--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/symbian-qemu-0.9.1-12/python-2.6.1/Doc/howto/regex.rst Fri Jul 31 15:01:17 2009 +0100
@@ -0,0 +1,1360 @@
+.. _regex-howto:
+
+****************************
+ Regular Expression HOWTO
+****************************
+
+:Author: A.M. Kuchling
+:Release: 0.05
+
+.. TODO:
+ Document lookbehind assertions
+ Better way of displaying a RE, a string, and what it matches
+ Mention optional argument to match.groups()
+ Unicode (at least a reference)
+
+
+.. topic:: Abstract
+
+ This document is an introductory tutorial to using regular expressions in Python
+ with the :mod:`re` module. It provides a gentler introduction than the
+ corresponding section in the Library Reference.
+
+
+Introduction
+============
+
+The :mod:`re` module was added in Python 1.5, and provides Perl-style regular
+expression patterns. Earlier versions of Python came with the :mod:`regex`
+module, which provided Emacs-style patterns. The :mod:`regex` module was
+removed completely in Python 2.5.
+
+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 :mod:`re` 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.
+
+
+Simple Patterns
+===============
+
+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.
+
+
+Matching Characters
+-------------------
+
+Most letters and characters will simply match themselves. For example, the
+regular expression ``test`` will match the string ``test`` exactly. (You can
+enable a case-insensitive mode that would let this RE match ``Test`` or ``TEST``
+as well; more about this later.)
+
+There are exceptions to this rule; some characters are special
+:dfn:`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. ::
+
+ . ^ $ * + ? { [ ] \ | ( )
+
+The first metacharacters we'll look at are ``[`` and ``]``. 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 ``'-'``. For
+example, ``[abc]`` will match any of the characters ``a``, ``b``, or ``c``; this
+is the same as ``[a-c]``, which uses a range to express the same set of
+characters. If you wanted to match only lowercase letters, your RE would be
+``[a-z]``.
+
+Metacharacters are not active inside classes. For example, ``[akm$]`` will
+match any of the characters ``'a'``, ``'k'``, ``'m'``, or ``'$'``; ``'$'`` 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 :dfn:`complementing`
+the set. This is indicated by including a ``'^'`` as the first character of the
+class; ``'^'`` outside a character class will simply match the ``'^'``
+character. For example, ``[^5]`` will match any character except ``'5'``.
+
+Perhaps the most important metacharacter is the backslash, ``\``. 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 ``[``
+or ``\``, you can precede them with a backslash to remove their special
+meaning: ``\[`` or ``\\``.
+
+Some of the special sequences beginning with ``'\'`` 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. The following predefined
+special sequences are available:
+
+``\d``
+ Matches any decimal digit; this is equivalent to the class ``[0-9]``.
+
+``\D``
+ Matches any non-digit character; this is equivalent to the class ``[^0-9]``.
+
+``\s``
+ Matches any whitespace character; this is equivalent to the class ``[
+ \t\n\r\f\v]``.
+
+``\S``
+ Matches any non-whitespace character; this is equivalent to the class ``[^
+ \t\n\r\f\v]``.
+
+``\w``
+ Matches any alphanumeric character; this is equivalent to the class
+ ``[a-zA-Z0-9_]``.
+
+``\W``
+ Matches any non-alphanumeric character; this is equivalent to the class
+ ``[^a-zA-Z0-9_]``.
+
+These sequences can be included inside a character class. For example,
+``[\s,.]`` is a character class that will match any whitespace character, or
+``','`` or ``'.'``.
+
+The final metacharacter in this section is ``.``. It matches anything except a
+newline character, and there's an alternate mode (``re.DOTALL``) where it will
+match even a newline. ``'.'`` is often used where you want to match "any
+character".
+
+
+Repeating Things
+----------------
+
+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 ``*``. ``*``
+doesn't match the literal character ``*``; instead, it specifies that the
+previous character can be matched zero or more times, instead of exactly once.
+
+For example, ``ca*t`` will match ``ct`` (0 ``a`` characters), ``cat`` (1 ``a``),
+``caaat`` (3 ``a`` characters), and so forth. The RE engine has various
+internal limitations stemming from the size of C's ``int`` type that will
+prevent it from matching over 2 billion ``a`` characters; you probably don't
+have enough memory to construct a string that large, so you shouldn't run into
+that limit.
+
+Repetitions such as ``*`` are :dfn:`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
+few repetitions.
+
+A step-by-step example will make this more obvious. Let's consider the
+expression ``a[bcd]*b``. This matches the letter ``'a'``, zero or more letters
+from the class ``[bcd]``, and finally ends with a ``'b'``. Now imagine matching
+this RE against the string ``abcbd``.
+
++------+-----------+---------------------------------+
+| Step | Matched | Explanation |
++======+===========+=================================+
+| 1 | ``a`` | The ``a`` in the RE matches. |
++------+-----------+---------------------------------+
+| 2 | ``abcbd`` | The engine matches ``[bcd]*``, |
+| | | going as far as it can, which |
+| | | is to the end of the string. |
++------+-----------+---------------------------------+
+| 3 | *Failure* | The engine tries to match |
+| | | ``b``, but the current position |
+| | | is at the end of the string, so |
+| | | it fails. |
++------+-----------+---------------------------------+
+| 4 | ``abcb`` | Back up, so that ``[bcd]*`` |
+| | | matches one less character. |
++------+-----------+---------------------------------+
+| 5 | *Failure* | Try ``b`` again, but the |
+| | | current position is at the last |
+| | | character, which is a ``'d'``. |
++------+-----------+---------------------------------+
+| 6 | ``abc`` | Back up again, so that |
+| | | ``[bcd]*`` is only matching |
+| | | ``bc``. |
++------+-----------+---------------------------------+
+| 6 | ``abcb`` | Try ``b`` again. This time |
+| | | the character at the |
+| | | current position is ``'b'``, so |
+| | | it succeeds. |
++------+-----------+---------------------------------+
+
+The end of the RE has now been reached, and it has matched ``abcb``. 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
+``[bcd]*``, and if that subsequently fails, the engine will conclude that the
+string doesn't match the RE at all.
