1 .. _regex-howto:

     2 

     3 ****************************

     4   Regular Expression HOWTO  

     5 ****************************

     6 

     7 :Author: A.M. Kuchling

     8 :Release: 0.05

     9 

    10 .. TODO:

    11    Document lookbehind assertions

    12    Better way of displaying a RE, a string, and what it matches

    13    Mention optional argument to match.groups()

    14    Unicode (at least a reference)

    15 

    16 

    17 .. topic:: Abstract

    18 

    19    This document is an introductory tutorial to using regular expressions in Python

    20    with the :mod:`re` module.  It provides a gentler introduction than the

    21    corresponding section in the Library Reference.

    22 

    23 

    24 Introduction

    25 ============

    26 

    27 The :mod:`re` module was added in Python 1.5, and provides Perl-style regular

    28 expression patterns.  Earlier versions of Python came with the :mod:`regex`

    29 module, which provided Emacs-style patterns.  The :mod:`regex` module was

    30 removed completely in Python 2.5.

    31 

    32 Regular expressions (called REs, or regexes, or regex patterns) are essentially

    33 a tiny, highly specialized programming language embedded inside Python and made

    34 available through the :mod:`re` module. Using this little language, you specify

    35 the rules for the set of possible strings that you want to match; this set might

    36 contain English sentences, or e-mail addresses, or TeX commands, or anything you

    37 like.  You can then ask questions such as "Does this string match the pattern?",

    38 or "Is there a match for the pattern anywhere in this string?".  You can also

    39 use REs to modify a string or to split it apart in various ways.

    40 

    41 Regular expression patterns are compiled into a series of bytecodes which are

    42 then executed by a matching engine written in C.  For advanced use, it may be

    43 necessary to pay careful attention to how the engine will execute a given RE,

    44 and write the RE in a certain way in order to produce bytecode that runs faster.

    45 Optimization isn't covered in this document, because it requires that you have a

    46 good understanding of the matching engine's internals.

    47 

    48 The regular expression language is relatively small and restricted, so not all

    49 possible string processing tasks can be done using regular expressions.  There

    50 are also tasks that *can* be done with regular expressions, but the expressions

    51 turn out to be very complicated.  In these cases, you may be better off writing

    52 Python code to do the processing; while Python code will be slower than an

    53 elaborate regular expression, it will also probably be more understandable.

    54 

    55 

    56 Simple Patterns

    57 ===============

    58 

    59 We'll start by learning about the simplest possible regular expressions.  Since

    60 regular expressions are used to operate on strings, we'll begin with the most

    61 common task: matching characters.

    62 

    63 For a detailed explanation of the computer science underlying regular

    64 expressions (deterministic and non-deterministic finite automata), you can refer

    65 to almost any textbook on writing compilers.

    66 

    67 

    68 Matching Characters

    69 -------------------

    70 

    71 Most letters and characters will simply match themselves.  For example, the

    72 regular expression ``test`` will match the string ``test`` exactly.  (You can

    73 enable a case-insensitive mode that would let this RE match ``Test`` or ``TEST``

    74 as well; more about this later.)

    75 

    76 There are exceptions to this rule; some characters are special

    77 :dfn:`metacharacters`, and don't match themselves.  Instead, they signal that

    78 some out-of-the-ordinary thing should be matched, or they affect other portions

    79 of the RE by repeating them or changing their meaning.  Much of this document is

    80 devoted to discussing various metacharacters and what they do.

    81 

    82 Here's a complete list of the metacharacters; their meanings will be discussed

    83 in the rest of this HOWTO. ::

    84 

    85    . ^ $ * + ? { [ ] \ | ( )

    86 

    87 The first metacharacters we'll look at are ``[`` and ``]``. They're used for

    88 specifying a character class, which is a set of characters that you wish to

    89 match.  Characters can be listed individually, or a range of characters can be

    90 indicated by giving two characters and separating them by a ``'-'``.  For

    91 example, ``[abc]`` will match any of the characters ``a``, ``b``, or ``c``; this

    92 is the same as ``[a-c]``, which uses a range to express the same set of

    93 characters.  If you wanted to match only lowercase letters, your RE would be

    94 ``[a-z]``.

    95 

    96 Metacharacters are not active inside classes.  For example, ``[akm$]`` will

    97 match any of the characters ``'a'``, ``'k'``, ``'m'``, or ``'$'``; ``'$'`` is

    98 usually a metacharacter, but inside a character class it's stripped of its

    99 special nature.

   100 

   101 You can match the characters not listed within the class by :dfn:`complementing`

   102 the set.  This is indicated by including a ``'^'`` as the first character of the

   103 class; ``'^'`` outside a character class will simply match the ``'^'``

   104 character.  For example, ``[^5]`` will match any character except ``'5'``.

   105 

   106 Perhaps the most important metacharacter is the backslash, ``\``.   As in Python

   107 string literals, the backslash can be followed by various characters to signal

   108 various special sequences.  It's also used to escape all the metacharacters so

   109 you can still match them in patterns; for example, if you need to match a ``[``

   110 or  ``\``, you can precede them with a backslash to remove their special

   111 meaning: ``\[`` or ``\\``.

   112 

   113 Some of the special sequences beginning with ``'\'`` represent predefined sets

   114 of characters that are often useful, such as the set of digits, the set of

   115 letters, or the set of anything that isn't whitespace.  The following predefined

   116 special sequences are available:

   117 

   118 ``\d``

   119    Matches any decimal digit; this is equivalent to the class ``[0-9]``.

   120 

   121 ``\D``

   122    Matches any non-digit character; this is equivalent to the class ``[^0-9]``.

   123 

   124 ``\s``

   125    Matches any whitespace character; this is equivalent to the class ``[

   126    \t\n\r\f\v]``.

   127 

   128 ``\S``

   129    Matches any non-whitespace character; this is equivalent to the class ``[^

   130    \t\n\r\f\v]``.

   131 

   132 ``\w``

   133    Matches any alphanumeric character; this is equivalent to the class

   134    ``[a-zA-Z0-9_]``.

   135 

   136 ``\W``

   137    Matches any non-alphanumeric character; this is equivalent to the class

   138    ``[^a-zA-Z0-9_]``.

   139 

   140 These sequences can be included inside a character class.  For example,

   141 ``[\s,.]`` is a character class that will match any whitespace character, or

   142 ``','`` or ``'.'``.

   143 

   144 The final metacharacter in this section is ``.``.  It matches anything except a

   145 newline character, and there's an alternate mode (``re.DOTALL``) where it will

   146 match even a newline.  ``'.'`` is often used where you want to match "any

   147 character".

