--- rest25/library/re.rst	2008-12-19 16:58:07.000000000 +0900
+++ rest262/library/re.rst	2009-04-07 00:06:53.000000000 +0900
@@ -1,107 +1,111 @@
 
 :mod:`re` --- Regular expression operations
 ===========================================
 
 .. module:: re
+   :synopsis: Regular expression operations.
 .. moduleauthor:: Fredrik Lundh <fredrik@pythonware.com>
 .. sectionauthor:: Andrew M. Kuchling <amk@amk.ca>
 
 
-
-
-This module provides regular expression matching operations similar to those
-found in Perl.  Regular expression pattern strings may not contain null bytes,
-but can specify the null byte using the ``\number`` notation.  Both patterns and
-strings to be searched can be Unicode strings as well as 8-bit strings.  The
-:mod:`re` module is always available.
-
-Regular expressions use the backslash character (``'\'``) to indicate special
-forms or to allow special characters to be used without invoking their special
-meaning.  This collides with Python's usage of the same character for the same
-purpose in string literals; for example, to match a literal backslash, one might
-have to write ``'\\\\'`` as the pattern string, because the regular expression
-must be ``\\``, and each backslash must be expressed as ``\\`` inside a regular
-Python string literal.
+This module provides regular expression matching operations similar to
+those found in Perl. Both patterns and strings to be searched can be
+Unicode strings as well as 8-bit strings.
+
+Regular expressions use the backslash character (``'\'``) to indicate
+special forms or to allow special characters to be used without invoking
+their special meaning.  This collides with Python's usage of the same
+character for the same purpose in string literals; for example, to match
+a literal backslash, one might have to write ``'\\\\'`` as the pattern
+string, because the regular expression must be ``\\``, and each
+backslash must be expressed as ``\\`` inside a regular Python string
+literal.
 
 The solution is to use Python's raw string notation for regular expression
 patterns; 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. Usually patterns will be expressed in Python code using this raw string
-notation.
+newline.  Usually patterns will be expressed in Python code using this raw
+string notation.
 
+It is important to note that most regular expression operations are available as
+module-level functions and :class:`RegexObject` methods.  The functions are
+shortcuts that don't require you to compile a regex object first, but miss some
+fine-tuning parameters.
 
 .. seealso::
 
    Mastering Regular Expressions
       Book on regular expressions by Jeffrey Friedl, published by O'Reilly.  The
-      second  edition of the book no longer covers Python at all,  but the first
+      second edition of the book no longer covers Python at all, but the first
       edition covered writing good regular expression patterns in great detail.
 
 
 .. _re-syntax:
 
 Regular Expression Syntax
 -------------------------
 
 A regular expression (or RE) specifies a set of strings that matches it; the
 functions in this module let you check if a particular string matches a given
 regular expression (or if a given regular expression matches a particular
 string, which comes down to the same thing).
 
 Regular expressions can be concatenated to form new regular expressions; if *A*
 and *B* are both regular expressions, then *AB* is also a regular expression.
 In general, if a string *p* matches *A* and another string *q* matches *B*, the
 string *pq* will match AB.  This holds unless *A* or *B* contain low precedence
 operations; boundary conditions between *A* and *B*; or have numbered group
 references.  Thus, complex expressions can easily be constructed from simpler
 primitive expressions like the ones described here.  For details of the theory
 and implementation of regular expressions, consult the Friedl book referenced
 above, or almost any textbook about compiler construction.
 
 A brief explanation of the format of regular expressions follows.  For further
-information and a gentler presentation, consult the Regular Expression HOWTO,
-accessible from `<http://www.python.org/doc/howto/>`_.
+information and a gentler presentation, consult the :ref:`regex-howto`.
 
 Regular expressions can contain both special and ordinary characters. Most
 ordinary characters, like ``'A'``, ``'a'``, or ``'0'``, are the simplest regular
 expressions; they simply match themselves.  You can concatenate ordinary
 characters, so ``last`` matches the string ``'last'``.  (In the rest of this
 section, we'll write RE's in ``this special style``, usually without quotes, and
 strings to be matched ``'in single quotes'``.)
 