+
+Another repeating metacharacter is ``+``, which matches one or more times. Pay
+careful attention to the difference between ``*`` and ``+``; ``*`` matches
+*zero* or more times, so whatever's being repeated may not be present at all,
+while ``+`` requires at least *one* occurrence. To use a similar example,
+``ca+t`` will match ``cat`` (1 ``a``), ``caaat`` (3 ``a``'s), but won't match
+``ct``.
+
+There are two more repeating qualifiers. The question mark character, ``?``,
+matches either once or zero times; you can think of it as marking something as
+being optional. For example, ``home-?brew`` matches either ``homebrew`` or
+``home-brew``.
+
+The most complicated repeated qualifier is ``{m,n}``, where *m* and *n* are
+decimal integers. This qualifier means there must be at least *m* repetitions,
+and at most *n*. For example, ``a/{1,3}b`` will match ``a/b``, ``a//b``, and
+``a///b``. It won't match ``ab``, which has no slashes, or ``a////b``, 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 --- actually, the upper bound
+is the 2-billion limit mentioned earlier, but that might as well be infinity.
+
+Readers of a reductionist bent may notice that the three other qualifiers can
+all be expressed using this notation. ``{0,}`` is the same as ``*``, ``{1,}``
+is equivalent to ``+``, and ``{0,1}`` is the same as ``?``. It's better to use
+``*``, ``+``, or ``?`` when you can, simply because they're shorter and easier
+to read.
+
+
+Using Regular Expressions
+=========================
+
+Now that we've looked at some simple regular expressions, how do we actually use
+them in Python? The :mod:`re` module provides an interface to the regular
+expression engine, allowing you to compile REs into objects and then perform
+matches with them.
+
+
+Compiling Regular Expressions
+-----------------------------
+
+Regular expressions are compiled into :class:`RegexObject` instances, which have
+methods for various operations such as searching for pattern matches or
+performing string substitutions. ::
+
+ >>> import re
+ >>> p = re.compile('ab*')
+ >>> print p
+ <re.RegexObject instance at 80b4150>
+
+:func:`re.compile` 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::
+
+ >>> p = re.compile('ab*', re.IGNORECASE)
+
+The RE is passed to :func:`re.compile` 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 :mod:`re` module is simply a C extension module
+included with Python, just like the :mod:`socket` or :mod:`zlib` modules.
+
+Putting REs in strings keeps the Python language simpler, but has one
+disadvantage which is the topic of the next section.
+
+
+The Backslash Plague
+--------------------
+
+As stated earlier, regular expressions use the backslash character (``'\'``) 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 ``\section``, 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 ``\\section``. The resulting string that must be passed
+to :func:`re.compile` must be ``\\section``. However, to express this as a
+Python string literal, both backslashes must be escaped *again*.
+
++-------------------+------------------------------------------+
+| Characters | Stage |
++===================+==========================================+
+| ``\section`` | Text string to be matched |
++-------------------+------------------------------------------+
+| ``\\section`` | Escaped backslash for :func:`re.compile` |
++-------------------+------------------------------------------+
+| ``"\\\\section"`` | Escaped backslashes for a string literal |
++-------------------+------------------------------------------+
+
+In short, to match a literal backslash, one has to write ``'\\\\'`` as the RE
+string, because the regular expression must be ``\\``, and each backslash must
+be expressed as ``\\`` 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
+``'r'``, so ``r"\n"`` is a two-character string containing ``'\'`` and ``'n'``,
+while ``"\n"`` is a one-character string containing a newline. Regular
+expressions will often be written in Python code using this raw string notation.
+
++-------------------+------------------+
+| Regular String | Raw string |
++===================+==================+
+| ``"ab*"`` | ``r"ab*"`` |
++-------------------+------------------+
+| ``"\\\\section"`` | ``r"\\section"`` |
++-------------------+------------------+
+| ``"\\w+\\s+\\1"`` | ``r"\w+\s+\1"`` |
++-------------------+------------------+
+
+
+Performing Matches
+------------------
+
+Once you have an object representing a compiled regular expression, what do you
+do with it? :class:`RegexObject` instances have several methods and attributes.
+Only the most significant ones will be covered here; consult the :mod:`re` docs
+for a complete listing.
+
++------------------+-----------------------------------------------+
+| Method/Attribute | Purpose |
++==================+===============================================+
+| ``match()`` | Determine if the RE matches at the beginning |
+| | of the string. |
++------------------+-----------------------------------------------+
+| ``search()`` | Scan through a string, looking for any |
+| | location where this RE matches. |
++------------------+-----------------------------------------------+
+| ``findall()`` | Find all substrings where the RE matches, and |
+| | returns them as a list. |
++------------------+-----------------------------------------------+
+| ``finditer()`` | Find all substrings where the RE matches, and |
+| | returns them as an :term:`iterator`. |
++------------------+-----------------------------------------------+
+
+:meth:`match` and :meth:`search` return ``None`` if no match can be found. If
+they're successful, a ``MatchObject`` 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 :mod:`re`
+module. If you have Tkinter available, you may also want to look at
+:file:`Tools/scripts/redemo.py`, a demonstration program included with the
+Python distribution. It allows you to enter REs and strings, and displays
+whether the RE matches or fails. :file:`redemo.py` can be quite useful when
+trying to debug a complicated RE. Phil Schwartz's `Kodos
+<http://kodos.sourceforge.net/>`_ is also an interactive tool for developing and
+testing RE patterns.