   148 

   149 

   150 Repeating Things

   151 ----------------

   152 

   153 Being able to match varying sets of characters is the first thing regular

   154 expressions can do that isn't already possible with the methods available on

   155 strings.  However, if that was the only additional capability of regexes, they

   156 wouldn't be much of an advance. Another capability is that you can specify that

   157 portions of the RE must be repeated a certain number of times.

   158 

   159 The first metacharacter for repeating things that we'll look at is ``*``.  ``*``

   160 doesn't match the literal character ``*``; instead, it specifies that the

   161 previous character can be matched zero or more times, instead of exactly once.

   162 

   163 For example, ``ca*t`` will match ``ct`` (0 ``a`` characters), ``cat`` (1 ``a``),

   164 ``caaat`` (3 ``a`` characters), and so forth.  The RE engine has various

   165 internal limitations stemming from the size of C's ``int`` type that will

   166 prevent it from matching over 2 billion ``a`` characters; you probably don't

   167 have enough memory to construct a string that large, so you shouldn't run into

   168 that limit.

   169 

   170 Repetitions such as ``*`` are :dfn:`greedy`; when repeating a RE, the matching

   171 engine will try to repeat it as many times as possible. If later portions of the

   172 pattern don't match, the matching engine will then back up and try again with

   173 few repetitions.

   174 

   175 A step-by-step example will make this more obvious.  Let's consider the

   176 expression ``a[bcd]*b``.  This matches the letter ``'a'``, zero or more letters

   177 from the class ``[bcd]``, and finally ends with a ``'b'``.  Now imagine matching

   178 this RE against the string ``abcbd``.

   179 

   180 +------+-----------+---------------------------------+

   181 | Step | Matched   | Explanation                     |

   182 +======+===========+=================================+

   183 | 1    | ``a``     | The ``a`` in the RE matches.    |

   184 +------+-----------+---------------------------------+

   185 | 2    | ``abcbd`` | The engine matches ``[bcd]*``,  |

   186 |      |           | going as far as it can, which   |

   187 |      |           | is to the end of the string.    |

   188 +------+-----------+---------------------------------+

   189 | 3    | *Failure* | The engine tries to match       |

   190 |      |           | ``b``, but the current position |

   191 |      |           | is at the end of the string, so |

   192 |      |           | it fails.                       |

   193 +------+-----------+---------------------------------+

   194 | 4    | ``abcb``  | Back up, so that  ``[bcd]*``    |

   195 |      |           | matches one less character.     |

   196 +------+-----------+---------------------------------+

   197 | 5    | *Failure* | Try ``b`` again, but the        |

   198 |      |           | current position is at the last |

   199 |      |           | character, which is a ``'d'``.  |

   200 +------+-----------+---------------------------------+

   201 | 6    | ``abc``   | Back up again, so that          |

   202 |      |           | ``[bcd]*`` is only matching     |

   203 |      |           | ``bc``.                         |

   204 +------+-----------+---------------------------------+

   205 | 6    | ``abcb``  | Try ``b`` again.  This time     |

   206 |      |           | the character at the            |

   207 |      |           | current position is ``'b'``, so |

   208 |      |           | it succeeds.                    |

   209 +------+-----------+---------------------------------+

   210 

   211 The end of the RE has now been reached, and it has matched ``abcb``.  This

   212 demonstrates how the matching engine goes as far as it can at first, and if no

   213 match is found it will then progressively back up and retry the rest of the RE

   214 again and again.  It will back up until it has tried zero matches for

   215 ``[bcd]*``, and if that subsequently fails, the engine will conclude that the

   216 string doesn't match the RE at all.

   217 

   218 Another repeating metacharacter is ``+``, which matches one or more times.  Pay

   219 careful attention to the difference between ``*`` and ``+``; ``*`` matches

   220 *zero* or more times, so whatever's being repeated may not be present at all,

   221 while ``+`` requires at least *one* occurrence.  To use a similar example,

   222 ``ca+t`` will match ``cat`` (1 ``a``), ``caaat`` (3 ``a``'s), but won't match

   223 ``ct``.

   224 

   225 There are two more repeating qualifiers.  The question mark character, ``?``,

   226 matches either once or zero times; you can think of it as marking something as

   227 being optional.  For example, ``home-?brew`` matches either ``homebrew`` or

   228 ``home-brew``.

   229 

   230 The most complicated repeated qualifier is ``{m,n}``, where *m* and *n* are

   231 decimal integers.  This qualifier means there must be at least *m* repetitions,

   232 and at most *n*.  For example, ``a/{1,3}b`` will match ``a/b``, ``a//b``, and

   233 ``a///b``.  It won't match ``ab``, which has no slashes, or ``a////b``, which

   234 has four.

   235 

   236 You can omit either *m* or *n*; in that case, a reasonable value is assumed for

   237 the missing value.  Omitting *m* is interpreted as a lower limit of 0, while

   238 omitting *n* results in an upper bound of infinity --- actually, the upper bound

   239 is the 2-billion limit mentioned earlier, but that might as well be infinity.

   240 

   241 Readers of a reductionist bent may notice that the three other qualifiers can

   242 all be expressed using this notation.  ``{0,}`` is the same as ``*``, ``{1,}``

   243 is equivalent to ``+``, and ``{0,1}`` is the same as ``?``.  It's better to use

   244 ``*``, ``+``, or ``?`` when you can, simply because they're shorter and easier

   245 to read.

   246 

   247 

   248 Using Regular Expressions

   249 =========================

   250 

   251 Now that we've looked at some simple regular expressions, how do we actually use

   252 them in Python?  The :mod:`re` module provides an interface to the regular

   253 expression engine, allowing you to compile REs into objects and then perform

   254 matches with them.

   255 

   256 

   257 Compiling Regular Expressions

   258 -----------------------------

   259 

   260 Regular expressions are compiled into :class:`RegexObject` instances, which have

   261 methods for various operations such as searching for pattern matches or

   262 performing string substitutions. ::

   263 

   264    >>> import re

   265    >>> p = re.compile('ab*')

   266    >>> print p

   267    <re.RegexObject instance at 80b4150>

   268 

   269 :func:`re.compile` also accepts an optional *flags* argument, used to enable

   270 various special features and syntax variations.  We'll go over the available

   271 settings later, but for now a single example will do::

   272 

   273    >>> p = re.compile('ab*', re.IGNORECASE)

   274 

   275 The RE is passed to :func:`re.compile` as a string.  REs are handled as strings

   276 because regular expressions aren't part of the core Python language, and no

   277 special syntax was created for expressing them.  (There are applications that

   278 don't need REs at all, so there's no need to bloat the language specification by

   279 including them.) Instead, the :mod:`re` module is simply a C extension module

   280 included with Python, just like the :mod:`socket` or :mod:`zlib` modules.

