module-re" title="re: Regular expression operations."><code class="xref py py-mod docutils literal notranslate">re</code></a>
module. If you have <a class="reference internal" href="../library/tkinter.html#module-tkinter" title="tkinter: Interface to Tcl/Tk for graphical user interfaces"><code class="xref py py-mod docutils literal notranslate">tkinter</code></a> available, you may also want to look at
<a class="reference external" href="https://github.com/python/cpython/tree/3.6/Tools/demo/redemo.py">Tools/demo/redemo.py</a>, a demonstration program included with the
Python distribution. It allows you to enter REs and strings, and displays
whether the RE matches or fails. <code class="file docutils literal notranslate">redemo.py</code> can be quite useful when
trying to debug a complicated RE.
This HOWTO uses the standard Python interpreter for its examples. First, run the
Python interpreter, import the <a class="reference internal" href="../library/re.html#module-re" title="re: Regular expression operations."><code class="xref py py-mod docutils literal notranslate">re</code></a> module, and compile a RE:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; import re
&gt;&gt;&gt; p = re.compile(&#39;[a-z]+&#39;)
&gt;&gt;&gt; p
re.compile(&#39;[a-z]+&#39;)
</pre></div>
</div>
Now, you can try matching various strings against the RE <code class="docutils literal notranslate">[a-z]+</code>. An empty
string shouldn’t match at all, since <code class="docutils literal notranslate">+</code> means ‘one or more repetitions’.
<code class="xref py py-meth docutils literal notranslate">match()</code> should return <code class="docutils literal notranslate">None</code> in this case, which will cause the
interpreter to print no output. You can explicitly print the result of
<code class="xref py py-meth docutils literal notranslate">match()</code> to make this clear.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; p.match(&quot;&quot;)
&gt;&gt;&gt; print(p.match(&quot;&quot;))
None
</pre></div>
</div>
Now, let’s try it on a string that it should match, such as <code class="docutils literal notranslate">tempo</code>. In this
case, <code class="xref py py-meth docutils literal notranslate">match()</code> will return a <a class="reference internal" href="../library/re.html#match-objects">match object</a>, so you
should store the result in a variable for later use.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; m = p.match(&#39;tempo&#39;)
&gt;&gt;&gt; m
&lt;_sre.SRE_Match object; span=(0, 5), match=&#39;tempo&#39;&gt;
</pre></div>
</div>
Now you can query the <a class="reference internal" href="../library/re.html#match-objects">match object</a> for information
about the matching string. Match object instances
also have several methods and attributes; the most important ones are:
<table border="1" class="docutils">
<colgroup>
<col width="29%" />
<col width="71%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Method/Attribute</th>
<th class="head">Purpose</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><code class="docutils literal notranslate">group()</code></td>
<td>Return the string matched by the RE</td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate">start()</code></td>
<td>Return the starting position of the match</td>
</tr>
<tr class="row-even"><td><code class="docutils literal notranslate">end()</code></td>
<td>Return the ending position of the match</td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate">span()</code></td>
<td>Return a tuple containing the (start, end)
positions of the match</td>
</tr>
</tbody>
</table>
Trying these methods will soon clarify their meaning:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; m.group()
&#39;tempo&#39;
&gt;&gt;&gt; m.start(), m.end()
(0, 5)
&gt;&gt;&gt; m.span()
(0, 5)
</pre></div>
</div>
<a class="reference internal" href="../library/re.html#re.match.group" title="re.match.group"><code class="xref py py-meth docutils literal notranslate">group()</code></a> returns the substring that was matched by the RE. <a class="reference internal" href="../library/re.html#re.match.start" title="re.match.start"><code class="xref py py-meth docutils literal notranslate">start()</code></a>
and <a class="reference internal" href="../library/re.html#re.match.end" title="re.match.end"><code class="xref py py-meth docutils literal notranslate">end()</code></a> return the starting and ending index of the match. <a class="reference internal" href="../library/re.html#re.match.span" title="re.match.span"><code class="xref py py-meth docutils literal notranslate">span()</code></a>
returns both start and end indexes in a single tuple. Since the <code class="xref py py-meth docutils literal notranslate">match()</code>
method only checks if the RE matches at the start of a string, <code class="xref py py-meth docutils literal notranslate">start()</code>
will always be zero. However, the <code class="xref py py-meth docutils literal notranslate">search()</code> method of patterns
scans through the string, so the match may not start at zero in that
case.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; print(p.match(&#39;::: message&#39;))
None
&gt;&gt;&gt; m = p.search(&#39;::: message&#39;); print(m)
&lt;_sre.SRE_Match object; span=(4, 11), match=&#39;message&#39;&gt;
&gt;&gt;&gt; m.group()
&#39;message&#39;
&gt;&gt;&gt; m.span()
(4, 11)
</pre></div>
</div>
In actual programs, the most common style is to store the
<a class="reference internal" href="../library/re.html#match-objects">match object</a> in a variable, and then check if it was
<code class="docutils literal notranslate">None</code>. This usually looks like:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>p = re.compile( ... )
m = p.match( &#39;string goes here&#39; )
if m:
 print(&#39;Match found: &#39;, m.group())
else:
 print(&#39;No match&#39;)
</pre></div>
</div>
Two pattern methods return all of the matches for a pattern.
<code class="xref py py-meth docutils literal notranslate">findall()</code> returns a list of matching strings:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; p = re.compile(r&#39;\d+&#39;)
&gt;&gt;&gt; p.findall(&#39;12 drummers drumming, 11 pipers piping, 10 lords a-leaping&#39;)
[&#39;12&#39;, &#39;11&#39;, &#39;10&#39;]
</pre></div>
</div>
The <code class="docutils literal notranslate">r</code> prefix, making the literal a raw string literal, is needed in this
example because escape sequences in a normal “cooked” string literal that are
not recognized by Python, as opposed to regular expressions, now result in a
<a class="reference internal" href="../library/exceptions.html#DeprecationWarning" title="DeprecationWarning"><code class="xref py py-exc docutils literal notranslate">DeprecationWarning</code></a> and will eventually become a <a class="reference internal" href="../library/exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal notranslate">SyntaxError</code></a>. See
<a class="reference internal" href="#the-backslash-plague">The Backslash Plague</a>.
<code class="xref py py-meth docutils literal notranslate">findall()</code> has to create the entire list before it can be returned as the
result. The <code class="xref py py-meth docutils literal notranslate">finditer()</code> method returns a sequence of
<a class="reference internal" href="../library/re.html#match-objects">match object</a> instances as an <a class="reference internal" href="../glossary.html#term-iterator">iterator</a>:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; iterator = p.finditer(&#39;12 drummers drumming, 11 ... 10 ...&#39;)
&gt;&gt;&gt; iterator 
&lt;callable_iterator object at 0x...&gt;
&gt;&gt;&gt; for match in iterator:
... print(match.span())
...
(0, 2)
(22, 24)
(29, 31)
</pre></div>
</div>
</div>
<div class="section" id="module-level-functions">
<h3>Module-Level Functions<a class="headerlink" href="#module-level-functions" title="Permalink to this headline">¶</a></h3>
You don’t have to create a pattern object and call its methods; the
<a class="reference internal" href="../library/re.html#module-re" title="re: Regular expression operations."><code class="xref py py-mod docutils literal notranslate">re</code></a> module also provides top-level functions called <a class="reference internal" href="../library/re.html#re.match" title="re.match"><code class="xref py py-func docutils literal notranslate">match()</code></a>,
<a class="reference internal" href="../library/re.html#re.search" title="re.search"><code class="xref py py-func docutils literal notranslate">search()</code></a>, <a class="reference internal" href="../library/re.html#re.findall" title="re.findall"><code class="xref py py-func docutils literal notranslate">findall()</code></a>, <a class="reference internal" href="../library/re.html#re.sub" title="re.sub"><code class="xref py py-func docutils literal notranslate">sub()</code></a>, and so forth. These functions
take the same arguments as the corresponding pattern method with
the RE string added as the first argument, and still return either <code class="docutils literal notranslate">None</code> or a
<a class="reference internal" href="../library/re.html#match-objects">match object</a> instance.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; print(re.match(r&#39;From\s+&#39;, &#39;Fromage amk&#39;))
None
&gt;&gt;&gt; re.match(r&#39;From\s+&#39;, &#39;From amk Thu May 14 19:12:10 1998&#39;) 
&lt;_sre.SRE_Match object; span=(0, 5), match=&#39;From &#39;&gt;
</pre></div>
</div>
Under the hood, these functions simply create a pattern object for you
and call the appropriate method on it. They also store the compiled
object in a cache, so future calls using the same RE won’t need to
parse the pattern again and again.
Should you use these module-level functions, or should you get the
pattern and call its methods yourself? If you’re accessing a regex
within a loop, pre-compiling it will save a few function calls.
Outside of loops, there’s not much difference thanks to the internal
cache.
</div>
<div class="section" id="compilation-flags">
<h3>Compilation Flags<a class="headerlink" href="#compilation-flags" title="Permalink to this headline">¶</a></h3>
Compilation flags let you modify some aspects of how regular expressions work.
Flags are available in the <a class="reference internal" href="../library/re.html#module-re" title="re: Regular expression operations."><code class="xref py py-mod docutils literal notranslate">re</code></a> module under two names, a long name such as
<code class="xref py py-const docutils literal notranslate">IGNORECASE</code> and a short, one-letter form such as <code class="xref py py-const docutils literal notranslate">I</code>. (If you’re
familiar with Perl’s pattern modifiers, the one-letter forms use the same
letters; the short form of <a class="reference internal" href="../library/re.html#re.VERBOSE" title="re.VERBOSE"><code class="xref py py-const docutils literal notranslate">re.VERBOSE</code></a> is <a class="reference internal" href="../library/re.html#re.X" title="re.X"><code class="xref py py-const docutils literal notranslate">re.X</code></a>, for example.)
Multiple flags can be specified by bitwise OR-ing them; <code class="docutils literal notranslate">re.I | re.M</code> sets
both the <code class="xref py py-const docutils literal notranslate">I</code> and <code class="xref py py-const docutils literal notranslate">M</code> flags, for example.
Here’s a table of the available flags, followed by a more detailed explanation
of each one.
<table border="1" class="docutils">
<colgroup>
<col width="43%" />
<col width="57%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Flag</th>
<th class="head">Meaning</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><code class="xref py py-const docutils literal notranslate">ASCII</code>, <code class="xref py py-const docutils literal notranslate">A</code></td>
<td>Makes several escapes like <code class="docutils literal notranslate">\w</code>, <code class="docutils literal notranslate">\b</code>,
<code class="docutils literal notranslate">\s</code> and <code class="docutils literal notranslate">\d</code> match only on ASCII
characters with the respective property.</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate">DOTALL</code>, <code class="xref py py-const docutils literal notranslate">S</code></td>
<td>Make <code class="docutils literal notranslate">.</code> match any character, including
newlines.</td>
</tr>
<tr class="row-even"><td><code class="xref py py-const docutils literal notranslate">IGNORECASE</code>, <code class="xref py py-const docutils literal notranslate">I</code></td>
<td>Do case-insensitive matches.</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate">LOCALE</code>, <code class="xref py py-const docutils literal notranslate">L</code></td>
<td>Do a locale-aware match.</td>
</tr>
<tr class="row-even"><td><code class="xref py py-const docutils literal notranslate">MULTILINE</code>, <code class="xref py py-const docutils literal notranslate">M</code></td>
<td>Multi-line matching, affecting <code class="docutils literal notranslate">^</code> and
<code class="docutils literal notranslate">$</code>.</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate">VERBOSE</code>, <code class="xref py py-const docutils literal notranslate">X</code>
(for ‘extended’)</td>
<td>Enable verbose REs, which can be organized
more cleanly and understandably.</td>
</tr>
</tbody>
</table>
<dl class="data">
<dt>
<code class="descname">I</code></dt>
<dt>
<code class="descname">IGNORECASE</code></dt>
<dd>Perform case-insensitive matching; character class and literal strings will
match letters by ignoring case. For example, <code class="docutils literal notranslate">[A-Z]</code> will match lowercase
letters, too. Full Unicode matching also works unless the <code class="xref py py-const docutils literal notranslate">ASCII</code>
flag is used to disable non-ASCII matches. When the Unicode patterns
<code class="docutils literal notranslate">[a-z]</code> or <code class="docutils literal notranslate">[A-Z]</code> are used in combination with the <code class="xref py py-const docutils literal notranslate">IGNORECASE</code>
flag, they will match the 52 ASCII letters and 4 additional non-ASCII
letters: ‘İ’ (U+0130, Latin capital letter I with dot above), ‘ı’ (U+0131,
Latin small letter dotless i), ‘ſ’ (U+017F, Latin small letter long s) and
‘K’ (U+212A, Kelvin sign). <code class="docutils literal notranslate">Spam</code> will match <code class="docutils literal notranslate">'Spam'</code>, <code class="docutils literal notranslate">'spam'</code>,
<code class="docutils literal notranslate">'spAM'</code>, or <code class="docutils literal notranslate">'ſpam'</code> (the latter is matched only in Unicode mode).
This lowercasing doesn’t take the current locale into account;
it will if you also set the <code class="xref py py-const docutils literal notranslate">LOCALE</code> flag.
</dd></dl>