+
+This HOWTO uses the standard Python interpreter for its examples. First, run the
+Python interpreter, import the :mod:`re` module, and compile a RE::
+
+ Python 2.2.2 (#1, Feb 10 2003, 12:57:01)
+ >>> import re
+ >>> p = re.compile('[a-z]+')
+ >>> p
+ <_sre.SRE_Pattern object at 80c3c28>
+
+Now, you can try matching various strings against the RE ``[a-z]+``. An empty
+string shouldn't match at all, since ``+`` means 'one or more repetitions'.
+:meth:`match` should return ``None`` in this case, which will cause the
+interpreter to print no output. You can explicitly print the result of
+:meth:`match` to make this clear. ::
+
+ >>> p.match("")
+ >>> print p.match("")
+ None
+
+Now, let's try it on a string that it should match, such as ``tempo``. In this
+case, :meth:`match` will return a :class:`MatchObject`, so you should store the
+result in a variable for later use. ::
+
+ >>> m = p.match('tempo')
+ >>> print m
+ <_sre.SRE_Match object at 80c4f68>
+
+Now you can query the :class:`MatchObject` for information about the matching
+string. :class:`MatchObject` instances also have several methods and
+attributes; the most important ones are:
+
++------------------+--------------------------------------------+
+| Method/Attribute | Purpose |
++==================+============================================+
+| ``group()`` | Return the string matched by the RE |
++------------------+--------------------------------------------+
+| ``start()`` | Return the starting position of the match |
++------------------+--------------------------------------------+
+| ``end()`` | Return the ending position of the match |
++------------------+--------------------------------------------+
+| ``span()`` | Return a tuple containing the (start, end) |
+| | positions of the match |
++------------------+--------------------------------------------+
+
+Trying these methods will soon clarify their meaning::
+
+ >>> m.group()
+ 'tempo'
+ >>> m.start(), m.end()
+ (0, 5)
+ >>> m.span()
+ (0, 5)
+
+:meth:`group` returns the substring that was matched by the RE. :meth:`start`
+and :meth:`end` return the starting and ending index of the match. :meth:`span`
+returns both start and end indexes in a single tuple. Since the :meth:`match`
+method only checks if the RE matches at the start of a string, :meth:`start`
+will always be zero. However, the :meth:`search` method of :class:`RegexObject`
+instances scans through the string, so the match may not start at zero in that
+case. ::
+
+ >>> print p.match('::: message')
+ None
+ >>> m = p.search('::: message') ; print m
+ <re.MatchObject instance at 80c9650>
+ >>> m.group()
+ 'message'
+ >>> m.span()
+ (4, 11)
+
+In actual programs, the most common style is to store the :class:`MatchObject`
+in a variable, and then check if it was ``None``. This usually looks like::
+
+ p = re.compile( ... )
+ m = p.match( 'string goes here' )
+ if m:
+ print 'Match found: ', m.group()
+ else:
+ print 'No match'
+
+Two :class:`RegexObject` methods return all of the matches for a pattern.
+:meth:`findall` returns a list of matching strings::
+
+ >>> p = re.compile('\d+')
+ >>> p.findall('12 drummers drumming, 11 pipers piping, 10 lords a-leaping')
+ ['12', '11', '10']
+
+:meth:`findall` has to create the entire list before it can be returned as the
+result. The :meth:`finditer` method returns a sequence of :class:`MatchObject`
+instances as an :term:`iterator`. [#]_ ::
+
+ >>> iterator = p.finditer('12 drummers drumming, 11 ... 10 ...')
+ >>> iterator
+ <callable-iterator object at 0x401833ac>
+ >>> for match in iterator:
+ ... print match.span()
+ ...
+ (0, 2)
+ (22, 24)
+ (29, 31)
+
+
+Module-Level Functions
+----------------------
+
+You don't have to create a :class:`RegexObject` and call its methods; the
+:mod:`re` module also provides top-level functions called :func:`match`,
+:func:`search`, :func:`findall`, :func:`sub`, and so forth. These functions
+take the same arguments as the corresponding :class:`RegexObject` method, with
+the RE string added as the first argument, and still return either ``None`` or a
+:class:`MatchObject` instance. ::
+
+ >>> print re.match(r'From\s+', 'Fromage amk')
+ None
+ >>> re.match(r'From\s+', 'From amk Thu May 14 19:12:10 1998')
+ <re.MatchObject instance at 80c5978>
+
+Under the hood, these functions simply produce a :class:`RegexObject` 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 are faster.
+
+Should you use these module-level functions, or should you get the
+:class:`RegexObject` and call its methods yourself? That choice depends on how
+frequently the RE will be used, and on your personal coding style. If the RE is
+being used at only one point in the code, then the module functions are probably
+more convenient. If a program contains a lot of regular expressions, or re-uses
+the same ones in several locations, then it might be worthwhile to collect all
+the definitions in one place, in a section of code that compiles all the REs
+ahead of time. To take an example from the standard library, here's an extract
+from :file:`xmllib.py`::
+
+ ref = re.compile( ... )
+ entityref = re.compile( ... )
+ charref = re.compile( ... )
+ starttagopen = re.compile( ... )
+
+I generally prefer to work with the compiled object, even for one-time uses, but
+few people will be as much of a purist about this as I am.