-Some characters, like ``'|'`` or ``'('``, are special. Special characters either
-stand for classes of ordinary characters, or affect how the regular expressions
-around them are interpreted.
+Some characters, like ``'|'`` or ``'('``, are special. Special
+characters either stand for classes of ordinary characters, or affect
+how the regular expressions around them are interpreted. Regular
+expression pattern strings may not contain null bytes, but can specify
+the null byte using the ``\number`` notation, e.g., ``'\x00'``.
 
-The special characters are:
 
-.. % 
+The special characters are:
 
 ``'.'``
    (Dot.)  In the default mode, this matches any character except a newline.  If
    the :const:`DOTALL` flag has been specified, this matches any character
    including a newline.
 
 ``'^'``
    (Caret.)  Matches the start of the string, and in :const:`MULTILINE` mode also
    matches immediately after each newline.
 
 ``'$'``
    Matches the end of the string or just before the newline at the end of the
    string, and in :const:`MULTILINE` mode also matches before a newline.  ``foo``
    matches both 'foo' and 'foobar', while the regular expression ``foo$`` matches
-   only 'foo'.  More interestingly, searching for ``foo.$`` in 'foo1\\nfoo2\\n'
-   matches 'foo2' normally, but 'foo1' in :const:`MULTILINE` mode.
+   only 'foo'.  More interestingly, searching for ``foo.$`` in ``'foo1\nfoo2\n'``
+   matches 'foo2' normally, but 'foo1' in :const:`MULTILINE` mode; searching for
+   a single ``$`` in ``'foo\n'`` will find two (empty) matches: one just before
+   the newline, and one at the end of the string.
 
 ``'*'``
    Causes the resulting RE to match 0 or more repetitions of the preceding RE, as
    many repetitions as are possible.  ``ab*`` will match 'a', 'ab', or 'a' followed
    by any number of 'b's.
 
 ``'+'``
    Causes the resulting RE to match 1 or more repetitions of the preceding RE.
    ``ab+`` will match 'a' followed by any non-zero number of 'b's; it will not
    match just 'a'.
 
 ``'?'``
@@ -124,59 +128,68 @@
 
 ``{m,n}``
    Causes the resulting RE to match from *m* to *n* repetitions of the preceding
    RE, attempting to match as many repetitions as possible.  For example,
    ``a{3,5}`` will match from 3 to 5 ``'a'`` characters.  Omitting *m* specifies a
    lower bound of zero,  and omitting *n* specifies an infinite upper bound.  As an
    example, ``a{4,}b`` will match ``aaaab`` or a thousand ``'a'`` characters
    followed by a ``b``, but not ``aaab``. The comma may not be omitted or the
    modifier would be confused with the previously described form.
 
 ``{m,n}?``
    Causes the resulting RE to match from *m* to *n* repetitions of the preceding
-   RE, attempting to match as *few* repetitions as possible.  This is the non-
-   greedy version of the previous qualifier.  For example, on the 6-character
-   string ``'aaaaaa'``, ``a{3,5}`` will match 5 ``'a'`` characters, while
-   ``a{3,5}?`` will only match 3 characters.
+   RE, attempting to match as *few* repetitions as possible.  This is the
+   non-greedy version of the previous qualifier.  For example, on the
+   6-character string ``'aaaaaa'``, ``a{3,5}`` will match 5 ``'a'`` characters,
+   while ``a{3,5}?`` will only match 3 characters.
 
 ``'\'``
    Either escapes special characters (permitting you to match characters like
    ``'*'``, ``'?'``, and so forth), or signals a special sequence; special
    sequences are discussed below.
 