<dl class="data">
<dt>
<code class="descname">L</code></dt>
<dt>
<code class="descname">LOCALE</code></dt>
<dd>Make <code class="docutils literal notranslate">\w</code>, <code class="docutils literal notranslate">\W</code>, <code class="docutils literal notranslate">\b</code>, <code class="docutils literal notranslate">\B</code> and case-insensitive matching dependent
on the current locale instead of the Unicode database.
Locales are a feature of the C library intended to help in writing programs
that take account of language differences. For example, if you’re
processing encoded French text, you’d want to be able to write <code class="docutils literal notranslate">\w+</code> to
match words, but <code class="docutils literal notranslate">\w</code> only matches the character class <code class="docutils literal notranslate">[A-Za-z]</code> in
bytes patterns; it won’t match bytes corresponding to <code class="docutils literal notranslate">é</code> or <code class="docutils literal notranslate">ç</code>.
If your system is configured properly and a French locale is selected,
certain C functions will tell the program that the byte corresponding to
<code class="docutils literal notranslate">é</code> should also be considered a letter.
Setting the <code class="xref py py-const docutils literal notranslate">LOCALE</code> flag when compiling a regular expression will cause
the resulting compiled object to use these C functions for <code class="docutils literal notranslate">\w</code>; this is
slower, but also enables <code class="docutils literal notranslate">\w+</code> to match French words as you’d expect.
The use of this flag is discouraged in Python 3 as the locale mechanism
is very unreliable, it only handles one “culture” at a time, and it only
works with 8-bit locales. Unicode matching is already enabled by default
in Python 3 for Unicode (str) patterns, and it is able to handle different
locales/languages.
</dd></dl>

<dl class="data">
<dt>
<code class="descname">M</code></dt>
<dt>
<code class="descname">MULTILINE</code></dt>
<dd>(<code class="docutils literal notranslate">^</code> and <code class="docutils literal notranslate">$</code> haven’t been explained yet; they’ll be introduced in section
<a class="reference internal" href="#more-metacharacters">More Metacharacters</a>.)
Usually <code class="docutils literal notranslate">^</code> matches only at the beginning of the string, and <code class="docutils literal notranslate">$</code> matches
only at the end of the string and immediately before the newline (if any) at the
end of the string. When this flag is specified, <code class="docutils literal notranslate">^</code> matches at the beginning
of the string and at the beginning of each line within the string, immediately
following each newline. Similarly, the <code class="docutils literal notranslate">$</code> metacharacter matches either at
the end of the string and at the end of each line (immediately preceding each
newline).
</dd></dl>

<dl class="data">
<dt>
<code class="descname">S</code></dt>
<dt>
<code class="descname">DOTALL</code></dt>
<dd>Makes the <code class="docutils literal notranslate">'.'</code> special character match any character at all, including a
newline; without this flag, <code class="docutils literal notranslate">'.'</code> will match anything except a newline.
</dd></dl>

<dl class="data">
<dt>
<code class="descname">A</code></dt>
<dt>
<code class="descname">ASCII</code></dt>
<dd>Make <code class="docutils literal notranslate">\w</code>, <code class="docutils literal notranslate">\W</code>, <code class="docutils literal notranslate">\b</code>, <code class="docutils literal notranslate">\B</code>, <code class="docutils literal notranslate">\s</code> and <code class="docutils literal notranslate">\S</code> perform ASCII-only
matching instead of full Unicode matching. This is only meaningful for
Unicode patterns, and is ignored for byte patterns.
</dd></dl>