+
+
+Compilation Flags
+-----------------
+
+Compilation flags let you modify some aspects of how regular expressions work.
+Flags are available in the :mod:`re` module under two names, a long name such as
+:const:`IGNORECASE` and a short, one-letter form such as :const:`I`. (If you're
+familiar with Perl's pattern modifiers, the one-letter forms use the same
+letters; the short form of :const:`re.VERBOSE` is :const:`re.X`, for example.)
+Multiple flags can be specified by bitwise OR-ing them; ``re.I | re.M`` sets
+both the :const:`I` and :const:`M` flags, for example.
+
+Here's a table of the available flags, followed by a more detailed explanation
+of each one.
+
++---------------------------------+--------------------------------------------+
+| Flag | Meaning |
++=================================+============================================+
+| :const:`DOTALL`, :const:`S` | Make ``.`` match any character, including |
+| | newlines |
++---------------------------------+--------------------------------------------+
+| :const:`IGNORECASE`, :const:`I` | Do case-insensitive matches |
++---------------------------------+--------------------------------------------+
+| :const:`LOCALE`, :const:`L` | Do a locale-aware match |
++---------------------------------+--------------------------------------------+
+| :const:`MULTILINE`, :const:`M` | Multi-line matching, affecting ``^`` and |
+| | ``$`` |
++---------------------------------+--------------------------------------------+
+| :const:`VERBOSE`, :const:`X` | Enable verbose REs, which can be organized |
+| | more cleanly and understandably. |
++---------------------------------+--------------------------------------------+
+
+
+.. data:: I
+ IGNORECASE
+ :noindex:
+
+ Perform case-insensitive matching; character class and literal strings will
+ match letters by ignoring case. For example, ``[A-Z]`` will match lowercase
+ letters, too, and ``Spam`` will match ``Spam``, ``spam``, or ``spAM``. This
+ lowercasing doesn't take the current locale into account; it will if you also
+ set the :const:`LOCALE` flag.
+
+
+.. data:: L
+ LOCALE
+ :noindex:
+
+ Make ``\w``, ``\W``, ``\b``, and ``\B``, dependent on the current locale.
+
+ Locales are a feature of the C library intended to help in writing programs that
+ take account of language differences. For example, if you're processing French
+ text, you'd want to be able to write ``\w+`` to match words, but ``\w`` only
+ matches the character class ``[A-Za-z]``; it won't match ``'é'`` or ``'ç'``. If
+ your system is configured properly and a French locale is selected, certain C
+ functions will tell the program that ``'é'`` should also be considered a letter.
+ Setting the :const:`LOCALE` flag when compiling a regular expression will cause
+ the resulting compiled object to use these C functions for ``\w``; this is
+ slower, but also enables ``\w+`` to match French words as you'd expect.
+
+
+.. data:: M
+ MULTILINE
+ :noindex:
+
+ (``^`` and ``$`` haven't been explained yet; they'll be introduced in section
+ :ref:`more-metacharacters`.)
+
+ Usually ``^`` matches only at the beginning of the string, and ``$`` matches
+ only at the end of the string and immediately before the newline (if any) at the
+ end of the string. When this flag is specified, ``^`` matches at the beginning
+ of the string and at the beginning of each line within the string, immediately
+ following each newline. Similarly, the ``$`` metacharacter matches either at
+ the end of the string and at the end of each line (immediately preceding each
+ newline).
+
+
+.. data:: S
+ DOTALL
+ :noindex:
+
+ Makes the ``'.'`` special character match any character at all, including a
+ newline; without this flag, ``'.'`` will match anything *except* a newline.
+
+
+.. data:: X
+ VERBOSE
+ :noindex:
+
+ 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 ``'#'`` that's neither in a character class or preceded by an unescaped
+ backslash.
+
+ For example, here's a RE that uses :const:`re.VERBOSE`; see how much easier it
+ is to read? ::
+
+ charref = re.compile(r"""
+ &[#] # Start of a numeric entity reference
+ (
+ 0[0-7]+ # Octal form
+ | [0-9]+ # Decimal form
+ | x[0-9a-fA-F]+ # Hexadecimal form
+ )
+ ; # Trailing semicolon
+ """, re.VERBOSE)
+
+ Without the verbose setting, the RE would look like this::
+
+ charref = re.compile("&#(0[0-7]+"
+ "|[0-9]+"
+ "|x[0-9a-fA-F]+);")
+
+ 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 :const:`re.VERBOSE`.
+
+
+More Pattern Power
+==================
+
+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.
+
+
+.. _more-metacharacters:
+
+More Metacharacters
+-------------------
+
+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 :dfn:`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, ``\b`` is an assertion that the current position is located at a word
+boundary; the position isn't changed by the ``\b`` 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.
+
+``|``
+ Alternation, or the "or" operator. If A and B are regular expressions,
+ ``A|B`` will match any string that matches either ``A`` or ``B``. ``|`` has very
+ low precedence in order to make it work reasonably when you're alternating
+ multi-character strings. ``Crow|Servo`` will match either ``Crow`` or ``Servo``,
+ not ``Cro``, a ``'w'`` or an ``'S'``, and ``ervo``.