    If you're not using a raw string to express the pattern, remember that Python
    also uses the backslash as an escape sequence in string literals; if the escape
    sequence isn't recognized by Python's parser, the backslash and subsequent
    character are included in the resulting string.  However, if Python would
    recognize the resulting sequence, the backslash should be repeated twice.  This
    is complicated and hard to understand, so it's highly recommended that you use
    raw strings for all but the simplest expressions.
 
 ``[]``
    Used to indicate a set of characters.  Characters can be listed individually, or
    a range of characters can be indicated by giving two characters and separating
    them by a ``'-'``.  Special characters are not active inside sets.  For example,
-   ``[akm$]`` will match any of the characters ``'a'``, ``'k'``, ``'m'``, or
-   ``'$'``; ``[a-z]`` will match any lowercase letter, and ``[a-zA-Z0-9]`` matches
-   any letter or digit.  Character classes such as ``\w`` or ``\S`` (defined below)
-   are also acceptable inside a range.  If you want to include a ``']'`` or a
-   ``'-'`` inside a set, precede it with a backslash, or place it as the first
-   character.  The pattern ``[]]`` will match ``']'``, for example.
+   ``[akm$]`` will match any of the characters ``'a'``, ``'k'``,
+   ``'m'``, or ``'$'``; ``[a-z]`` will match any lowercase letter, and
+   ``[a-zA-Z0-9]`` matches any letter or digit.  Character classes such
+   as ``\w`` or ``\S`` (defined below) are also acceptable inside a
+   range, although the characters they match depends on whether :const:`LOCALE`
+   or  :const:`UNICODE` mode is in force.  If you want to include a
+   ``']'`` or a ``'-'`` inside a set, precede it with a backslash, or
+   place it as the first character.  The pattern ``[]]`` will match
+   ``']'``, for example.
 
    You can match the characters not within a range by :dfn:`complementing` the set.
    This is indicated by including a ``'^'`` as the first character of the set;
    ``'^'`` elsewhere will simply match the ``'^'`` character.  For example,
    ``[^5]`` will match any character except ``'5'``, and ``[^^]`` will match any
    character except ``'^'``.
 
+   Note that inside ``[]`` the special forms and special characters lose
+   their meanings and only the syntaxes described here are valid. For
+   example, ``+``, ``*``, ``(``, ``)``, and so on are treated as
+   literals inside ``[]``, and backreferences cannot be used inside
+   ``[]``.
+
 ``'|'``
    ``A|B``, where A and B can be arbitrary REs, creates a regular expression that
    will match either A or B.  An arbitrary number of REs can be separated by the
    ``'|'`` in this way.  This can be used inside groups (see below) as well.  As
    the target string is scanned, REs separated by ``'|'`` are tried from left to
    right. When one pattern completely matches, that branch is accepted. This means
    that once ``A`` matches, ``B`` will not be tested further, even if it would
    produce a longer overall match.  In other words, the ``'|'`` operator is never
    greedy.  To match a literal ``'|'``, use ``\|``, or enclose it inside a
    character class, as in ``[|]``.
 
 ``(...)``
@@ -185,54 +198,60 @@
    has been performed, and can be matched later in the string with the ``\number``
    special sequence, described below.  To match the literals ``'('`` or ``')'``,
    use ``\(`` or ``\)``, or enclose them inside a character class: ``[(] [)]``.
 
 ``(?...)``
    This is an extension notation (a ``'?'`` following a ``'('`` is not meaningful
    otherwise).  The first character after the ``'?'`` determines what the meaning
    and further syntax of the construct is. Extensions usually do not create a new
    group; ``(?P<name>...)`` is the only exception to this rule. Following are the
    currently supported extensions.
 