<dl class="data">
<dt>
<code class="descname">X</code></dt>
<dt>
<code class="descname">VERBOSE</code></dt>
<dd>This flag allows you to write regular expressions that are more readable by
granting you more flexibility in how you can format them. When this flag has
been specified, whitespace within the RE string is ignored, except when the
whitespace is in a character class or preceded by an unescaped backslash; this
lets you organize and indent the RE more clearly. This flag also lets you put
comments within a RE that will be ignored by the engine; comments are marked by
a <code class="docutils literal notranslate">'#'</code> that’s neither in a character class or preceded by an unescaped
backslash.
For example, here’s a RE that uses <a class="reference internal" href="../library/re.html#re.VERBOSE" title="re.VERBOSE"><code class="xref py py-const docutils literal notranslate">re.VERBOSE</code></a>; see how much easier it
is to read?
<div class="highlight-python3 notranslate"><div class="highlight"><pre>charref = re.compile(r&quot;&quot;&quot;
 &amp;[#] # Start of a numeric entity reference
 (
 0[0-7]+ # Octal form
 | [0-9]+ # Decimal form
 | x[0-9a-fA-F]+ # Hexadecimal form
 )
 ; # Trailing semicolon
&quot;&quot;&quot;, re.VERBOSE)
</pre></div>
</div>
Without the verbose setting, the RE would look like this:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>charref = re.compile(&quot;&amp;#(0[0-7]+&quot;
 &quot;|[0-9]+&quot;
 &quot;|x[0-9a-fA-F]+);&quot;)
</pre></div>
</div>
In the above example, Python’s automatic concatenation of string literals has
been used to break up the RE into smaller pieces, but it’s still more difficult
to understand than the version using <a class="reference internal" href="../library/re.html#re.VERBOSE" title="re.VERBOSE"><code class="xref py py-const docutils literal notranslate">re.VERBOSE</code></a>.
</dd></dl>