   281 

   282 Putting REs in strings keeps the Python language simpler, but has one

   283 disadvantage which is the topic of the next section.

   284 

   285 

   286 The Backslash Plague

   287 --------------------

   288 

   289 As stated earlier, regular expressions use the backslash character (``'\'``) to

   290 indicate special forms or to allow special characters to be used without

   291 invoking their special meaning. This conflicts with Python's usage of the same

   292 character for the same purpose in string literals.

   293 

   294 Let's say you want to write a RE that matches the string ``\section``, which

   295 might be found in a LaTeX file.  To figure out what to write in the program

   296 code, start with the desired string to be matched.  Next, you must escape any

   297 backslashes and other metacharacters by preceding them with a backslash,

   298 resulting in the string ``\\section``.  The resulting string that must be passed

   299 to :func:`re.compile` must be ``\\section``.  However, to express this as a

   300 Python string literal, both backslashes must be escaped *again*.

   301 

   302 +-------------------+------------------------------------------+

   303 | Characters        | Stage                                    |

   304 +===================+==========================================+

   305 | ``\section``      | Text string to be matched                |

   306 +-------------------+------------------------------------------+

   307 | ``\\section``     | Escaped backslash for :func:`re.compile` |

   308 +-------------------+------------------------------------------+

   309 | ``"\\\\section"`` | Escaped backslashes for a string literal |

   310 +-------------------+------------------------------------------+

   311 

   312 In short, to match a literal backslash, one has to write ``'\\\\'`` as the RE

   313 string, because the regular expression must be ``\\``, and each backslash must

   314 be expressed as ``\\`` inside a regular Python string literal.  In REs that

   315 feature backslashes repeatedly, this leads to lots of repeated backslashes and

   316 makes the resulting strings difficult to understand.

   317 

   318 The solution is to use Python's raw string notation for regular expressions;

   319 backslashes are not handled in any special way in a string literal prefixed with

   320 ``'r'``, so ``r"\n"`` is a two-character string containing ``'\'`` and ``'n'``,

   321 while ``"\n"`` is a one-character string containing a newline. Regular

   322 expressions will often be written in Python code using this raw string notation.

   323 

   324 +-------------------+------------------+

   325 | Regular String    | Raw string       |

   326 +===================+==================+

   327 | ``"ab*"``         | ``r"ab*"``       |

   328 +-------------------+------------------+

   329 | ``"\\\\section"`` | ``r"\\section"`` |

   330 +-------------------+------------------+

   331 | ``"\\w+\\s+\\1"`` | ``r"\w+\s+\1"``  |

   332 +-------------------+------------------+

   333 

   334 

   335 Performing Matches

   336 ------------------

   337 

   338 Once you have an object representing a compiled regular expression, what do you

   339 do with it?  :class:`RegexObject` instances have several methods and attributes.

   340 Only the most significant ones will be covered here; consult the :mod:`re` docs

   341 for a complete listing.

   342 

   343 +------------------+-----------------------------------------------+

   344 | Method/Attribute | Purpose                                       |

   345 +==================+===============================================+

   346 | ``match()``      | Determine if the RE matches at the beginning  |

   347 |                  | of the string.                                |

   348 +------------------+-----------------------------------------------+

   349 | ``search()``     | Scan through a string, looking for any        |

   350 |                  | location where this RE matches.               |

   351 +------------------+-----------------------------------------------+

   352 | ``findall()``    | Find all substrings where the RE matches, and |

   353 |                  | returns them as a list.                       |

   354 +------------------+-----------------------------------------------+

   355 | ``finditer()``   | Find all substrings where the RE matches, and |

   356 |                  | returns them as an :term:`iterator`.          |

   357 +------------------+-----------------------------------------------+

   358 

   359 :meth:`match` and :meth:`search` return ``None`` if no match can be found.  If

   360 they're successful, a ``MatchObject`` instance is returned, containing

   361 information about the match: where it starts and ends, the substring it matched,

   362 and more.

   363 

   364 You can learn about this by interactively experimenting with the :mod:`re`

   365 module.  If you have Tkinter available, you may also want to look at

   366 :file:`Tools/scripts/redemo.py`, a demonstration program included with the

   367 Python distribution.  It allows you to enter REs and strings, and displays

   368 whether the RE matches or fails. :file:`redemo.py` can be quite useful when

   369 trying to debug a complicated RE.  Phil Schwartz's `Kodos

   370 <http://kodos.sourceforge.net/>`_ is also an interactive tool for developing and

   371 testing RE patterns.

   372 

   373 This HOWTO uses the standard Python interpreter for its examples. First, run the

   374 Python interpreter, import the :mod:`re` module, and compile a RE::

   375 

   376    Python 2.2.2 (#1, Feb 10 2003, 12:57:01)

   377    >>> import re

   378    >>> p = re.compile('[a-z]+')

   379    >>> p

   380    <_sre.SRE_Pattern object at 80c3c28>

   381 

   382 Now, you can try matching various strings against the RE ``[a-z]+``.  An empty

   383 string shouldn't match at all, since ``+`` means 'one or more repetitions'.

   384 :meth:`match` should return ``None`` in this case, which will cause the

   385 interpreter to print no output.  You can explicitly print the result of

   386 :meth:`match` to make this clear. ::

   387 

   388    >>> p.match("")

   389    >>> print p.match("")

   390    None

   391 

   392 Now, let's try it on a string that it should match, such as ``tempo``.  In this

   393 case, :meth:`match` will return a :class:`MatchObject`, so you should store the

   394 result in a variable for later use. ::

   395 

   396    >>> m = p.match('tempo')

   397    >>> print m

   398    <_sre.SRE_Match object at 80c4f68>

   399 

   400 Now you can query the :class:`MatchObject` for information about the matching

   401 string.   :class:`MatchObject` instances also have several methods and

   402 attributes; the most important ones are:

   403 

   404 +------------------+--------------------------------------------+

   405 | Method/Attribute | Purpose                                    |

   406 +==================+============================================+

   407 | ``group()``      | Return the string matched by the RE        |

   408 +------------------+--------------------------------------------+

   409 | ``start()``      | Return the starting position of the match  |

   410 +------------------+--------------------------------------------+

   411 | ``end()``        | Return the ending position of the match    |

   412 +------------------+--------------------------------------------+

   413 | ``span()``       | Return a tuple containing the (start, end) |

   414 |                  | positions  of the match                    |

   415 +------------------+--------------------------------------------+

   416 

   417 Trying these methods will soon clarify their meaning::

   418 

   419    >>> m.group()