+
+ To match a literal ``'|'``, use ``\|``, or enclose it inside a character class,
+ as in ``[|]``.
+
+``^``
+ Matches at the beginning of lines. Unless the :const:`MULTILINE` flag has been
+ set, this will only match at the beginning of the string. In :const:`MULTILINE`
+ mode, this also matches immediately after each newline within the string.
+
+ For example, if you wish to match the word ``From`` only at the beginning of a
+ line, the RE to use is ``^From``. ::
+
+ >>> print re.search('^From', 'From Here to Eternity')
+ <re.MatchObject instance at 80c1520>
+ >>> print re.search('^From', 'Reciting From Memory')
+ None
+
+ .. To match a literal \character{\^}, use \regexp{\e\^} or enclose it
+ .. inside a character class, as in \regexp{[{\e}\^]}.
+
+``$``
+ Matches at the end of a line, which is defined as either the end of the string,
+ or any location followed by a newline character. ::
+
+ >>> print re.search('}$', '{block}')
+ <re.MatchObject instance at 80adfa8>
+ >>> print re.search('}$', '{block} ')
+ None
+ >>> print re.search('}$', '{block}\n')
+ <re.MatchObject instance at 80adfa8>
+
+ To match a literal ``'$'``, use ``\$`` or enclose it inside a character class,
+ as in ``[$]``.
+
+``\A``
+ Matches only at the start of the string. When not in :const:`MULTILINE` mode,
+ ``\A`` and ``^`` are effectively the same. In :const:`MULTILINE` mode, they're
+ different: ``\A`` still matches only at the beginning of the string, but ``^``
+ may match at any location inside the string that follows a newline character.
+
+``\Z``
+ Matches only at the end of the string.
+
+``\b``
+ 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 ``class`` only when it's a complete word; it won't
+ match when it's contained inside another word. ::
+
+ >>> p = re.compile(r'\bclass\b')
+ >>> print p.search('no class at all')
+ <re.MatchObject instance at 80c8f28>
+ >>> print p.search('the declassified algorithm')
+ None
+ >>> print p.search('one subclass is')
+ None
+
+ 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, ``\b`` is the backspace
+ character, ASCII value 8. If you're not using raw strings, then Python will
+ convert the ``\b`` 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 ``'r'``
+ in front of the RE string. ::
+
+ >>> p = re.compile('\bclass\b')
+ >>> print p.search('no class at all')
+ None
+ >>> print p.search('\b' + 'class' + '\b')
+ <re.MatchObject instance at 80c3ee0>
+
+ Second, inside a character class, where there's no use for this assertion,
+ ``\b`` represents the backspace character, for compatibility with Python's
+ string literals.
+
+``\B``
+ Another zero-width assertion, this is the opposite of ``\b``, only matching when
+ the current position is not at a word boundary.
+
+
+Grouping
+--------
+
+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 ``':'``, like this::
+
+ From: author@example.com
+ User-Agent: Thunderbird 1.5.0.9 (X11/20061227)
+ MIME-Version: 1.0
+ To: editor@example.com
+
+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 ``'('``, ``')'`` metacharacters. ``'('`` and ``')'``
+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 ``*``, ``+``, ``?``, or
+``{m,n}``. For example, ``(ab)*`` will match zero or more repetitions of
+``ab``. ::
+
+ >>> p = re.compile('(ab)*')
+ >>> print p.match('ababababab').span()
+ (0, 10)
+
+Groups indicated with ``'('``, ``')'`` also capture the starting and ending
+index of the text that they match; this can be retrieved by passing an argument
+to :meth:`group`, :meth:`start`, :meth:`end`, and :meth:`span`. Groups are
+numbered starting with 0. Group 0 is always present; it's the whole RE, so
+:class:`MatchObject` 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. ::
+
+ >>> p = re.compile('(a)b')
+ >>> m = p.match('ab')
+ >>> m.group()
+ 'ab'
+ >>> m.group(0)
+ 'ab'
+
+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. ::
+
+ >>> p = re.compile('(a(b)c)d')
+ >>> m = p.match('abcd')
+ >>> m.group(0)
+ 'abcd'
+ >>> m.group(1)
+ 'abc'
+ >>> m.group(2)
+ 'b'
+
+:meth:`group` can be passed multiple group numbers at a time, in which case it
+will return a tuple containing the corresponding values for those groups. ::
+
+ >>> m.group(2,1,2)
+ ('b', 'abc', 'b')
+
+The :meth:`groups` method returns a tuple containing the strings for all the
+subgroups, from 1 up to however many there are. ::
+
+ >>> m.groups()
+ ('abc', 'b')
+
+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, ``\1`` 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. ::
+
+ >>> p = re.compile(r'(\b\w+)\s+\1')
+ >>> p.search('Paris in the the spring').group()
+ 'the the'
+
+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.
+
+
+Non-capturing and Named Groups
+------------------------------
+
+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 added several additional features to standard regular expressions, and
+the Python :mod:`re` module supports most of them. It would have been
+difficult to choose new single-keystroke metacharacters or new special sequences
+beginning with ``\`` to represent the new features without making Perl's regular
+expressions confusingly different from standard REs. If you chose ``&`` as a
+new metacharacter, for example, old expressions would be assuming that ``&`` was
+a regular character and wouldn't have escaped it by writing ``\&`` or ``[&]``.