</div>
</div>
<div class="section" id="more-pattern-power">
<h2>More Pattern Power<a class="headerlink" href="#more-pattern-power" title="Permalink to this headline">¶</a></h2>
So far we’ve only covered a part of the features of regular expressions. In
this section, we’ll cover some new metacharacters, and how to use groups to
retrieve portions of the text that was matched.
<div class="section" id="more-metacharacters">
<h3>More Metacharacters<a class="headerlink" href="#more-metacharacters" title="Permalink to this headline">¶</a></h3>
There are some metacharacters that we haven’t covered yet. Most of them will be
covered in this section.
Some of the remaining metacharacters to be discussed are zero-width
assertions. They don’t cause the engine to advance through the string;
instead, they consume no characters at all, and simply succeed or fail. For
example, <code class="docutils literal notranslate">\b</code> is an assertion that the current position is located at a word
boundary; the position isn’t changed by the <code class="docutils literal notranslate">\b</code> at all. This means that
zero-width assertions should never be repeated, because if they match once at a
given location, they can obviously be matched an infinite number of times.
<dl class="docutils">
<dt><code class="docutils literal notranslate">|</code></dt>
<dd>Alternation, or the “or” operator. If A and B are regular expressions,
<code class="docutils literal notranslate">A|B</code> will match any string that matches either A or B. <code class="docutils literal notranslate">|</code> has very
low precedence in order to make it work reasonably when you’re alternating
multi-character strings. <code class="docutils literal notranslate">Crow|Servo</code> will match either <code class="docutils literal notranslate">'Crow'</code> or <code class="docutils literal notranslate">'Servo'</code>,
not <code class="docutils literal notranslate">'Cro'</code>, a <code class="docutils literal notranslate">'w'</code> or an <code class="docutils literal notranslate">'S'</code>, and <code class="docutils literal notranslate">'ervo'</code>.
To match a literal <code class="docutils literal notranslate">'|'</code>, use <code class="docutils literal notranslate">\|</code>, or enclose it inside a character class,
as in <code class="docutils literal notranslate">[|]</code>.
</dd>
<dt><code class="docutils literal notranslate">^</code></dt>
<dd>Matches at the beginning of lines. Unless the <code class="xref py py-const docutils literal notranslate">MULTILINE</code> flag has been
set, this will only match at the beginning of the string. In <code class="xref py py-const docutils literal notranslate">MULTILINE</code>
mode, this also matches immediately after each newline within the string.
For example, if you wish to match the word <code class="docutils literal notranslate">From</code> only at the beginning of a
line, the RE to use is <code class="docutils literal notranslate">^From</code>.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; print(re.search(&#39;^From&#39;, &#39;From Here to Eternity&#39;)) 
&lt;_sre.SRE_Match object; span=(0, 4), match=&#39;From&#39;&gt;
&gt;&gt;&gt; print(re.search(&#39;^From&#39;, &#39;Reciting From Memory&#39;))
None
</pre></div>
</div>
To match a literal <code class="docutils literal notranslate">'^'</code>, use <code class="docutils literal notranslate">\^</code>.
</dd>
<dt><code class="docutils literal notranslate">$</code></dt>
<dd>Matches at the end of a line, which is defined as either the end of the string,
or any location followed by a newline character.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; print(re.search(&#39;}$&#39;, &#39;{block}&#39;)) 
&lt;_sre.SRE_Match object; span=(6, 7), match=&#39;}&#39;&gt;
&gt;&gt;&gt; print(re.search(&#39;}$&#39;, &#39;{block} &#39;))
None
&gt;&gt;&gt; print(re.search(&#39;}$&#39;, &#39;{block}\n&#39;)) 
&lt;_sre.SRE_Match object; span=(6, 7), match=&#39;}&#39;&gt;
</pre></div>
</div>
To match a literal <code class="docutils literal notranslate">'$'</code>, use <code class="docutils literal notranslate">\$</code> or enclose it inside a character class,
as in <code class="docutils literal notranslate">[$]</code>.
</dd>
<dt><code class="docutils literal notranslate">\A</code></dt>
<dd>Matches only at the start of the string. When not in <code class="xref py py-const docutils literal notranslate">MULTILINE</code> mode,
<code class="docutils literal notranslate">\A</code> and <code class="docutils literal notranslate">^</code> are effectively the same. In <code class="xref py py-const docutils literal notranslate">MULTILINE</code> mode, they’re
different: <code class="docutils literal notranslate">\A</code> still matches only at the beginning of the string, but <code class="docutils literal notranslate">^</code>
may match at any location inside the string that follows a newline character.</dd>
<dt><code class="docutils literal notranslate">\Z</code></dt>
<dd>Matches only at the end of the string.</dd>
<dt><code class="docutils literal notranslate">\b</code></dt>
<dd>Word boundary. This is a zero-width assertion that matches only at the
beginning or end of a word. A word is defined as a sequence of alphanumeric
characters, so the end of a word is indicated by whitespace or a
non-alphanumeric character.
The following example matches <code class="docutils literal notranslate">class</code> only when it’s a complete word; it won’t
match when it’s contained inside another word.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; p = re.compile(r&#39;\bclass\b&#39;)
&gt;&gt;&gt; print(p.search(&#39;no class at all&#39;))
&lt;_sre.SRE_Match object; span=(3, 8), match=&#39;class&#39;&gt;
&gt;&gt;&gt; print(p.search(&#39;the declassified algorithm&#39;))
None
&gt;&gt;&gt; print(p.search(&#39;one subclass is&#39;))
None
</pre></div>
</div>
There are two subtleties you should remember when using this special sequence.
First, this is the worst collision between Python’s string literals and regular
expression sequences. In Python’s string literals, <code class="docutils literal notranslate">\b</code> is the backspace
character, ASCII value 8. If you’re not using raw strings, then Python will
convert the <code class="docutils literal notranslate">\b</code> to a backspace, and your RE won’t match as you expect it to.
The following example looks the same as our previous RE, but omits the <code class="docutils literal notranslate">'r'</code>
in front of the RE string.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; p = re.compile(&#39;\bclass\b&#39;)
&gt;&gt;&gt; print(p.search(&#39;no class at all&#39;))
None
&gt;&gt;&gt; print(p.search(&#39;\b&#39; + &#39;class&#39; + &#39;\b&#39;))
&lt;_sre.SRE_Match object; span=(0, 7), match=&#39;\x08class\x08&#39;&gt;
</pre></div>
</div>
Second, inside a character class, where there’s no use for this assertion,
<code class="docutils literal notranslate">\b</code> represents the backspace character, for compatibility with Python’s
string literals.
</dd>
<dt><code class="docutils literal notranslate">\B</code></dt>
<dd>Another zero-width assertion, this is the opposite of <code class="docutils literal notranslate">\b</code>, only matching when
the current position is not at a word boundary.</dd>
</dl>
</div>
<div class="section" id="grouping">
<h3>Grouping<a class="headerlink" href="#grouping" title="Permalink to this headline">¶</a></h3>
Frequently you need to obtain more information than just whether the RE matched
or not. Regular expressions are often used to dissect strings by writing a RE
divided into several subgroups which match different components of interest.
For example, an RFC-822 header line is divided into a header name and a value,
separated by a <code class="docutils literal notranslate">':'</code>, like this:
<div class="highlight-none notranslate"><div class="highlight"><pre>From: author@example.com
User-Agent: Thunderbird 1.5.0.9 (X11/20061227)
MIME-Version: 1.0
To: editor@example.com
</pre></div>
</div>
This can be handled by writing a regular expression which matches an entire
header line, and has one group which matches the header name, and another group
which matches the header’s value.
Groups are marked by the <code class="docutils literal notranslate">'('</code>, <code class="docutils literal notranslate">')'</code> metacharacters. <code class="docutils literal notranslate">'('</code> and <code class="docutils literal notranslate">')'</code>
have much the same meaning as they do in mathematical expressions; they group
together the expressions contained inside them, and you can repeat the contents
of a group with a repeating qualifier, such as <code class="docutils literal notranslate">*</code>, <code class="docutils literal notranslate">+</code>, <code class="docutils literal notranslate">?</code>, or
<code class="docutils literal notranslate">{m,n}</code>. For example, <code class="docutils literal notranslate">(ab)*</code> will match zero or more repetitions of
<code class="docutils literal notranslate">ab</code>.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; p = re.compile(&#39;(ab)*&#39;)
&gt;&gt;&gt; print(p.match(&#39;ababababab&#39;).span())
(0, 10)
</pre></div>
</div>
Groups indicated with <code class="docutils literal notranslate">'('</code>, <code class="docutils literal notranslate">')'</code> also capture the starting and ending
index of the text that they match; this can be retrieved by passing an argument
to <a class="reference internal" href="../library/re.html#re.match.group" title="re.match.group"><code class="xref py py-meth docutils literal notranslate">group()</code></a>, <a class="reference internal" href="../library/re.html#re.match.start" title="re.match.start"><code class="xref py py-meth docutils literal notranslate">start()</code></a>, <a class="reference internal" href="../library/re.html#re.match.end" title="re.match.end"><code class="xref py py-meth docutils literal notranslate">end()</code></a>, and
<a class="reference internal" href="../library/re.html#re.match.span" title="re.match.span"><code class="xref py py-meth docutils literal notranslate">span()</code></a>. Groups are
numbered starting with 0. Group 0 is always present; it’s the whole RE, so
<a class="reference internal" href="../library/re.html#match-objects">match object</a> methods all have group 0 as their default
argument. Later we’ll see how to express groups that don’t capture the span
of text that they match.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; p = re.compile(&#39;(a)b&#39;)
&gt;&gt;&gt; m = p.match(&#39;ab&#39;)
&gt;&gt;&gt; m.group()
&#39;ab&#39;
&gt;&gt;&gt; m.group(0)
&#39;ab&#39;
</pre></div>
</div>
Subgroups are numbered from left to right, from 1 upward. Groups can be nested;
to determine the number, just count the opening parenthesis characters, going
from left to right.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; p = re.compile(&#39;(a(b)c)d&#39;)
&gt;&gt;&gt; m = p.match(&#39;abcd&#39;)
&gt;&gt;&gt; m.group(0)
&#39;abcd&#39;
&gt;&gt;&gt; m.group(1)
&#39;abc&#39;
&gt;&gt;&gt; m.group(2)
&#39;b&#39;
</pre></div>
</div>
<a class="reference internal" href="../library/re.html#re.match.group" title="re.match.group"><code class="xref py py-meth docutils literal notranslate">group()</code></a> can be passed multiple group numbers at a time, in which case it
will return a tuple containing the corresponding values for those groups.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; m.group(2,1,2)
(&#39;b&#39;, &#39;abc&#39;, &#39;b&#39;)
</pre></div>
</div>
The <a class="reference internal" href="../library/re.html#re.match.groups" title="re.match.groups"><code class="xref py py-meth docutils literal notranslate">groups()</code></a> method returns a tuple containing the strings for all the
subgroups, from 1 up to however many there are.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; m.groups()
(&#39;abc&#39;, &#39;b&#39;)
</pre></div>
</div>
Backreferences in a pattern allow you to specify that the contents of an earlier
capturing group must also be found at the current location in the string. For
example, <code class="docutils literal notranslate">\1</code> will succeed if the exact contents of group 1 can be found at
the current position, and fails otherwise. Remember that Python’s string
literals also use a backslash followed by numbers to allow including arbitrary
characters in a string, so be sure to use a raw string when incorporating
backreferences in a RE.
For example, the following RE detects doubled words in a string.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; p = re.compile(r&#39;\b(\w+)\s+\1\b&#39;)
&gt;&gt;&gt; p.search(&#39;Paris in the the spring&#39;).group()
&#39;the the&#39;
</pre></div>
</div>
Backreferences like this aren’t often useful for just searching through a string
— there are few text formats which repeat data in this way — but you’ll soon
find out that they’re very useful when performing string substitutions.
</div>
<div class="section" id="non-capturing-and-named-groups">
<h3>Non-capturing and Named Groups<a class="headerlink" href="#non-capturing-and-named-groups" title="Permalink to this headline">¶</a></h3>
Elaborate REs may use many groups, both to capture substrings of interest, and
to group and structure the RE itself. In complex REs, it becomes difficult to
keep track of the group numbers. There are two features which help with this
problem. Both of them use a common syntax for regular expression extensions, so
we’ll look at that first.
Perl 5 is well known for its powerful additions to standard regular expressions.
For these new features the Perl developers couldn’t choose new single-keystroke metacharacters
or new special sequences beginning with <code class="docutils literal notranslate">\</code> without making Perl’s regular
expressions confusingly different from standard REs. If they chose <code class="docutils literal notranslate">&amp;</code> as a
new metacharacter, for example, old expressions would be assuming that <code class="docutils literal notranslate">&amp;</code> was
a regular character and wouldn’t have escaped it by writing <code class="docutils literal notranslate">\&amp;</code> or <code class="docutils literal notranslate">[&amp;]</code>.
The solution chosen by the Perl developers was to use <code class="docutils literal notranslate">(?...)</code> as the
extension syntax. <code class="docutils literal notranslate">?</code> immediately after a parenthesis was a syntax error
because the <code class="docutils literal notranslate">?</code> would have nothing to repeat, so this didn’t introduce any
compatibility problems. The characters immediately after the <code class="docutils literal notranslate">?</code> indicate
what extension is being used, so <code class="docutils literal notranslate">(?=foo)</code> is one thing (a positive lookahead
assertion) and <code class="docutils literal notranslate">(?:foo)</code> is something else (a non-capturing group containing
the subexpression <code class="docutils literal notranslate">foo</code>).
Python supports several of Perl’s extensions and adds an extension
syntax to Perl’s extension syntax. If the first character after the
question mark is a <code class="docutils literal notranslate">P</code>, you know that it’s an extension that’s
specific to Python.
Now that we’ve looked at the general extension syntax, we can return
to the features that simplify working with groups in complex REs.
Sometimes you’ll want to use a group to denote a part of a regular expression,
but aren’t interested in retrieving the group’s contents. You can make this fact
explicit by using a non-capturing group: <code class="docutils literal notranslate">(?:...)</code>, where you can replace the
<code class="docutils literal notranslate">...</code> with any other regular expression.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; m = re.match(&quot;([abc])+&quot;, &quot;abc&quot;)
&gt;&gt;&gt; m.groups()
(&#39;c&#39;,)
&gt;&gt;&gt; m = re.match(&quot;(?:[abc])+&quot;, &quot;abc&quot;)
&gt;&gt;&gt; m.groups()
()
</pre></div>
</div>
Except for the fact that you can’t retrieve the contents of what the group
matched, a non-capturing group behaves exactly the same as a capturing group;
you can put anything inside it, repeat it with a repetition metacharacter such
as <code class="docutils literal notranslate">*</code>, and nest it within other groups (capturing or non-capturing).
<code class="docutils literal notranslate">(?:...)</code> is particularly useful when modifying an existing pattern, since you
can add new groups without changing how all the other groups are numbered. It
should be mentioned that there’s no performance difference in searching between
capturing and non-capturing groups; neither form is any faster than the other.
A more significant feature is named groups: instead of referring to them by
numbers, groups can be referenced by a name.
The syntax for a named group is one of the Python-specific extensions:
<code class="docutils literal notranslate">(?P&lt;name&gt;...)</code>. name is, obviously, the name of the group. Named groups
behave exactly like capturing groups, and additionally associate a name
with a group. The <a class="reference internal" href="../library/re.html#match-objects">match object</a> methods that deal with
capturing groups all accept either integers that refer to the group by number
or strings that contain the desired group’s name. Named groups are still
given numbers, so you can retrieve information about a group in two ways:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; p = re.compile(r&#39;(?P&lt;word&gt;\b\w+\b)&#39;)
&gt;&gt;&gt; m = p.search( &#39;(((( Lots of punctuation )))&#39; )
&gt;&gt;&gt; m.group(&#39;word&#39;)
&#39;Lots&#39;
&gt;&gt;&gt; m.group(1)
&#39;Lots&#39;
</pre></div>
</div>
Named groups are handy because they let you use easily-remembered names, instead
of having to remember numbers. Here’s an example RE from the <a class="reference internal" href="../library/imaplib.html#module-imaplib" title="imaplib: IMAP4 protocol client (requires sockets)."><code class="xref py py-mod docutils literal notranslate">imaplib</code></a>
module:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>InternalDate = re.compile(r&#39;INTERNALDATE &quot;&#39;
 r&#39;(?P&lt;day&gt;[ 123][0-9])-(?P&lt;mon&gt;[A-Z][a-z][a-z])-&#39;
 r&#39;(?P&lt;year&gt;[0-9][0-9][0-9][0-9])&#39;
 r&#39; (?P&lt;hour&gt;[0-9][0-9]):(?P&lt;min&gt;[0-9][0-9]):(?P&lt;sec&gt;[0-9][0-9])&#39;
 r&#39; (?P&lt;zonen&gt;[-+])(?P&lt;zoneh&gt;[0-9][0-9])(?P&lt;zonem&gt;[0-9][0-9])&#39;
 r&#39;&quot;&#39;)
</pre></div>
</div>
It’s obviously much easier to retrieve <code class="docutils literal notranslate">m.group('zonem')</code>, instead of having
to remember to retrieve group 9.
The syntax for backreferences in an expression such as <code class="docutils literal notranslate">(...)\1</code> refers to the
number of the group. There’s naturally a variant that uses the group name
instead of the number. This is another Python extension: <code class="docutils literal notranslate">(?P=name)</code> indicates
that the contents of the group called name should again be matched at the
current point. The regular expression for finding doubled words,
<code class="docutils literal notranslate">\b(\w+)\s+\1\b</code> can also be written as <code class="docutils literal notranslate">\b(?P&lt;word&gt;\w+)\s+(?P=word)\b</code>:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; p = re.compile(r&#39;\b(?P&lt;word&gt;\w+)\s+(?P=word)\b&#39;)
&gt;&gt;&gt; p.search(&#39;Paris in the the spring&#39;).group()
&#39;the the&#39;
</pre></div>
</div>
</div>
<div class="section" id="lookahead-assertions">
<h3>Lookahead Assertions<a class="headerlink" href="#lookahead-assertions" title="Permalink to this headline">¶</a></h3>
Another zero-width assertion is the lookahead assertion. Lookahead assertions
are available in both positive and negative form, and look like this:
<dl class="docutils">
<dt><code class="docutils literal notranslate">(?=...)</code></dt>
<dd>Positive lookahead assertion. This succeeds if the contained regular
expression, represented here by <code class="docutils literal notranslate">...</code>, successfully matches at the current
location, and fails otherwise. But, once the contained expression has been
tried, the matching engine doesn’t advance at all; the rest of the pattern is
tried right where the assertion started.</dd>
<dt><code class="docutils literal notranslate">(?!...)</code></dt>
<dd>Negative lookahead assertion. This is the opposite of the positive assertion;
it succeeds if the contained expression doesn’t match at the current position
in the string.</dd>
</dl>
To make this concrete, let’s look at a case where a lookahead is useful.
Consider a simple pattern to match a filename and split it apart into a base
name and an extension, separated by a <code class="docutils literal notranslate">.</code>. For example, in <code class="docutils literal notranslate">news.rc</code>,
<code class="docutils literal notranslate">news</code> is the base name, and <code class="docutils literal notranslate">rc</code> is the filename’s extension.
The pattern to match this is quite simple:
<code class="docutils literal notranslate">.*[.].*$</code>
Notice that the <code class="docutils literal notranslate">.</code> needs to be treated specially because it’s a
metacharacter, so it’s inside a character class to only match that
specific character. Also notice the trailing <code class="docutils literal notranslate">$</code>; this is added to
ensure that all the rest of the string must be included in the
extension. This regular expression matches <code class="docutils literal notranslate">foo.bar</code> and
<code class="docutils literal notranslate">autoexec.bat</code> and <code class="docutils literal notranslate">sendmail.cf</code> and <code class="docutils literal notranslate">printers.conf</code>.
Now, consider complicating the problem a bit; what if you want to match
filenames where the extension is not <code class="docutils literal notranslate">bat</code>? Some incorrect attempts:
<code class="docutils literal notranslate">.*[.][^b].*$</code> The first attempt above tries to exclude <code class="docutils literal notranslate">bat</code> by requiring
that the first character of the extension is not a <code class="docutils literal notranslate">b</code>. This is wrong,
because the pattern also doesn’t match <code class="docutils literal notranslate">foo.bar</code>.
<code class="docutils literal notranslate">.*[.]([^b]..|.[^a].|..[^t])$</code>
The expression gets messier when you try to patch up the first solution by
requiring one of the following cases to match: the first character of the
extension isn’t <code class="docutils literal notranslate">b</code>; the second character isn’t <code class="docutils literal notranslate">a</code>; or the third character
isn’t <code class="docutils literal notranslate">t</code>. This accepts <code class="docutils literal notranslate">foo.bar</code> and rejects <code class="docutils literal notranslate">autoexec.bat</code>, but it
requires a three-letter extension and won’t accept a filename with a two-letter
extension such as <code class="docutils literal notranslate">sendmail.cf</code>. We’ll complicate the pattern again in an
effort to fix it.
<code class="docutils literal notranslate">.*[.]([^b].?.?|.[^a]?.?|..?[^t]?)$</code>
In the third attempt, the second and third letters are all made optional in
order to allow matching extensions shorter than three characters, such as
<code class="docutils literal notranslate">sendmail.cf</code>.
The pattern’s getting really complicated now, which makes it hard to read and
understand. Worse, if the problem changes and you want to exclude both <code class="docutils literal notranslate">bat</code>
and <code class="docutils literal notranslate">exe</code> as extensions, the pattern would get even more complicated and
confusing.
A negative lookahead cuts through all this confusion:
<code class="docutils literal notranslate">.*[.](?!bat$)[^.]*$</code> The negative lookahead means: if the expression <code class="docutils literal notranslate">bat</code>
doesn’t match at this point, try the rest of the pattern; if <code class="docutils literal notranslate">bat$</code> does
match, the whole pattern will fail. The trailing <code class="docutils literal notranslate">$</code> is required to ensure
that something like <code class="docutils literal notranslate">sample.batch</code>, where the extension only starts with
<code class="docutils literal notranslate">bat</code>, will be allowed. The <code class="docutils literal notranslate">[^.]*</code> makes sure that the pattern works
when there are multiple dots in the filename.
Excluding another filename extension is now easy; simply add it as an
alternative inside the assertion. The following pattern excludes filenames that
end in either <code class="docutils literal notranslate">bat</code> or <code class="docutils literal notranslate">exe</code>:
<code class="docutils literal notranslate">.*[.](?!bat$|exe$)[^.]*$</code>
</div>
</div>
<div class="section" id="modifying-strings">
<h2>Modifying Strings<a class="headerlink" href="#modifying-strings" title="Permalink to this headline">¶</a></h2>
Up to this point, we’ve simply performed searches against a static string.
Regular expressions are also commonly used to modify strings in various ways,
using the following pattern methods:
<table border="1" class="docutils">
<colgroup>
<col width="28%" />
<col width="72%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Method/Attribute</th>
<th class="head">Purpose</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><code class="docutils literal notranslate">split()</code></td>
<td>Split the string into a list, splitting it
wherever the RE matches</td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate">sub()</code></td>
<td>Find all substrings where the RE matches, and
replace them with a different string</td>
</tr>
<tr class="row-even"><td><code class="docutils literal notranslate">subn()</code></td>
<td>Does the same thing as <code class="xref py py-meth docutils literal notranslate">sub()</code>, but
returns the new string and the number of
replacements</td>
</tr>
</tbody>
</table>
<div class="section" id="splitting-strings">
<h3>Splitting Strings<a class="headerlink" href="#splitting-strings" title="Permalink to this headline">¶</a></h3>
The <code class="xref py py-meth docutils literal notranslate">split()</code> method of a pattern splits a string apart
wherever the RE matches, returning a list of the pieces. It’s similar to the
<a class="reference internal" href="../library/stdtypes.html#str.split" title="str.split"><code class="xref py py-meth docutils literal notranslate">split()</code></a> method of strings but provides much more generality in the
delimiters that you can split by; string <code class="xref py py-meth docutils literal notranslate">split()</code> only supports splitting by
whitespace or by a fixed string. As you’d expect, there’s a module-level
<a class="reference internal" href="../library/re.html#re.split" title="re.split"><code class="xref py py-func docutils literal notranslate">re.split()</code></a> function, too.
<dl class="method">
<dt>
<code class="descclassname">.</code><code class="descname">split</code>(string[, maxsplit=0])</dt>
<dd>Split string by the matches of the regular expression. If capturing
parentheses are used in the RE, then their contents will also be returned as
part of the resulting list. If maxsplit is nonzero, at most maxsplit splits
are performed.
</dd></dl>