 ``(?iLmsux)``
-   (One or more letters from the set ``'i'``, ``'L'``, ``'m'``, ``'s'``, ``'u'``,
-   ``'x'``.)  The group matches the empty string; the letters set the corresponding
-   flags (:const:`re.I`, :const:`re.L`, :const:`re.M`, :const:`re.S`,
-   :const:`re.U`, :const:`re.X`) for the entire regular expression.  This is useful
-   if you wish to include the flags as part of the regular expression, instead of
-   passing a *flag* argument to the :func:`compile` function.
+   (One or more letters from the set ``'i'``, ``'L'``, ``'m'``, ``'s'``,
+   ``'u'``, ``'x'``.)  The group matches the empty string; the letters
+   set the corresponding flags: :const:`re.I` (ignore case),
+   :const:`re.L` (locale dependent), :const:`re.M` (multi-line),
+   :const:`re.S` (dot matches all), :const:`re.U` (Unicode dependent),
+   and :const:`re.X` (verbose), for the entire regular expression. (The
+   flags are described in :ref:`contents-of-module-re`.) This
+   is useful if you wish to include the flags as part of the regular
+   expression, instead of passing a *flag* argument to the
+   :func:`compile` function.
 
    Note that the ``(?x)`` flag changes how the expression is parsed. It should be
    used first in the expression string, or after one or more whitespace characters.
    If there are non-whitespace characters before the flag, the results are
    undefined.
 
 ``(?:...)``
    A non-grouping version of regular parentheses. Matches whatever regular
    expression is inside the parentheses, but the substring matched by the group
    *cannot* be retrieved after performing a match or referenced later in the
    pattern.
 
 ``(?P<name>...)``
    Similar to regular parentheses, but the substring matched by the group is
-   accessible via the symbolic group name *name*.  Group names must be valid Python
-   identifiers, and each group name must be defined only once within a regular
-   expression.  A symbolic group is also a numbered group, just as if the group
-   were not named.  So the group named 'id' in the example above can also be
-   referenced as the numbered group 1.
+   accessible within the rest of the regular expression via the symbolic group
+   name *name*.  Group names must be valid Python identifiers, and each group
+   name must be defined only once within a regular expression.  A symbolic group
+   is also a numbered group, just as if the group were not named.  So the group
+   named ``id`` in the example below can also be referenced as the numbered group
+   ``1``.
 
    For example, if the pattern is ``(?P<id>[a-zA-Z_]\w*)``, the group can be
    referenced by its name in arguments to methods of match objects, such as
-   ``m.group('id')`` or ``m.end('id')``, and also by name in pattern text (for
-   example, ``(?P=id)``) and replacement text (such as ``\g<id>``).
+   ``m.group('id')`` or ``m.end('id')``, and also by name in the regular
+   expression itself (using ``(?P=id)``) and replacement text given to
+   ``.sub()`` (using ``\g<id>``).
 
 ``(?P=name)``
    Matches whatever text was matched by the earlier group named *name*.
 
 ``(?#...)``
    A comment; the contents of the parentheses are simply ignored.
 
 ``(?=...)``
    Matches if ``...`` matches next, but doesn't consume any of the string.  This is
    called a lookahead assertion.  For example, ``Isaac (?=Asimov)`` will match
    ``'Isaac '`` only if it's followed by ``'Asimov'``.
 
@@ -241,84 +260,82 @@
    For example, ``Isaac (?!Asimov)`` will match ``'Isaac '`` only if it's *not*
    followed by ``'Asimov'``.
 
 ``(?<=...)``
    Matches if the current position in the string is preceded by a match for ``...``
    that ends at the current position.  This is called a :dfn:`positive lookbehind
    assertion`. ``(?<=abc)def`` will find a match in ``abcdef``, since the
    lookbehind will back up 3 characters and check if the contained pattern matches.
    The contained pattern must only match strings of some fixed length, meaning that
    ``abc`` or ``a|b`` are allowed, but ``a*`` and ``a{3,4}`` are not.  Note that
    patterns which start with positive lookbehind assertions will never match at the
    beginning of the string being searched; you will most likely want to use the
-   :func:`search` function rather than the :func:`match` function::
+   :func:`search` function rather than the :func:`match` function:
 
       >>> import re
       >>> m = re.search('(?<=abc)def', 'abcdef')
       >>> m.group(0)
       'def'
 
-   This example looks for a word following a hyphen::
+   This example looks for a word following a hyphen:
 
       >>> m = re.search('(?<=-)\w+', 'spam-egg')
       >>> m.group(0)
       'egg'
 
 ``(?<!...)``
    Matches if the current position in the string is not preceded by a match for
    ``...``.  This is called a :dfn:`negative lookbehind assertion`.  Similar to
    positive lookbehind assertions, the contained pattern must only match strings of
    some fixed length.  Patterns which start with negative lookbehind assertions may
    match at the beginning of the string being searched.
 