+
+The solution chosen by the Perl developers was to use ``(?...)`` as the
+extension syntax. ``?`` immediately after a parenthesis was a syntax error
+because the ``?`` would have nothing to repeat, so this didn't introduce any
+compatibility problems. The characters immediately after the ``?`` indicate
+what extension is being used, so ``(?=foo)`` is one thing (a positive lookahead
+assertion) and ``(?:foo)`` is something else (a non-capturing group containing
+the subexpression ``foo``).
+
+Python adds an extension syntax to Perl's extension syntax. If the first
+character after the question mark is a ``P``, you know that it's an extension
+that's specific to Python. Currently there are two such extensions:
+``(?P<name>...)`` defines a named group, and ``(?P=name)`` is a backreference to
+a named group. If future versions of Perl 5 add similar features using a
+different syntax, the :mod:`re` module will be changed to support the new
+syntax, while preserving the Python-specific syntax for compatibility's sake.
+
+Now that we've looked at the general extension syntax, we can return to the
+features that simplify working with groups in complex REs. Since groups are
+numbered from left to right and a complex expression may use many groups, it can
+become difficult to keep track of the correct numbering. Modifying such a
+complex RE is annoying, too: insert a new group near the beginning and you
+change the numbers of everything that follows it.
+
+Sometimes you'll want to use a group to collect 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: ``(?:...)``, where you can replace the
+``...`` with any other regular expression. ::
+
+ >>> m = re.match("([abc])+", "abc")
+ >>> m.groups()
+ ('c',)
+ >>> m = re.match("(?:[abc])+", "abc")
+ >>> m.groups()
+ ()
+
+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 ``*``, and nest it within other groups (capturing or non-capturing).
+``(?:...)`` 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:
+``(?P<name>...)``. *name* is, obviously, the name of the group. Named groups
+also behave exactly like capturing groups, and additionally associate a name
+with a group. The :class:`MatchObject` 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::
+
+ >>> p = re.compile(r'(?P<word>\b\w+\b)')
+ >>> m = p.search( '(((( Lots of punctuation )))' )
+ >>> m.group('word')
+ 'Lots'
+ >>> m.group(1)
+ 'Lots'
+
+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 :mod:`imaplib`
+module::
+
+ InternalDate = re.compile(r'INTERNALDATE "'
+ r'(?P<day>[ 123][0-9])-(?P<mon>[A-Z][a-z][a-z])-'
+ r'(?P<year>[0-9][0-9][0-9][0-9])'
+ r' (?P<hour>[0-9][0-9]):(?P<min>[0-9][0-9]):(?P<sec>[0-9][0-9])'
+ r' (?P<zonen>[-+])(?P<zoneh>[0-9][0-9])(?P<zonem>[0-9][0-9])'
+ r'"')
+
+It's obviously much easier to retrieve ``m.group('zonem')``, instead of having
+to remember to retrieve group 9.
+
+The syntax for backreferences in an expression such as ``(...)\1`` 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: ``(?P=name)`` indicates
+that the contents of the group called *name* should again be matched at the
+current point. The regular expression for finding doubled words,
+``(\b\w+)\s+\1`` can also be written as ``(?P<word>\b\w+)\s+(?P=word)``::
+
+ >>> p = re.compile(r'(?P<word>\b\w+)\s+(?P=word)')
+ >>> p.search('Paris in the the spring').group()
+ 'the the'
+
+
+Lookahead Assertions
+--------------------
+
+Another zero-width assertion is the lookahead assertion. Lookahead assertions
+are available in both positive and negative form, and look like this:
+
+``(?=...)``
+ Positive lookahead assertion. This succeeds if the contained regular
+ expression, represented here by ``...``, 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.
+
+``(?!...)``
+ 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.
+
+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 ``.``. For example, in ``news.rc``,
+``news`` is the base name, and ``rc`` is the filename's extension.
+
+The pattern to match this is quite simple:
+
+``.*[.].*$``
+
+Notice that the ``.`` needs to be treated specially because it's a
+metacharacter; I've put it inside a character class. Also notice the trailing
+``$``; this is added to ensure that all the rest of the string must be included
+in the extension. This regular expression matches ``foo.bar`` and
+``autoexec.bat`` and ``sendmail.cf`` and ``printers.conf``.
+
+Now, consider complicating the problem a bit; what if you want to match
+filenames where the extension is not ``bat``? Some incorrect attempts:
+
+``.*[.][^b].*$`` The first attempt above tries to exclude ``bat`` by requiring
+that the first character of the extension is not a ``b``. This is wrong,
+because the pattern also doesn't match ``foo.bar``.
+
+``.*[.]([^b]..|.[^a].|..[^t])$``
+
+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 ``b``; the second character isn't ``a``; or the third character
+isn't ``t``. This accepts ``foo.bar`` and rejects ``autoexec.bat``, but it
+requires a three-letter extension and won't accept a filename with a two-letter
+extension such as ``sendmail.cf``. We'll complicate the pattern again in an
+effort to fix it.
+
+``.*[.]([^b].?.?|.[^a]?.?|..?[^t]?)$``
+
+In the third attempt, the second and third letters are all made optional in
+order to allow matching extensions shorter than three characters, such as
+``sendmail.cf``.
+
+The pattern's getting really complicated now, which makes it hard to read and
+understand. Worse, if the problem changes and you want to exclude both ``bat``
+and ``exe`` as extensions, the pattern would get even more complicated and
+confusing.
+
+A negative lookahead cuts through all this confusion:
+
+``.*[.](?!bat$).*$`` The negative lookahead means: if the expression ``bat``
+doesn't match at this point, try the rest of the pattern; if ``bat$`` does
+match, the whole pattern will fail. The trailing ``$`` is required to ensure
+that something like ``sample.batch``, where the extension only starts with
+``bat``, will be allowed.