You can limit the number of splits made, by passing a value for maxsplit.
When maxsplit is nonzero, at most maxsplit splits will be made, and the
remainder of the string is returned as the final element of the list. In the
following example, the delimiter is any sequence of non-alphanumeric characters.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; p = re.compile(r&#39;\W+&#39;)
&gt;&gt;&gt; p.split(&#39;This is a test, short and sweet, of split().&#39;)
[&#39;This&#39;, &#39;is&#39;, &#39;a&#39;, &#39;test&#39;, &#39;short&#39;, &#39;and&#39;, &#39;sweet&#39;, &#39;of&#39;, &#39;split&#39;, &#39;&#39;]
&gt;&gt;&gt; p.split(&#39;This is a test, short and sweet, of split().&#39;, 3)
[&#39;This&#39;, &#39;is&#39;, &#39;a&#39;, &#39;test, short and sweet, of split().&#39;]
</pre></div>
</div>
Sometimes you’re not only interested in what the text between delimiters is, but
also need to know what the delimiter was. If capturing parentheses are used in
the RE, then their values are also returned as part of the list. Compare the
following calls:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; p = re.compile(r&#39;\W+&#39;)
&gt;&gt;&gt; p2 = re.compile(r&#39;(\W+)&#39;)
&gt;&gt;&gt; p.split(&#39;This... is a test.&#39;)
[&#39;This&#39;, &#39;is&#39;, &#39;a&#39;, &#39;test&#39;, &#39;&#39;]
&gt;&gt;&gt; p2.split(&#39;This... is a test.&#39;)
[&#39;This&#39;, &#39;... &#39;, &#39;is&#39;, &#39; &#39;, &#39;a&#39;, &#39; &#39;, &#39;test&#39;, &#39;.&#39;, &#39;&#39;]
</pre></div>
</div>
The module-level function <a class="reference internal" href="../library/re.html#re.split" title="re.split"><code class="xref py py-func docutils literal notranslate">re.split()</code></a> adds the RE to be used as the first
argument, but is otherwise the same.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; re.split(r&#39;[\W]+&#39;, &#39;Words, words, words.&#39;)
[&#39;Words&#39;, &#39;words&#39;, &#39;words&#39;, &#39;&#39;]
&gt;&gt;&gt; re.split(r&#39;([\W]+)&#39;, &#39;Words, words, words.&#39;)
[&#39;Words&#39;, &#39;, &#39;, &#39;words&#39;, &#39;, &#39;, &#39;words&#39;, &#39;.&#39;, &#39;&#39;]
&gt;&gt;&gt; re.split(r&#39;[\W]+&#39;, &#39;Words, words, words.&#39;, 1)
[&#39;Words&#39;, &#39;words, words.&#39;]
</pre></div>
</div>
</div>
<div class="section" id="search-and-replace">
<h3>Search and Replace<a class="headerlink" href="#search-and-replace" title="Permalink to this headline">¶</a></h3>
Another common task is to find all the matches for a pattern, and replace them
with a different string. The <code class="xref py py-meth docutils literal notranslate">sub()</code> method takes a replacement value,
which can be either a string or a function, and the string to be processed.
<dl class="method">
<dt>
<code class="descclassname">.</code><code class="descname">sub</code>(replacement, string[, count=0])</dt>
<dd>Returns the string obtained by replacing the leftmost non-overlapping
occurrences of the RE in string by the replacement replacement. If the
pattern isn’t found, string is returned unchanged.
The optional argument count is the maximum number of pattern occurrences to be
replaced; count must be a non-negative integer. The default value of 0 means
to replace all occurrences.
</dd></dl>