 ``(?(id/name)yes-pattern|no-pattern)``
    Will try to match with ``yes-pattern`` if the group with given *id* or *name*
-   exists, and with ``no-pattern`` if it doesn't. ``|no-pattern`` is optional and
+   exists, and with ``no-pattern`` if it doesn't. ``no-pattern`` is optional and
    can be omitted. For example,  ``(<)?(\w+@\w+(?:\.\w+)+)(?(1)>)`` is a poor email
    matching pattern, which will match with ``'<user@host.com>'`` as well as
    ``'user@host.com'``, but not with ``'<user@host.com'``.
 
    .. versionadded:: 2.4
 
 The special sequences consist of ``'\'`` and a character from the list below.
 If the ordinary character is not on the list, then the resulting RE will match
 the second character.  For example, ``\$`` matches the character ``'$'``.
 
-.. % 
-
 ``\number``
    Matches the contents of the group of the same number.  Groups are numbered
    starting from 1.  For example, ``(.+) \1`` matches ``'the the'`` or ``'55 55'``,
    but not ``'the end'`` (note the space after the group).  This special sequence
    can only be used to match one of the first 99 groups.  If the first digit of
    *number* is 0, or *number* is 3 octal digits long, it will not be interpreted as
    a group match, but as the character with octal value *number*. Inside the
    ``'['`` and ``']'`` of a character class, all numeric escapes are treated as
    characters.
 
 ``\A``
    Matches only at the start of the string.
 
 ``\b``
    Matches the empty string, but only at the beginning or end of a word.  A word is
    defined as a sequence of alphanumeric or underscore characters, so the end of a
    word is indicated by whitespace or a non-alphanumeric, non-underscore character.
    Note that  ``\b`` is defined as the boundary between ``\w`` and ``\ W``, so the
    precise set of characters deemed to be alphanumeric depends on the values of the
    ``UNICODE`` and ``LOCALE`` flags.  Inside a character range, ``\b`` represents
    the backspace character, for compatibility with Python's string literals.
 
 ``\B``
    Matches the empty string, but only when it is *not* at the beginning or end of a
-   word.  This is just the opposite of ``\ b``, so is also subject to the settings
+   word.  This is just the opposite of ``\b``, so is also subject to the settings
    of ``LOCALE`` and ``UNICODE``.
 
 ``\d``
    When the :const:`UNICODE` flag is not specified, matches any decimal digit; this
    is equivalent to the set ``[0-9]``.  With :const:`UNICODE`, it will match
    whatever is classified as a digit in the Unicode character properties database.
 
 ``\D``
    When the :const:`UNICODE` flag is not specified, matches any non-digit
    character; this is equivalent to the set  ``[^0-9]``.  With :const:`UNICODE`, it
    will match  anything other than character marked as digits in the Unicode
    character  properties database.
@@ -361,110 +378,104 @@
 Most of the standard escapes supported by Python string literals are also
 accepted by the regular expression parser::
 
    \a      \b      \f      \n
    \r      \t      \v      \x
    \\
 
 Octal escapes are included in a limited form: If the first digit is a 0, or if
 there are three octal digits, it is considered an octal escape. Otherwise, it is
 a group reference.  As for string literals, octal escapes are always at most
 three digits in length.
 