   420    'tempo'

   421    >>> m.start(), m.end()

   422    (0, 5)

   423    >>> m.span()

   424    (0, 5)

   425 

   426 :meth:`group` returns the substring that was matched by the RE.  :meth:`start`

   427 and :meth:`end` return the starting and ending index of the match. :meth:`span`

   428 returns both start and end indexes in a single tuple.  Since the :meth:`match`

   429 method only checks if the RE matches at the start of a string, :meth:`start`

   430 will always be zero.  However, the :meth:`search` method of :class:`RegexObject`

   431 instances scans through the string, so  the match may not start at zero in that

   432 case. ::

   433 

   434    >>> print p.match('::: message')

   435    None

   436    >>> m = p.search('::: message') ; print m

   437    <re.MatchObject instance at 80c9650>

   438    >>> m.group()

   439    'message'

   440    >>> m.span()

   441    (4, 11)

   442 

   443 In actual programs, the most common style is to store the :class:`MatchObject`

   444 in a variable, and then check if it was ``None``.  This usually looks like::

   445 

   446    p = re.compile( ... )

   447    m = p.match( 'string goes here' )

   448    if m:

   449        print 'Match found: ', m.group()

   450    else:

   451        print 'No match'

   452 

   453 Two :class:`RegexObject` methods return all of the matches for a pattern.

   454 :meth:`findall` returns a list of matching strings::

   455 

   456    >>> p = re.compile('\d+')

   457    >>> p.findall('12 drummers drumming, 11 pipers piping, 10 lords a-leaping')

   458    ['12', '11', '10']

   459 

   460 :meth:`findall` has to create the entire list before it can be returned as the

   461 result.  The :meth:`finditer` method returns a sequence of :class:`MatchObject`

   462 instances as an :term:`iterator`. [#]_ ::

   463 

   464    >>> iterator = p.finditer('12 drummers drumming, 11 ... 10 ...')

   465    >>> iterator

   466    <callable-iterator object at 0x401833ac>

   467    >>> for match in iterator:

   468    ...     print match.span()

   469    ...

   470    (0, 2)

   471    (22, 24)

   472    (29, 31)

   473 

   474 

   475 Module-Level Functions

   476 ----------------------

   477 

   478 You don't have to create a :class:`RegexObject` and call its methods; the

   479 :mod:`re` module also provides top-level functions called :func:`match`,

   480 :func:`search`, :func:`findall`, :func:`sub`, and so forth.  These functions

   481 take the same arguments as the corresponding :class:`RegexObject` method, with

   482 the RE string added as the first argument, and still return either ``None`` or a

   483 :class:`MatchObject` instance. ::

   484 

   485    >>> print re.match(r'From\s+', 'Fromage amk')

   486    None

   487    >>> re.match(r'From\s+', 'From amk Thu May 14 19:12:10 1998')

   488    <re.MatchObject instance at 80c5978>

   489 

   490 Under the hood, these functions simply produce a :class:`RegexObject` for you

   491 and call the appropriate method on it.  They also store the compiled object in a

   492 cache, so future calls using the same RE are faster.

   493 

   494 Should you use these module-level functions, or should you get the

   495 :class:`RegexObject` and call its methods yourself?  That choice depends on how

   496 frequently the RE will be used, and on your personal coding style.  If the RE is

   497 being used at only one point in the code, then the module functions are probably

   498 more convenient.  If a program contains a lot of regular expressions, or re-uses

   499 the same ones in several locations, then it might be worthwhile to collect all

   500 the definitions in one place, in a section of code that compiles all the REs

   501 ahead of time.  To take an example from the standard library, here's an extract

   502 from :file:`xmllib.py`::

   503 

   504    ref = re.compile( ... )

   505    entityref = re.compile( ... )

   506    charref = re.compile( ... )

   507    starttagopen = re.compile( ... )

   508 

   509 I generally prefer to work with the compiled object, even for one-time uses, but

   510 few people will be as much of a purist about this as I am.

   511 

   512 

   513 Compilation Flags

   514 -----------------

   515 

   516 Compilation flags let you modify some aspects of how regular expressions work.

   517 Flags are available in the :mod:`re` module under two names, a long name such as

   518 :const:`IGNORECASE` and a short, one-letter form such as :const:`I`.  (If you're

   519 familiar with Perl's pattern modifiers, the one-letter forms use the same

   520 letters; the short form of :const:`re.VERBOSE` is :const:`re.X`, for example.)

   521 Multiple flags can be specified by bitwise OR-ing them; ``re.I | re.M`` sets

   522 both the :const:`I` and :const:`M` flags, for example.

   523 

   524 Here's a table of the available flags, followed by a more detailed explanation

   525 of each one.

   526 

   527 +---------------------------------+--------------------------------------------+

   528 | Flag                            | Meaning                                    |

   529 +=================================+============================================+

   530 | :const:`DOTALL`, :const:`S`     | Make ``.`` match any character, including  |

   531 |                                 | newlines                                   |

   532 +---------------------------------+--------------------------------------------+

   533 | :const:`IGNORECASE`, :const:`I` | Do case-insensitive matches                |

   534 +---------------------------------+--------------------------------------------+

   535 | :const:`LOCALE`, :const:`L`     | Do a locale-aware match                    |

   536 +---------------------------------+--------------------------------------------+

   537 | :const:`MULTILINE`, :const:`M`  | Multi-line matching, affecting ``^`` and   |

   538 |                                 | ``$``                                      |

   539 +---------------------------------+--------------------------------------------+

   540 | :const:`VERBOSE`, :const:`X`    | Enable verbose REs, which can be organized |

   541 |                                 | more cleanly and understandably.           |

   542 +---------------------------------+--------------------------------------------+

   543 

   544 

   545 .. data:: I

   546           IGNORECASE

   547    :noindex:

   548 

   549    Perform case-insensitive matching; character class and literal strings will

   550    match letters by ignoring case.  For example, ``[A-Z]`` will match lowercase

   551    letters, too, and ``Spam`` will match ``Spam``, ``spam``, or ``spAM``. This

   552    lowercasing doesn't take the current locale into account; it will if you also

   553    set the :const:`LOCALE` flag.

   554 

   555 

   556 .. data:: L

   557           LOCALE

   558    :noindex:

   559 

   560    Make ``\w``, ``\W``, ``\b``, and ``\B``, dependent on the current locale.

   561 

   562    Locales are a feature of the C library intended to help in writing programs that

   563    take account of language differences.  For example, if you're processing French

   564    text, you'd want to be able to write ``\w+`` to match words, but ``\w`` only

   565    matches the character class ``[A-Za-z]``; it won't match ``'é'`` or ``'ç'``.  If

   566    your system is configured properly and a French locale is selected, certain C

   567    functions will tell the program that ``'é'`` should also be considered a letter.