Here’s a simple example of using the <code class="xref py py-meth docutils literal notranslate">sub()</code> method. It replaces colour
names with the word <code class="docutils literal notranslate">colour</code>:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; p = re.compile(&#39;(blue|white|red)&#39;)
&gt;&gt;&gt; p.sub(&#39;colour&#39;, &#39;blue socks and red shoes&#39;)
&#39;colour socks and colour shoes&#39;
&gt;&gt;&gt; p.sub(&#39;colour&#39;, &#39;blue socks and red shoes&#39;, count=1)
&#39;colour socks and red shoes&#39;
</pre></div>
</div>
The <code class="xref py py-meth docutils literal notranslate">subn()</code> method does the same work, but returns a 2-tuple containing the
new string value and the number of replacements that were performed:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; p = re.compile(&#39;(blue|white|red)&#39;)
&gt;&gt;&gt; p.subn(&#39;colour&#39;, &#39;blue socks and red shoes&#39;)
(&#39;colour socks and colour shoes&#39;, 2)
&gt;&gt;&gt; p.subn(&#39;colour&#39;, &#39;no colours at all&#39;)
(&#39;no colours at all&#39;, 0)
</pre></div>
</div>
Empty matches are replaced only when they’re not adjacent to a previous match.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; p = re.compile(&#39;x*&#39;)
&gt;&gt;&gt; p.sub(&#39;-&#39;, &#39;abxd&#39;)
&#39;-a-b-d-&#39;
</pre></div>
</div>
If replacement is a string, any backslash escapes in it are processed. That
is, <code class="docutils literal notranslate">\n</code> is converted to a single newline character, <code class="docutils literal notranslate">\r</code> is converted to a
carriage return, and so forth. Unknown escapes such as <code class="docutils literal notranslate">\&amp;</code> are left alone.
Backreferences, such as <code class="docutils literal notranslate">\6</code>, are replaced with the substring matched by the
corresponding group in the RE. This lets you incorporate portions of the
original text in the resulting replacement string.
This example matches the word <code class="docutils literal notranslate">section</code> followed by a string enclosed in
<code class="docutils literal notranslate">{</code>, <code class="docutils literal notranslate">}</code>, and changes <code class="docutils literal notranslate">section</code> to <code class="docutils literal notranslate">subsection</code>:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; p = re.compile(&#39;section{ ( [^}]* ) }&#39;, re.VERBOSE)
&gt;&gt;&gt; p.sub(r&#39;subsection{\1}&#39;,&#39;section{First} section{second}&#39;)
&#39;subsection{First} subsection{second}&#39;
</pre></div>
</div>
There’s also a syntax for referring to named groups as defined by the
<code class="docutils literal notranslate">(?P&lt;name&gt;...)</code> syntax. <code class="docutils literal notranslate">\g&lt;name&gt;</code> will use the substring matched by the
group named <code class="docutils literal notranslate">name</code>, and <code class="docutils literal notranslate">\g&lt;number&gt;</code> uses the corresponding group number.
<code class="docutils literal notranslate">\g&lt;2&gt;</code> is therefore equivalent to <code class="docutils literal notranslate">\2</code>, but isn’t ambiguous in a
replacement string such as <code class="docutils literal notranslate">\g&lt;2&gt;0</code>. (<code class="docutils literal notranslate">\20</code> would be interpreted as a
reference to group 20, not a reference to group 2 followed by the literal
character <code class="docutils literal notranslate">'0'</code>.) The following substitutions are all equivalent, but use all
three variations of the replacement string.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; p = re.compile(&#39;section{ (?P&lt;name&gt; [^}]* ) }&#39;, re.VERBOSE)
&gt;&gt;&gt; p.sub(r&#39;subsection{\1}&#39;,&#39;section{First}&#39;)
&#39;subsection{First}&#39;
&gt;&gt;&gt; p.sub(r&#39;subsection{\g&lt;1&gt;}&#39;,&#39;section{First}&#39;)
&#39;subsection{First}&#39;
&gt;&gt;&gt; p.sub(r&#39;subsection{\g&lt;name&gt;}&#39;,&#39;section{First}&#39;)
&#39;subsection{First}&#39;
</pre></div>
</div>
replacement can also be a function, which gives you even more control. If
replacement is a function, the function is called for every non-overlapping
occurrence of pattern. On each call, the function is passed a
<a class="reference internal" href="../library/re.html#match-objects">match object</a> argument for the match and can use this
information to compute the desired replacement string and return it.
In the following example, the replacement function translates decimals into
hexadecimal:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; def hexrepl(match):
... &quot;Return the hex string for a decimal number&quot;
... value = int(match.group())
... return hex(value)
...
&gt;&gt;&gt; p = re.compile(r&#39;\d+&#39;)
&gt;&gt;&gt; p.sub(hexrepl, &#39;Call 65490 for printing, 49152 for user code.&#39;)
&#39;Call 0xffd2 for printing, 0xc000 for user code.&#39;
</pre></div>
</div>
When using the module-level <a class="reference internal" href="../library/re.html#re.sub" title="re.sub"><code class="xref py py-func docutils literal notranslate">re.sub()</code></a> function, the pattern is passed as
the first argument. The pattern may be provided as an object or as a string; if
you need to specify regular expression flags, you must either use a
pattern object as the first parameter, or use embedded modifiers in the
pattern string, e.g. <code class="docutils literal notranslate">sub(&quot;(?i)b+&quot;, &quot;x&quot;, &quot;bbbb BBBB&quot;)</code> returns <code class="docutils literal notranslate">'x x'</code>.
</div>
</div>
<div class="section" id="common-problems">
<h2>Common Problems<a class="headerlink" href="#common-problems" title="Permalink to this headline">¶</a></h2>
Regular expressions are a powerful tool for some applications, but in some ways
their behaviour isn’t intuitive and at times they don’t behave the way you may
expect them to. This section will point out some of the most common pitfalls.
<div class="section" id="use-string-methods">
<h3>Use String Methods<a class="headerlink" href="#use-string-methods" title="Permalink to this headline">¶</a></h3>
Sometimes using the <a class="reference internal" href="../library/re.html#module-re" title="re: Regular expression operations."><code class="xref py py-mod docutils literal notranslate">re</code></a> module is a mistake. If you’re matching a fixed
string, or a single character class, and you’re not using any <a class="reference internal" href="../library/re.html#module-re" title="re: Regular expression operations."><code class="xref py py-mod docutils literal notranslate">re</code></a> features
such as the <a class="reference internal" href="../library/re.html#re.IGNORECASE" title="re.IGNORECASE"><code class="xref py py-const docutils literal notranslate">IGNORECASE</code></a> flag, then the full power of regular expressions
may not be required. Strings have several methods for performing operations with
fixed strings and they’re usually much faster, because the implementation is a
single small C loop that’s been optimized for the purpose, instead of the large,
more generalized regular expression engine.
One example might be replacing a single fixed string with another one; for
example, you might replace <code class="docutils literal notranslate">word</code> with <code class="docutils literal notranslate">deed</code>. <a class="reference internal" href="../library/re.html#re.sub" title="re.sub"><code class="xref py py-func docutils literal notranslate">re.sub()</code></a> seems like the
function to use for this, but consider the <a class="reference internal" href="../library/stdtypes.html#str.replace" title="str.replace"><code class="xref py py-meth docutils literal notranslate">replace()</code></a> method. Note that
<code class="xref py py-meth docutils literal notranslate">replace()</code> will also replace <code class="docutils literal notranslate">word</code> inside words, turning <code class="docutils literal notranslate">swordfish</code>
into <code class="docutils literal notranslate">sdeedfish</code>, but the naive RE <code class="docutils literal notranslate">word</code> would have done that, too. (To
avoid performing the substitution on parts of words, the pattern would have to
be <code class="docutils literal notranslate">\bword\b</code>, in order to require that <code class="docutils literal notranslate">word</code> have a word boundary on
either side. This takes the job beyond <code class="xref py py-meth docutils literal notranslate">replace()</code>’s abilities.)
Another common task is deleting every occurrence of a single character from a
string or replacing it with another single character. You might do this with
something like <code class="docutils literal notranslate">re.sub('\n', ' ', S)</code>, but <a class="reference internal" href="../library/stdtypes.html#str.translate" title="str.translate"><code class="xref py py-meth docutils literal notranslate">translate()</code></a> is capable of
doing both tasks and will be faster than any regular expression operation can
be.
In short, before turning to the <a class="reference internal" href="../library/re.html#module-re" title="re: Regular expression operations."><code class="xref py py-mod docutils literal notranslate">re</code></a> module, consider whether your problem
can be solved with a faster and simpler string method.
</div>
<div class="section" id="match-versus-search">
<h3>match() versus search()<a class="headerlink" href="#match-versus-search" title="Permalink to this headline">¶</a></h3>
The <a class="reference internal" href="../library/re.html#re.match" title="re.match"><code class="xref py py-func docutils literal notranslate">match()</code></a> function only checks if the RE matches at the beginning of the
string while <a class="reference internal" href="../library/re.html#re.search" title="re.search"><code class="xref py py-func docutils literal notranslate">search()</code></a> will scan forward through the string for a match.
It’s important to keep this distinction in mind. Remember, <code class="xref py py-func docutils literal notranslate">match()</code> will
only report a successful match which will start at 0; if the match wouldn’t
start at zero, <code class="xref py py-func docutils literal notranslate">match()</code> will not report it.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; print(re.match(&#39;super&#39;, &#39;superstition&#39;).span())
(0, 5)
&gt;&gt;&gt; print(re.match(&#39;super&#39;, &#39;insuperable&#39;))
None
</pre></div>
</div>
On the other hand, <a class="reference internal" href="../library/re.html#re.search" title="re.search"><code class="xref py py-func docutils literal notranslate">search()</code></a> will scan forward through the string,
reporting the first match it finds.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; print(re.search(&#39;super&#39;, &#39;superstition&#39;).span())
(0, 5)
&gt;&gt;&gt; print(re.search(&#39;super&#39;, &#39;insuperable&#39;).span())
(2, 7)
</pre></div>
</div>
Sometimes you’ll be tempted to keep using <a class="reference internal" href="../library/re.html#re.match" title="re.match"><code class="xref py py-func docutils literal notranslate">re.match()</code></a>, and just add <code class="docutils literal notranslate">.*</code>
to the front of your RE. Resist this temptation and use <a class="reference internal" href="../library/re.html#re.search" title="re.search"><code class="xref py py-func docutils literal notranslate">re.search()</code></a>
instead. The regular expression compiler does some analysis of REs in order to
speed up the process of looking for a match. One such analysis figures out what
the first character of a match must be; for example, a pattern starting with
<code class="docutils literal notranslate">Crow</code> must match starting with a <code class="docutils literal notranslate">'C'</code>. The analysis lets the engine
quickly scan through the string looking for the starting character, only trying
the full match if a <code class="docutils literal notranslate">'C'</code> is found.
Adding <code class="docutils literal notranslate">.*</code> defeats this optimization, requiring scanning to the end of the
string and then backtracking to find a match for the rest of the RE. Use
<a class="reference internal" href="../library/re.html#re.search" title="re.search"><code class="xref py py-func docutils literal notranslate">re.search()</code></a> instead.
</div>
<div class="section" id="greedy-versus-non-greedy">
<h3>Greedy versus Non-Greedy<a class="headerlink" href="#greedy-versus-non-greedy" title="Permalink to this headline">¶</a></h3>
When repeating a regular expression, as in <code class="docutils literal notranslate">a*</code>, the resulting action is to
consume as much of the pattern as possible. This fact often bites you when
you’re trying to match a pair of balanced delimiters, such as the angle brackets
surrounding an HTML tag. The naive pattern for matching a single HTML tag
doesn’t work because of the greedy nature of <code class="docutils literal notranslate">.*</code>.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; s = &#39;&lt;html&gt;&lt;head&gt;&lt;title&gt;Title&lt;/title&gt;&#39;
&gt;&gt;&gt; len(s)
32
&gt;&gt;&gt; print(re.match(&#39;&lt;.*&gt;&#39;, s).span())
(0, 32)
&gt;&gt;&gt; print(re.match(&#39;&lt;.*&gt;&#39;, s).group())
&lt;html&gt;&lt;head&gt;&lt;title&gt;Title&lt;/title&gt;
</pre></div>
</div>
The RE matches the <code class="docutils literal notranslate">'&lt;'</code> in <code class="docutils literal notranslate">'&lt;html&gt;'</code>, and the <code class="docutils literal notranslate">.*</code> consumes the rest of
the string. There’s still more left in the RE, though, and the <code class="docutils literal notranslate">&gt;</code> can’t
match at the end of the string, so the regular expression engine has to
backtrack character by character until it finds a match for the <code class="docutils literal notranslate">&gt;</code>. The
final match extends from the <code class="docutils literal notranslate">'&lt;'</code> in <code class="docutils literal notranslate">'&lt;html&gt;'</code> to the <code class="docutils literal notranslate">'&gt;'</code> in
<code class="docutils literal notranslate">'&lt;/title&gt;'</code>, which isn’t what you want.
In this case, the solution is to use the non-greedy qualifiers <code class="docutils literal notranslate">*?</code>, <code class="docutils literal notranslate">+?</code>,
<code class="docutils literal notranslate">??</code>, or <code class="docutils literal notranslate">{m,n}?</code>, which match as little text as possible. In the above
example, the <code class="docutils literal notranslate">'&gt;'</code> is tried immediately after the first <code class="docutils literal notranslate">'&lt;'</code> matches, and
when it fails, the engine advances a character at a time, retrying the <code class="docutils literal notranslate">'&gt;'</code>
at every step. This produces just the right result:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; print(re.match(&#39;&lt;.*?&gt;&#39;, s).group())
&lt;html&gt;
</pre></div>
</div>
(Note that parsing HTML or XML with regular expressions is painful.
Quick-and-dirty patterns will handle common cases, but HTML and XML have special
cases that will break the obvious regular expression; by the time you’ve written
a regular expression that handles all of the possible cases, the patterns will
be very complicated. Use an HTML or XML parser module for such tasks.)
</div>
<div class="section" id="using-re-verbose">
<h3>Using re.VERBOSE<a class="headerlink" href="#using-re-verbose" title="Permalink to this headline">¶</a></h3>
By now you’ve probably noticed that regular expressions are a very compact
notation, but they’re not terribly readable. REs of moderate complexity can
become lengthy collections of backslashes, parentheses, and metacharacters,
making them difficult to read and understand.
For such REs, specifying the <a class="reference internal" href="../library/re.html#re.VERBOSE" title="re.VERBOSE"><code class="xref py py-const docutils literal notranslate">re.VERBOSE</code></a> flag when compiling the regular
expression can be helpful, because it allows you to format the regular
expression more clearly.
The <code class="docutils literal notranslate">re.VERBOSE</code> flag has several effects. Whitespace in the regular
expression that isn’t inside a character class is ignored. This means that an
expression such as <code class="docutils literal notranslate">dog | cat</code> is equivalent to the less readable <code class="docutils literal notranslate">dog|cat</code>,
but <code class="docutils literal notranslate">[a b]</code> will still match the characters <code class="docutils literal notranslate">'a'</code>, <code class="docutils literal notranslate">'b'</code>, or a space. In
addition, you can also put comments inside a RE; comments extend from a <code class="docutils literal notranslate">#</code>
character to the next newline. When used with triple-quoted strings, this
enables REs to be formatted more neatly:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>pat = re.compile(r&quot;&quot;&quot;
 \s* # Skip leading whitespace
 (?P&lt;header&gt;[^:]+) # Header name
 \s* : # Whitespace, and a colon
 (?P&lt;value&gt;.*?) # The header&#39;s value -- *? used to
 # lose the following trailing whitespace
 \s*$ # Trailing whitespace to end-of-line
&quot;&quot;&quot;, re.VERBOSE)
</pre></div>
</div>
This is far more readable than:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>pat = re.compile(r&quot;\s*(?P&lt;header&gt;[^:]+)\s*:(?P&lt;value&gt;.*?)\s*$&quot;)
</pre></div>
</div>
</div>
</div>
<div class="section" id="feedback">
<h2>Feedback<a class="headerlink" href="#feedback" title="Permalink to this headline">¶</a></h2>
Regular expressions are a complicated topic. Did this document help you
understand them? Were there parts that were unclear, or Problems you
encountered that weren’t covered here? If so, please send suggestions for
improvements to the author.
The most complete book on regular expressions is almost certainly Jeffrey
Friedl’s Mastering Regular Expressions, published by O’Reilly. Unfortunately,
it exclusively concentrates on Perl and Java’s flavours of regular expressions,
and doesn’t contain any Python material at all, so it won’t be useful as a
reference for programming in Python. (The first edition covered Python’s
now-removed <code class="xref py py-mod docutils literal notranslate">regex</code> module, which won’t help you much.) Consider checking
it out from your library.
</div>
</div>