-.. % Note the lack of a period in the section title; it causes problems
-.. % with readers of the GNU info version.  See http://www.python.org/sf/581414.
-
 
 .. _matching-searching:
 
 Matching vs Searching
 ---------------------
 
 .. sectionauthor:: Fred L. Drake, Jr. <fdrake@acm.org>
 
 
 Python offers two different primitive operations based on regular expressions:
-match and search.  If you are accustomed to Perl's semantics, the search
-operation is what you're looking for.  See the :func:`search` function and
-corresponding method of compiled regular expression objects.
+**match** checks for a match only at the beginning of the string, while
+**search** checks for a match anywhere in the string (this is what Perl does
+by default).
 
-Note that match may differ from search using a regular expression beginning with
-``'^'``: ``'^'`` matches only at the start of the string, or in
+Note that match may differ from search even when using a regular expression
+beginning with ``'^'``: ``'^'`` matches only at the start of the string, or in
 :const:`MULTILINE` mode also immediately following a newline.  The "match"
 operation succeeds only if the pattern matches at the start of the string
 regardless of mode, or at the starting position given by the optional *pos*
 argument regardless of whether a newline precedes it.
 
-.. % Examples from Tim Peters:
-
-::
+   >>> re.match("c", "abcdef")  # No match
+   >>> re.search("c", "abcdef") # Match
+   <_sre.SRE_Match object at ...>
 
-   re.compile("a").match("ba", 1)           # succeeds
-   re.compile("^a").search("ba", 1)         # fails; 'a' not at start
-   re.compile("^a").search("\na", 1)        # fails; 'a' not at start
-   re.compile("^a", re.M).search("\na", 1)  # succeeds
-   re.compile("^a", re.M).search("ba", 1)   # fails; no preceding \n
 
+.. _contents-of-module-re:
 
 Module Contents
 ---------------
 
-.. _contents of module re:
-
 The module defines several functions, constants, and an exception. Some of the
 functions are simplified versions of the full featured methods for compiled
 regular expressions.  Most non-trivial applications always use the compiled
 form.
 
 
 .. function:: compile(pattern[, flags])
 
-   Compile a regular expression pattern into a regular expression object, which can
-   be used for matching using its :func:`match` and :func:`search` methods,
+   Compile a regular expression pattern into a regular expression object, which
+   can be used for matching using its :func:`match` and :func:`search` methods,
    described below.
 
    The expression's behaviour can be modified by specifying a *flags* value.
    Values can be any of the following variables, combined using bitwise OR (the
    ``|`` operator).
 
    The sequence ::
 
-      prog = re.compile(pat)
-      result = prog.match(str)
+      prog = re.compile(pattern)
+      result = prog.match(string)
 
    is equivalent to ::
 
-      result = re.match(pat, str)
+      result = re.match(pattern, string)
 
-   but the version using :func:`compile` is more efficient when the expression will
-   be used several times in a single program.
+   but using :func:`compile` and saving the resulting regular expression object
+   for reuse is more efficient when the expression will be used several times
+   in a single program.
 
-   .. % (The compiled version of the last pattern passed to
-   .. % \function{re.match()} or \function{re.search()} is cached, so
-   .. % programs that use only a single regular expression at a time needn't
-   .. % worry about compiling regular expressions.)
+   .. note::
+
+      The compiled versions of the most recent patterns passed to
+      :func:`re.match`, :func:`re.search` or :func:`re.compile` are cached, so
+      programs that use only a few regular expressions at a time needn't worry
+      about compiling regular expressions.
 
 
 .. data:: I
           IGNORECASE
 
    Perform case-insensitive matching; expressions like ``[A-Z]`` will match
    lowercase letters, too.  This is not affected by the current locale.
 
 
 .. data:: L
           LOCALE
 
-   Make ``\w``, ``\W``, ``\b``, ``\B``, ``\s`` and ``\S`` dependent on the current
-   locale.
+   Make ``\w``, ``\W``, ``\b``, ``\B``, ``\s`` and ``\S`` dependent on the
+   current locale.
 