   568    Setting the :const:`LOCALE` flag when compiling a regular expression will cause

   569    the resulting compiled object to use these C functions for ``\w``; this is

   570    slower, but also enables ``\w+`` to match French words as you'd expect.

   571 

   572 

   573 .. data:: M

   574           MULTILINE

   575    :noindex:

   576 

   577    (``^`` and ``$`` haven't been explained yet;  they'll be introduced in section

   578    :ref:`more-metacharacters`.)

   579 

   580    Usually ``^`` matches only at the beginning of the string, and ``$`` matches

   581    only at the end of the string and immediately before the newline (if any) at the

   582    end of the string. When this flag is specified, ``^`` matches at the beginning

   583    of the string and at the beginning of each line within the string, immediately

   584    following each newline.  Similarly, the ``$`` metacharacter matches either at

   585    the end of the string and at the end of each line (immediately preceding each

   586    newline).

   587 

   588 

   589 .. data:: S

   590           DOTALL

   591    :noindex:

   592 

   593    Makes the ``'.'`` special character match any character at all, including a

   594    newline; without this flag, ``'.'`` will match anything *except* a newline.

   595 

   596 

   597 .. data:: X

   598           VERBOSE

   599    :noindex:

   600 

   601    This flag allows you to write regular expressions that are more readable by

   602    granting you more flexibility in how you can format them.  When this flag has

   603    been specified, whitespace within the RE string is ignored, except when the

   604    whitespace is in a character class or preceded by an unescaped backslash; this

   605    lets you organize and indent the RE more clearly.  This flag also lets you put

   606    comments within a RE that will be ignored by the engine; comments are marked by

   607    a ``'#'`` that's neither in a character class or preceded by an unescaped

   608    backslash.

   609 

   610    For example, here's a RE that uses :const:`re.VERBOSE`; see how much easier it

   611    is to read? ::

   612 

   613       charref = re.compile(r"""

   614        &[#]		     # Start of a numeric entity reference

   615        (

   616            0[0-7]+         # Octal form

   617          | [0-9]+          # Decimal form

   618          | x[0-9a-fA-F]+   # Hexadecimal form

   619        )

   620        ;                   # Trailing semicolon

   621       """, re.VERBOSE)

   622 

   623    Without the verbose setting, the RE would look like this::

   624 

   625       charref = re.compile("&#(0[0-7]+"

   626                            "|[0-9]+"

   627                            "|x[0-9a-fA-F]+);")

   628 

   629    In the above example, Python's automatic concatenation of string literals has

   630    been used to break up the RE into smaller pieces, but it's still more difficult

   631    to understand than the version using :const:`re.VERBOSE`.

   632 

   633 

   634 More Pattern Power

   635 ==================

   636 

   637 So far we've only covered a part of the features of regular expressions.  In

   638 this section, we'll cover some new metacharacters, and how to use groups to

   639 retrieve portions of the text that was matched.

   640 

   641 

   642 .. _more-metacharacters:

   643 

   644 More Metacharacters

   645 -------------------

   646 

   647 There are some metacharacters that we haven't covered yet.  Most of them will be

   648 covered in this section.

   649 

   650 Some of the remaining metacharacters to be discussed are :dfn:`zero-width

   651 assertions`.  They don't cause the engine to advance through the string;

   652 instead, they consume no characters at all, and simply succeed or fail.  For

   653 example, ``\b`` is an assertion that the current position is located at a word

   654 boundary; the position isn't changed by the ``\b`` at all.  This means that

   655 zero-width assertions should never be repeated, because if they match once at a

   656 given location, they can obviously be matched an infinite number of times.

   657 

   658 ``|``

   659    Alternation, or the "or" operator.   If A and B are regular expressions,

   660    ``A|B`` will match any string that matches either ``A`` or ``B``. ``|`` has very

   661    low precedence in order to make it work reasonably when you're alternating

   662    multi-character strings. ``Crow|Servo`` will match either ``Crow`` or ``Servo``,

   663    not ``Cro``, a ``'w'`` or an ``'S'``, and ``ervo``.

   664 

   665    To match a literal ``'|'``, use ``\|``, or enclose it inside a character class,

   666    as in ``[|]``.

   667 

   668 ``^``

   669    Matches at the beginning of lines.  Unless the :const:`MULTILINE` flag has been

   670    set, this will only match at the beginning of the string.  In :const:`MULTILINE`

   671    mode, this also matches immediately after each newline within the string.

   672 

   673    For example, if you wish to match the word ``From`` only at the beginning of a

   674    line, the RE to use is ``^From``. ::

   675 

   676       >>> print re.search('^From', 'From Here to Eternity')

   677       <re.MatchObject instance at 80c1520>

   678       >>> print re.search('^From', 'Reciting From Memory')

   679       None

   680 

   681    .. To match a literal \character{\^}, use \regexp{\e\^} or enclose it

   682    .. inside a character class, as in \regexp{[{\e}\^]}.

   683 

   684 ``$``

   685    Matches at the end of a line, which is defined as either the end of the string,

   686    or any location followed by a newline character.     ::

   687 

   688       >>> print re.search('}$', '{block}')

   689       <re.MatchObject instance at 80adfa8>

   690       >>> print re.search('}$', '{block} ')

   691       None

   692       >>> print re.search('}$', '{block}\n')

   693       <re.MatchObject instance at 80adfa8>

   694 

   695    To match a literal ``'$'``, use ``\$`` or enclose it inside a character class,

   696    as in  ``[$]``.

   697 

   698 ``\A``

   699    Matches only at the start of the string.  When not in :const:`MULTILINE` mode,

   700    ``\A`` and ``^`` are effectively the same.  In :const:`MULTILINE` mode, they're

   701    different: ``\A`` still matches only at the beginning of the string, but ``^``

   702    may match at any location inside the string that follows a newline character.

   703 

   704 ``\Z``

   705    Matches only at the end of the string.

   706 

   707 ``\b``

   708    Word boundary.  This is a zero-width assertion that matches only at the

   709    beginning or end of a word.  A word is defined as a sequence of alphanumeric

   710    characters, so the end of a word is indicated by whitespace or a

   711    non-alphanumeric character.