</div>
 </div>
 </div>
 <div class="sphinxsidebar" role="navigation" aria-label="main navigation">
 <div class="sphinxsidebarwrapper">
 <h3><a href="../contents.html">Table Of Contents</a></h3>
 <ul>
<li><a class="reference internal" href="#">Regular Expression HOWTO</a><ul>
<li><a class="reference internal" href="#introduction">Introduction</a></li>
<li><a class="reference internal" href="#simple-patterns">Simple Patterns</a><ul>
<li><a class="reference internal" href="#matching-characters">Matching Characters</a></li>
<li><a class="reference internal" href="#repeating-things">Repeating Things</a></li>
</ul>
</li>
<li><a class="reference internal" href="#using-regular-expressions">Using Regular Expressions</a><ul>
<li><a class="reference internal" href="#compiling-regular-expressions">Compiling Regular Expressions</a></li>
<li><a class="reference internal" href="#the-backslash-plague">The Backslash Plague</a></li>
<li><a class="reference internal" href="#performing-matches">Performing Matches</a></li>
<li><a class="reference internal" href="#module-level-functions">Module-Level Functions</a></li>
<li><a class="reference internal" href="#compilation-flags">Compilation Flags</a></li>
</ul>
</li>
<li><a class="reference internal" href="#more-pattern-power">More Pattern Power</a><ul>
<li><a class="reference internal" href="#more-metacharacters">More Metacharacters</a></li>
<li><a class="reference internal" href="#grouping">Grouping</a></li>
<li><a class="reference internal" href="#non-capturing-and-named-groups">Non-capturing and Named Groups</a></li>
<li><a class="reference internal" href="#lookahead-assertions">Lookahead Assertions</a></li>
</ul>
</li>
<li><a class="reference internal" href="#modifying-strings">Modifying Strings</a><ul>
<li><a class="reference internal" href="#splitting-strings">Splitting Strings</a></li>
<li><a class="reference internal" href="#search-and-replace">Search and Replace</a></li>
</ul>
</li>
<li><a class="reference internal" href="#common-problems">Common Problems</a><ul>
<li><a class="reference internal" href="#use-string-methods">Use String Methods</a></li>
<li><a class="reference internal" href="#match-versus-search">match() versus search()</a></li>
<li><a class="reference internal" href="#greedy-versus-non-greedy">Greedy versus Non-Greedy</a></li>
<li><a class="reference internal" href="#using-re-verbose">Using re.VERBOSE</a></li>
</ul>
</li>
<li><a class="reference internal" href="#feedback">Feedback</a></li>
</ul>
</li>
</ul>