+
+Excluding another filename extension is now easy; simply add it as an
+alternative inside the assertion. The following pattern excludes filenames that
+end in either ``bat`` or ``exe``:
+
+``.*[.](?!bat$|exe$).*$``
+
+
+Modifying Strings
+=================
+
+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 :class:`RegexObject` methods:
+
++------------------+-----------------------------------------------+
+| Method/Attribute | Purpose |
++==================+===============================================+
+| ``split()`` | Split the string into a list, splitting it |
+| | wherever the RE matches |
++------------------+-----------------------------------------------+
+| ``sub()`` | Find all substrings where the RE matches, and |
+| | replace them with a different string |
++------------------+-----------------------------------------------+
+| ``subn()`` | Does the same thing as :meth:`sub`, but |
+| | returns the new string and the number of |
+| | replacements |
++------------------+-----------------------------------------------+
+
+
+Splitting Strings
+-----------------
+
+The :meth:`split` method of a :class:`RegexObject` splits a string apart
+wherever the RE matches, returning a list of the pieces. It's similar to the
+:meth:`split` method of strings but provides much more generality in the
+delimiters that you can split by; :meth:`split` only supports splitting by
+whitespace or by a fixed string. As you'd expect, there's a module-level
+:func:`re.split` function, too.
+
+
+.. method:: .split(string [, maxsplit=0])
+ :noindex:
+
+ 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.
+
+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.
+::
+
+ >>> p = re.compile(r'\W+')
+ >>> p.split('This is a test, short and sweet, of split().')
+ ['This', 'is', 'a', 'test', 'short', 'and', 'sweet', 'of', 'split', '']
+ >>> p.split('This is a test, short and sweet, of split().', 3)
+ ['This', 'is', 'a', 'test, short and sweet, of split().']
+
+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::
+
+ >>> p = re.compile(r'\W+')
+ >>> p2 = re.compile(r'(\W+)')
+ >>> p.split('This... is a test.')
+ ['This', 'is', 'a', 'test', '']
+ >>> p2.split('This... is a test.')
+ ['This', '... ', 'is', ' ', 'a', ' ', 'test', '.', '']
+
+The module-level function :func:`re.split` adds the RE to be used as the first
+argument, but is otherwise the same. ::
+
+ >>> re.split('[\W]+', 'Words, words, words.')
+ ['Words', 'words', 'words', '']
+ >>> re.split('([\W]+)', 'Words, words, words.')
+ ['Words', ', ', 'words', ', ', 'words', '.', '']
+ >>> re.split('[\W]+', 'Words, words, words.', 1)
+ ['Words', 'words, words.']
+
+
+Search and Replace
+------------------
+
+Another common task is to find all the matches for a pattern, and replace them
+with a different string. The :meth:`sub` method takes a replacement value,
+which can be either a string or a function, and the string to be processed.
+
+
+.. method:: .sub(replacement, string[, count=0])
+ :noindex:
+
+ 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.
+
+Here's a simple example of using the :meth:`sub` method. It replaces colour
+names with the word ``colour``::
+
+ >>> p = re.compile( '(blue|white|red)')
+ >>> p.sub( 'colour', 'blue socks and red shoes')
+ 'colour socks and colour shoes'
+ >>> p.sub( 'colour', 'blue socks and red shoes', count=1)
+ 'colour socks and red shoes'
+
+The :meth:`subn` method does the same work, but returns a 2-tuple containing the
+new string value and the number of replacements that were performed::
+
+ >>> p = re.compile( '(blue|white|red)')
+ >>> p.subn( 'colour', 'blue socks and red shoes')
+ ('colour socks and colour shoes', 2)
+ >>> p.subn( 'colour', 'no colours at all')
+ ('no colours at all', 0)
+
+Empty matches are replaced only when they're not adjacent to a previous match.
+::
+
+ >>> p = re.compile('x*')
+ >>> p.sub('-', 'abxd')
+ '-a-b-d-'
+
+If *replacement* is a string, any backslash escapes in it are processed. That
+is, ``\n`` is converted to a single newline character, ``\r`` is converted to a
+carriage return, and so forth. Unknown escapes such as ``\j`` are left alone.
+Backreferences, such as ``\6``, 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 ``section`` followed by a string enclosed in
+``{``, ``}``, and changes ``section`` to ``subsection``::
+
+ >>> p = re.compile('section{ ( [^}]* ) }', re.VERBOSE)
+ >>> p.sub(r'subsection{\1}','section{First} section{second}')
+ 'subsection{First} subsection{second}'
+
+There's also a syntax for referring to named groups as defined by the
+``(?P<name>...)`` syntax. ``\g<name>`` will use the substring matched by the
+group named ``name``, and ``\g<number>`` uses the corresponding group number.
+``\g<2>`` is therefore equivalent to ``\2``, but isn't ambiguous in a
+replacement string such as ``\g<2>0``. (``\20`` would be interpreted as a
+reference to group 20, not a reference to group 2 followed by the literal
+character ``'0'``.) The following substitutions are all equivalent, but use all
+three variations of the replacement string. ::
+
+ >>> p = re.compile('section{ (?P<name> [^}]* ) }', re.VERBOSE)
+ >>> p.sub(r'subsection{\1}','section{First}')
+ 'subsection{First}'
+ >>> p.sub(r'subsection{\g<1>}','section{First}')
+ 'subsection{First}'
+ >>> p.sub(r'subsection{\g<name>}','section{First}')
+ 'subsection{First}'
+
+*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
+:class:`MatchObject` 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::
+
+ >>> def hexrepl( match ):
+ ... "Return the hex string for a decimal number"
+ ... value = int( match.group() )
+ ... return hex(value)
+ ...