   712 

   713    The following example matches ``class`` only when it's a complete word; it won't

   714    match when it's contained inside another word. ::

   715 

   716       >>> p = re.compile(r'\bclass\b')

   717       >>> print p.search('no class at all')

   718       <re.MatchObject instance at 80c8f28>

   719       >>> print p.search('the declassified algorithm')

   720       None

   721       >>> print p.search('one subclass is')

   722       None

   723 

   724    There are two subtleties you should remember when using this special sequence.

   725    First, this is the worst collision between Python's string literals and regular

   726    expression sequences.  In Python's string literals, ``\b`` is the backspace

   727    character, ASCII value 8.  If you're not using raw strings, then Python will

   728    convert the ``\b`` to a backspace, and your RE won't match as you expect it to.

   729    The following example looks the same as our previous RE, but omits the ``'r'``

   730    in front of the RE string. ::

   731 

   732       >>> p = re.compile('\bclass\b')

   733       >>> print p.search('no class at all')

   734       None

   735       >>> print p.search('\b' + 'class' + '\b')  

   736       <re.MatchObject instance at 80c3ee0>

   737 

   738    Second, inside a character class, where there's no use for this assertion,

   739    ``\b`` represents the backspace character, for compatibility with Python's

   740    string literals.

   741 

   742 ``\B``

   743    Another zero-width assertion, this is the opposite of ``\b``, only matching when

   744    the current position is not at a word boundary.

   745 

   746 

   747 Grouping

   748 --------

   749 

   750 Frequently you need to obtain more information than just whether the RE matched

   751 or not.  Regular expressions are often used to dissect strings by writing a RE

   752 divided into several subgroups which match different components of interest.

   753 For example, an RFC-822 header line is divided into a header name and a value,

   754 separated by a ``':'``, like this::

   755 

   756    From: author@example.com

   757    User-Agent: Thunderbird 1.5.0.9 (X11/20061227)

   758    MIME-Version: 1.0

   759    To: editor@example.com

   760 

   761 This can be handled by writing a regular expression which matches an entire

   762 header line, and has one group which matches the header name, and another group

   763 which matches the header's value.

   764 

   765 Groups are marked by the ``'('``, ``')'`` metacharacters. ``'('`` and ``')'``

   766 have much the same meaning as they do in mathematical expressions; they group

   767 together the expressions contained inside them, and you can repeat the contents

   768 of a group with a repeating qualifier, such as ``*``, ``+``, ``?``, or

   769 ``{m,n}``.  For example, ``(ab)*`` will match zero or more repetitions of

   770 ``ab``. ::

   771 

   772    >>> p = re.compile('(ab)*')

   773    >>> print p.match('ababababab').span()

   774    (0, 10)

   775 

   776 Groups indicated with ``'('``, ``')'`` also capture the starting and ending

   777 index of the text that they match; this can be retrieved by passing an argument

   778 to :meth:`group`, :meth:`start`, :meth:`end`, and :meth:`span`.  Groups are

   779 numbered starting with 0.  Group 0 is always present; it's the whole RE, so

   780 :class:`MatchObject` methods all have group 0 as their default argument.  Later

   781 we'll see how to express groups that don't capture the span of text that they

   782 match. ::

   783 

   784    >>> p = re.compile('(a)b')

   785    >>> m = p.match('ab')

   786    >>> m.group()

   787    'ab'

   788    >>> m.group(0)

   789    'ab'

   790 

   791 Subgroups are numbered from left to right, from 1 upward.  Groups can be nested;

   792 to determine the number, just count the opening parenthesis characters, going

   793 from left to right. ::

   794 

   795    >>> p = re.compile('(a(b)c)d')

   796    >>> m = p.match('abcd')

   797    >>> m.group(0)

   798    'abcd'

   799    >>> m.group(1)

   800    'abc'

   801    >>> m.group(2)

   802    'b'

   803 

   804 :meth:`group` can be passed multiple group numbers at a time, in which case it

   805 will return a tuple containing the corresponding values for those groups. ::

   806 

   807    >>> m.group(2,1,2)

   808    ('b', 'abc', 'b')

   809 

   810 The :meth:`groups` method returns a tuple containing the strings for all the

   811 subgroups, from 1 up to however many there are. ::

   812 

   813    >>> m.groups()

   814    ('abc', 'b')

   815 

   816 Backreferences in a pattern allow you to specify that the contents of an earlier

   817 capturing group must also be found at the current location in the string.  For

   818 example, ``\1`` will succeed if the exact contents of group 1 can be found at

   819 the current position, and fails otherwise.  Remember that Python's string

   820 literals also use a backslash followed by numbers to allow including arbitrary

   821 characters in a string, so be sure to use a raw string when incorporating

   822 backreferences in a RE.

   823 

   824 For example, the following RE detects doubled words in a string. ::

   825 

   826    >>> p = re.compile(r'(\b\w+)\s+\1')

   827    >>> p.search('Paris in the the spring').group()

   828    'the the'

   829 

   830 Backreferences like this aren't often useful for just searching through a string

   831 --- there are few text formats which repeat data in this way --- but you'll soon

   832 find out that they're *very* useful when performing string substitutions.

   833 

   834 

   835 Non-capturing and Named Groups

   836 ------------------------------

   837 

   838 Elaborate REs may use many groups, both to capture substrings of interest, and

   839 to group and structure the RE itself.  In complex REs, it becomes difficult to

   840 keep track of the group numbers.  There are two features which help with this

   841 problem.  Both of them use a common syntax for regular expression extensions, so

   842 we'll look at that first.

   843 

   844 Perl 5 added several additional features to standard regular expressions, and

   845 the Python :mod:`re` module supports most of them.   It would have been

   846 difficult to choose new single-keystroke metacharacters or new special sequences

   847 beginning with ``\`` to represent the new features without making Perl's regular

   848 expressions confusingly different from standard REs.  If you chose ``&`` as a

   849 new metacharacter, for example, old expressions would be assuming that ``&`` was

   850 a regular character and wouldn't have escaped it by writing ``\&`` or ``[&]``.

   851 

   852 The solution chosen by the Perl developers was to use ``(?...)`` as the

   853 extension syntax.  ``?`` immediately after a parenthesis was a syntax error

   854 because the ``?`` would have nothing to repeat, so this didn't introduce any

   855 compatibility problems.  The characters immediately after the ``?``  indicate

   856 what extension is being used, so ``(?=foo)`` is one thing (a positive lookahead

   857 assertion) and ``(?:foo)`` is something else (a non-capturing group containing

   858 the subexpression ``foo``).

   859 

   860 Python adds an extension syntax to Perl's extension syntax.  If the first

   861 character after the question mark is a ``P``, you know that it's an extension

   862 that's specific to Python.  Currently there are two such extensions:

   863 ``(?P<name>...)`` defines a named group, and ``(?P=name)`` is a backreference to

   864 a named group.  If future versions of Perl 5 add similar features using a

   865 different syntax, the :mod:`re` module will be changed to support the new

   866 syntax, while preserving the Python-specific syntax for compatibility's sake.