<h4>Previous topic</h4>
 <a href="logging-cookbook.html"
 title="previous chapter">Logging Cookbook</a>
 <h4>Next topic</h4>
 <a href="sockets.html"
 title="next chapter">Socket Programming HOWTO</a>
 <div role="note" aria-label="source link">
 <h3>This Page</h3>
 <ul class="this-page-menu">
 <li><a href="../bugs.html">Report a Bug</a></li>
 <li>
 <a href="https://github.com/python/cpython/blob/3.6/Doc/howto/regex.rst"
 rel="nofollow">Show Source
 </a>
 </li>
 </ul>
 </div>
 </div>
 </div>
 <div class="clearer"></div>
 </div> 
 <div class="related" role="navigation" aria-label="related navigation">
 <h3>Navigation</h3>
 <ul>
 <li class="right" style="margin-right: 10px">
 <a href="../genindex.html" title="General Index"
 >index</a></li>
 <li class="right" >
 <a href="../py-modindex.html" title="Python Module Index"
 >modules</a> |</li>
 <li class="right" >
 <a href="sockets.html" title="Socket Programming HOWTO"
 >next</a> |</li>
 <li class="right" >
 <a href="logging-cookbook.html" title="Logging Cookbook"
 >previous</a> |</li>
 <li><img src="../_static/py.png" alt=""
 style="vertical-align: middle; margin-top: -1px"/></li>
 <li><a href="https://www.python.org/">Python</a> &#187;</li>
 <li>
 <a href="../index.html">3.6.7 Documentation</a> &#187;
 </li>

<li class="nav-item nav-item-1"><a href="index.html" >Python HOWTOs</a> &#187;</li>
 <li class="right">

</ul>
 </div> 
 <div class="footer">
 &copy; <a href="../copyright.html">Copyright</a> 2001-2023, Python Software Foundation.
 
 The Python Software Foundation is a non-profit corporation.
 <a href="https://www.python.org/psf/donations/">Please donate.</a>
 
 Last updated on Dec 18, 2023.
 <a href="../bugs.html">Found a bug</a>?
 
 Created using <a href="http://sphinx.pocoo.org/">Sphinx</a> 1.7.6.
 </div>

</body>
</html>

MR SKB