+ >>> p = re.compile(r'\d+')
+ >>> p.sub(hexrepl, 'Call 65490 for printing, 49152 for user code.')
+ 'Call 0xffd2 for printing, 0xc000 for user code.'
+
+When using the module-level :func:`re.sub` function, the pattern is passed as
+the first argument. The pattern may be a string or a :class:`RegexObject`; if
+you need to specify regular expression flags, you must either use a
+:class:`RegexObject` as the first parameter, or use embedded modifiers in the
+pattern, e.g. ``sub("(?i)b+", "x", "bbbb BBBB")`` returns ``'x x'``.
+
+
+Common Problems
+===============
+
+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.
+
+
+Use String Methods
+------------------
+
+Sometimes using the :mod:`re` module is a mistake. If you're matching a fixed
+string, or a single character class, and you're not using any :mod:`re` features
+such as the :const:`IGNORECASE` 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 ``word`` with ``deed``. ``re.sub()`` seems like the
+function to use for this, but consider the :meth:`replace` method. Note that
+:func:`replace` will also replace ``word`` inside words, turning ``swordfish``
+into ``sdeedfish``, but the naive RE ``word`` would have done that, too. (To
+avoid performing the substitution on parts of words, the pattern would have to
+be ``\bword\b``, in order to require that ``word`` have a word boundary on
+either side. This takes the job beyond :meth:`replace`'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 ``re.sub('\n', ' ', S)``, but :meth:`translate` is capable of
+doing both tasks and will be faster than any regular expression operation can
+be.
+
+In short, before turning to the :mod:`re` module, consider whether your problem
+can be solved with a faster and simpler string method.
+
+
+match() versus search()
+-----------------------
+
+The :func:`match` function only checks if the RE matches at the beginning of the
+string while :func:`search` will scan forward through the string for a match.
+It's important to keep this distinction in mind. Remember, :func:`match` will
+only report a successful match which will start at 0; if the match wouldn't
+start at zero, :func:`match` will *not* report it. ::
+
+ >>> print re.match('super', 'superstition').span()
+ (0, 5)
+ >>> print re.match('super', 'insuperable')
+ None
+
+On the other hand, :func:`search` will scan forward through the string,
+reporting the first match it finds. ::
+
+ >>> print re.search('super', 'superstition').span()
+ (0, 5)
+ >>> print re.search('super', 'insuperable').span()
+ (2, 7)
+
+Sometimes you'll be tempted to keep using :func:`re.match`, and just add ``.*``
+to the front of your RE. Resist this temptation and use :func:`re.search`
+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
+``Crow`` must match starting with a ``'C'``. The analysis lets the engine
+quickly scan through the string looking for the starting character, only trying
+the full match if a ``'C'`` is found.
+
+Adding ``.*`` 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
+:func:`re.search` instead.
+
+
+Greedy versus Non-Greedy
+------------------------
+
+When repeating a regular expression, as in ``a*``, 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 ``.*``. ::
+
+ >>> s = '<html><head><title>Title</title>'
+ >>> len(s)
+ 32
+ >>> print re.match('<.*>', s).span()
+ (0, 32)
+ >>> print re.match('<.*>', s).group()
+ <html><head><title>Title</title>
+
+The RE matches the ``'<'`` in ``<html>``, and the ``.*`` consumes the rest of
+the string. There's still more left in the RE, though, and the ``>`` 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 ``>``. The
+final match extends from the ``'<'`` in ``<html>`` to the ``'>'`` in
+``</title>``, which isn't what you want.
+
+In this case, the solution is to use the non-greedy qualifiers ``*?``, ``+?``,
+``??``, or ``{m,n}?``, which match as *little* text as possible. In the above
+example, the ``'>'`` is tried immediately after the first ``'<'`` matches, and
+when it fails, the engine advances a character at a time, retrying the ``'>'``
+at every step. This produces just the right result::
+
+ >>> print re.match('<.*?>', s).group()
+ <html>
+
+(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.)
+
+
+Not Using re.VERBOSE
+--------------------
+
+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 ``re.VERBOSE`` flag when compiling the regular
+expression can be helpful, because it allows you to format the regular
+expression more clearly.
+
+The ``re.VERBOSE`` 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 ``dog | cat`` is equivalent to the less readable ``dog|cat``,
+but ``[a b]`` will still match the characters ``'a'``, ``'b'``, or a space. In
+addition, you can also put comments inside a RE; comments extend from a ``#``
+character to the next newline. When used with triple-quoted strings, this
+enables REs to be formatted more neatly::
+
+ pat = re.compile(r"""
+ \s* # Skip leading whitespace
+ (?P<header>[^:]+) # Header name
+ \s* : # Whitespace, and a colon
+ (?P<value>.*?) # The header's value -- *? used to
+ # lose the following trailing whitespace
+ \s*$ # Trailing whitespace to end-of-line
+ """, re.VERBOSE)
+
+This is far more readable than::
+
+ pat = re.compile(r"\s*(?P<header>[^:]+)\s*:(?P<value>.*?)\s*$")
+
+
+Feedback
+========
+
+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 :mod:`regex` module, which won't help you much.) Consider checking
+it out from your library.
+
+
+.. rubric:: Footnotes
+
+.. [#] Introduced in Python 2.2.2.
+