   867 

   868 Now that we've looked at the general extension syntax, we can return to the

   869 features that simplify working with groups in complex REs. Since groups are

   870 numbered from left to right and a complex expression may use many groups, it can

   871 become difficult to keep track of the correct numbering.  Modifying such a

   872 complex RE is annoying, too: insert a new group near the beginning and you

   873 change the numbers of everything that follows it.

   874 

   875 Sometimes you'll want to use a group to collect a part of a regular expression,

   876 but aren't interested in retrieving the group's contents. You can make this fact

   877 explicit by using a non-capturing group: ``(?:...)``, where you can replace the

   878 ``...`` with any other regular expression. ::

   879 

   880    >>> m = re.match("([abc])+", "abc")

   881    >>> m.groups()

   882    ('c',)

   883    >>> m = re.match("(?:[abc])+", "abc")

   884    >>> m.groups()

   885    ()

   886 

   887 Except for the fact that you can't retrieve the contents of what the group

   888 matched, a non-capturing group behaves exactly the same as a capturing group;

   889 you can put anything inside it, repeat it with a repetition metacharacter such

   890 as ``*``, and nest it within other groups (capturing or non-capturing).

   891 ``(?:...)`` is particularly useful when modifying an existing pattern, since you

   892 can add new groups without changing how all the other groups are numbered.  It

   893 should be mentioned that there's no performance difference in searching between

   894 capturing and non-capturing groups; neither form is any faster than the other.

   895 

   896 A more significant feature is named groups: instead of referring to them by

   897 numbers, groups can be referenced by a name.

   898 

   899 The syntax for a named group is one of the Python-specific extensions:

   900 ``(?P<name>...)``.  *name* is, obviously, the name of the group.  Named groups

   901 also behave exactly like capturing groups, and additionally associate a name

   902 with a group.  The :class:`MatchObject` methods that deal with capturing groups

   903 all accept either integers that refer to the group by number or strings that

   904 contain the desired group's name.  Named groups are still given numbers, so you

   905 can retrieve information about a group in two ways::

   906 

   907    >>> p = re.compile(r'(?P<word>\b\w+\b)')

   908    >>> m = p.search( '(((( Lots of punctuation )))' )

   909    >>> m.group('word')

   910    'Lots'

   911    >>> m.group(1)

   912    'Lots'

   913 

   914 Named groups are handy because they let you use easily-remembered names, instead

   915 of having to remember numbers.  Here's an example RE from the :mod:`imaplib`

   916 module::

   917 

   918    InternalDate = re.compile(r'INTERNALDATE "'

   919            r'(?P<day>[ 123][0-9])-(?P<mon>[A-Z][a-z][a-z])-'

   920    	r'(?P<year>[0-9][0-9][0-9][0-9])'

   921            r' (?P<hour>[0-9][0-9]):(?P<min>[0-9][0-9]):(?P<sec>[0-9][0-9])'

   922            r' (?P<zonen>[-+])(?P<zoneh>[0-9][0-9])(?P<zonem>[0-9][0-9])'

   923            r'"')

   924 

   925 It's obviously much easier to retrieve ``m.group('zonem')``, instead of having

   926 to remember to retrieve group 9.

   927 

   928 The syntax for backreferences in an expression such as ``(...)\1`` refers to the

   929 number of the group.  There's naturally a variant that uses the group name

   930 instead of the number. This is another Python extension: ``(?P=name)`` indicates

   931 that the contents of the group called *name* should again be matched at the

   932 current point.  The regular expression for finding doubled words,

   933 ``(\b\w+)\s+\1`` can also be written as ``(?P<word>\b\w+)\s+(?P=word)``::

   934 

   935    >>> p = re.compile(r'(?P<word>\b\w+)\s+(?P=word)')

   936    >>> p.search('Paris in the the spring').group()

   937    'the the'

   938 

   939 

   940 Lookahead Assertions

   941 --------------------

   942 

   943 Another zero-width assertion is the lookahead assertion.  Lookahead assertions

   944 are available in both positive and negative form, and  look like this:

   945 

   946 ``(?=...)``

   947    Positive lookahead assertion.  This succeeds if the contained regular

   948    expression, represented here by ``...``, successfully matches at the current

   949    location, and fails otherwise. But, once the contained expression has been

   950    tried, the matching engine doesn't advance at all; the rest of the pattern is

   951    tried right where the assertion started.

   952 

   953 ``(?!...)``

   954    Negative lookahead assertion.  This is the opposite of the positive assertion;

   955    it succeeds if the contained expression *doesn't* match at the current position

   956    in the string.

   957 

   958 To make this concrete, let's look at a case where a lookahead is useful.

   959 Consider a simple pattern to match a filename and split it apart into a base

   960 name and an extension, separated by a ``.``.  For example, in ``news.rc``,

   961 ``news`` is the base name, and ``rc`` is the filename's extension.

   962 

   963 The pattern to match this is quite simple:

   964 

   965 ``.*[.].*$``

   966 

   967 Notice that the ``.`` needs to be treated specially because it's a

   968 metacharacter; I've put it inside a character class.  Also notice the trailing

   969 ``$``; this is added to ensure that all the rest of the string must be included

   970 in the extension.  This regular expression matches ``foo.bar`` and

   971 ``autoexec.bat`` and ``sendmail.cf`` and ``printers.conf``.

   972 

   973 Now, consider complicating the problem a bit; what if you want to match

   974 filenames where the extension is not ``bat``? Some incorrect attempts:

   975 

   976 ``.*[.][^b].*$``  The first attempt above tries to exclude ``bat`` by requiring

   977 that the first character of the extension is not a ``b``.  This is wrong,

   978 because the pattern also doesn't match ``foo.bar``.

   979 

   980 ``.*[.]([^b]..|.[^a].|..[^t])$``

   981 

   982 The expression gets messier when you try to patch up the first solution by

   983 requiring one of the following cases to match: the first character of the

   984 extension isn't ``b``; the second character isn't ``a``; or the third character

   985 isn't ``t``.  This accepts ``foo.bar`` and rejects ``autoexec.bat``, but it

   986 requires a three-letter extension and won't accept a filename with a two-letter

   987 extension such as ``sendmail.cf``.  We'll complicate the pattern again in an

   988 effort to fix it.

   989 

   990 ``.*[.]([^b].?.?|.[^a]?.?|..?[^t]?)$``

   991 

   992 In the third attempt, the second and third letters are all made optional in

   993 order to allow matching extensions shorter than three characters, such as

   994 ``sendmail.cf``.

   995 

   996 The pattern's getting really complicated now, which makes it hard to read and

   997 understand.  Worse, if the problem changes and you want to exclude both ``bat``

   998 and ``exe`` as extensions, the pattern would get even more complicated and

   999 confusing.

  1000 

  1001 A negative lookahead cuts through all this confusion:

  1002 

  1003 ``.*[.](?!bat$).*$``  The negative lookahead means: if the expression ``bat``

  1004 doesn't match at this point, try the rest of the pattern; if ``bat$`` does

  1005 match, the whole pattern will fail.  The trailing ``$`` is required to ensure

  1006 that something like ``sample.batch``, where the extension only starts with

  1007 ``bat``, will be allowed.

  1008 

  1009 Excluding another filename extension is now easy; simply add it as an

  1010 alternative inside the assertion.  The following pattern excludes filenames that

  1011 end in either ``bat`` or ``exe``:

  1012 

  1013 ``.*[.](?!bat$|exe$).*$``

  1014 

  1015 

  1016 Modifying Strings

  1017 =================

  1018 

  1019 Up to this point, we've simply performed searches against a static string.

  1020 Regular expressions are also commonly used to modify strings in various ways,

  1021 using the following :class:`RegexObject` methods:

  1022 

  1023 +------------------+-----------------------------------------------+

  1024 | Method/Attribute | Purpose                                       |

  1025 +==================+===============================================+

  1026 | ``split()``      | Split the string into a list, splitting it    |

  1027 |                  | wherever the RE matches                       |

  1028 +------------------+-----------------------------------------------+

  1029 | ``sub()``        | Find all substrings where the RE matches, and |

  1030 |                  | replace them with a different string          |

  1031 +------------------+-----------------------------------------------+

  1032 | ``subn()``       | Does the same thing as :meth:`sub`,  but      |

  1033 |                  | returns the new string and the number of      |

  1034 |                  | replacements                                  |

  1035 +------------------+-----------------------------------------------+

  1036 

  1037 

  1038 Splitting Strings

  1039 -----------------

  1040 

  1041 The :meth:`split` method of a :class:`RegexObject` splits a string apart

  1042 wherever the RE matches, returning a list of the pieces. It's similar to the

  1043 :meth:`split` method of strings but provides much more generality in the

  1044 delimiters that you can split by; :meth:`split` only supports splitting by

  1045 whitespace or by a fixed string.  As you'd expect, there's a module-level

  1046 :func:`re.split` function, too.

  1047 

  1048 

  1049 .. method:: .split(string [, maxsplit=0])

  1050    :noindex:

  1051 

  1052    Split *string* by the matches of the regular expression.  If capturing

  1053    parentheses are used in the RE, then their contents will also be returned as

  1054    part of the resulting list.  If *maxsplit* is nonzero, at most *maxsplit* splits

  1055    are performed.

  1056 

  1057 You can limit the number of splits made, by passing a value for *maxsplit*.

  1058 When *maxsplit* is nonzero, at most *maxsplit* splits will be made, and the

  1059 remainder of the string is returned as the final element of the list.  In the

  1060 following example, the delimiter is any sequence of non-alphanumeric characters.

  1061 ::

  1062 

  1063    >>> p = re.compile(r'\W+')

  1064    >>> p.split('This is a test, short and sweet, of split().')

  1065    ['This', 'is', 'a', 'test', 'short', 'and', 'sweet', 'of', 'split', '']

  1066    >>> p.split('This is a test, short and sweet, of split().', 3)

  1067    ['This', 'is', 'a', 'test, short and sweet, of split().']

  1068 

  1069 Sometimes you're not only interested in what the text between delimiters is, but

  1070 also need to know what the delimiter was.  If capturing parentheses are used in

  1071 the RE, then their values are also returned as part of the list.  Compare the

  1072 following calls::

  1073 

  1074    >>> p = re.compile(r'\W+')

  1075    >>> p2 = re.compile(r'(\W+)')

  1076    >>> p.split('This... is a test.')

  1077    ['This', 'is', 'a', 'test', '']

  1078    >>> p2.split('This... is a test.')

  1079    ['This', '... ', 'is', ' ', 'a', ' ', 'test', '.', '']

  1080 

  1081 The module-level function :func:`re.split` adds the RE to be used as the first

  1082 argument, but is otherwise the same.   ::

  1083 

  1084    >>> re.split('[\W]+', 'Words, words, words.')

  1085    ['Words', 'words', 'words', '']

  1086    >>> re.split('([\W]+)', 'Words, words, words.')

  1087    ['Words', ', ', 'words', ', ', 'words', '.', '']

  1088    >>> re.split('[\W]+', 'Words, words, words.', 1)

  1089    ['Words', 'words, words.']

  1090 

  1091 

  1092 Search and Replace

  1093 ------------------

  1094 

  1095 Another common task is to find all the matches for a pattern, and replace them

  1096 with a different string.  The :meth:`sub` method takes a replacement value,

  1097 which can be either a string or a function, and the string to be processed.

  1098 

  1099 

  1100 .. method:: .sub(replacement, string[, count=0])

  1101    :noindex:

  1102 

  1103    Returns the string obtained by replacing the leftmost non-overlapping

  1104    occurrences of the RE in *string* by the replacement *replacement*.  If the

  1105    pattern isn't found, *string* is returned unchanged.

  1106 

  1107    The optional argument *count* is the maximum number of pattern occurrences to be

  1108    replaced; *count* must be a non-negative integer.  The default value of 0 means

  1109    to replace all occurrences.

  1110 

  1111 Here's a simple example of using the :meth:`sub` method.  It replaces colour

  1112 names with the word ``colour``::

  1113 

  1114    >>> p = re.compile( '(blue|white|red)')

  1115    >>> p.sub( 'colour', 'blue socks and red shoes')

  1116    'colour socks and colour shoes'

  1117    >>> p.sub( 'colour', 'blue socks and red shoes', count=1)

  1118    'colour socks and red shoes'

  1119 

  1120 The :meth:`subn` method does the same work, but returns a 2-tuple containing the

  1121 new string value and the number of replacements  that were performed::

  1122 

  1123    >>> p = re.compile( '(blue|white|red)')

  1124    >>> p.subn( 'colour', 'blue socks and red shoes')

  1125    ('colour socks and colour shoes', 2)

  1126    >>> p.subn( 'colour', 'no colours at all')

  1127    ('no colours at all', 0)

  1128 

  1129 Empty matches are replaced only when they're not adjacent to a previous match.

  1130 ::

  1131 

  1132    >>> p = re.compile('x*')

  1133    >>> p.sub('-', 'abxd')

  1134    '-a-b-d-'

  1135