 
 .. data:: M
           MULTILINE
 
    When specified, the pattern character ``'^'`` matches at the beginning of the
    string and at the beginning of each line (immediately following each newline);
    and the pattern character ``'$'`` matches at the end of the string and at the
    end of each line (immediately preceding each newline).  By default, ``'^'``
    matches only at the beginning of the string, and ``'$'`` only at the end of the
    string and immediately before the newline (if any) at the end of the string.
 
@@ -485,109 +496,139 @@
    .. versionadded:: 2.0
 
 
 .. data:: X
           VERBOSE
 
    This flag allows you to write regular expressions that look nicer. Whitespace
    within the pattern is ignored, except when in a character class or preceded by
    an unescaped backslash, and, when a line contains a ``'#'`` neither in a
    character class or preceded by an unescaped backslash, all characters from the
    leftmost such ``'#'`` through the end of the line are ignored.
 
-   .. % XXX should add an example here
+   That means that the two following regular expression objects that match a
+   decimal number are functionally equal::
+
+      a = re.compile(r"""\d +  # the integral part
+                         \.    # the decimal point
+                         \d *  # some fractional digits""", re.X)
+      b = re.compile(r"\d+\.\d*")
 
 
 .. function:: search(pattern, string[, flags])
 
    Scan through *string* looking for a location where the regular expression
    *pattern* produces a match, and return a corresponding :class:`MatchObject`
    instance. Return ``None`` if no position in the string matches the pattern; note
    that this is different from finding a zero-length match at some point in the
    string.
 
 
 .. function:: match(pattern, string[, flags])
 
    If zero or more characters at the beginning of *string* match the regular
    expression *pattern*, return a corresponding :class:`MatchObject` instance.
    Return ``None`` if the string does not match the pattern; note that this is
    different from a zero-length match.
 
    .. note::
 
-      If you want to locate a match anywhere in *string*, use :meth:`search` instead.
+      If you want to locate a match anywhere in *string*, use :meth:`search`
+      instead.
 
 
-.. function:: split(pattern, string[, maxsplit\ ``= 0``])
+.. function:: split(pattern, string[, maxsplit=0])
 
    Split *string* by the occurrences of *pattern*.  If capturing parentheses are
    used in *pattern*, then the text of all groups in the pattern are also returned
    as part of the resulting list. If *maxsplit* is nonzero, at most *maxsplit*
    splits occur, and the remainder of the string is returned as the final element
    of the list.  (Incompatibility note: in the original Python 1.5 release,
-   *maxsplit* was ignored.  This has been fixed in later releases.) ::
+   *maxsplit* was ignored.  This has been fixed in later releases.)
 
       >>> 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.']
 
+   If there are capturing groups in the separator and it matches at the start of
+   the string, the result will start with an empty string.  The same holds for
+   the end of the string:
+
+      >>> re.split('(\W+)', '...words, words...')
+      ['', '...', 'words', ', ', 'words', '...', '']
+
+   That way, separator components are always found at the same relative
+   indices within the result list (e.g., if there's one capturing group
+   in the separator, the 0th, the 2nd and so forth).
+
+   Note that *split* will never split a string on an empty pattern match.
+   For example:
+
+      >>> re.split('x*', 'foo')
+      ['foo']
+      >>> re.split("(?m)^$", "foo\n\nbar\n")
+      ['foo\n\nbar\n']
+
 
 .. function:: findall(pattern, string[, flags])
 
-   Return a list of all non-overlapping matches of *pattern* in *string*.  If one
-   or more groups are present in the pattern, return a list of groups; this will be
-   a list of tuples if the pattern has more than one group.  Empty matches are
-   included in the result unless they touch the beginning of another match.
+   Return all non-overlapping matches of *pattern* in *string*, as a list of
+   strings.  The *string* is scanned left-to-right, and matches are returned in
+   the order found.  If one or more groups are present in the pattern, return a
+   list of groups; this will be a list of tuples if the pattern has more than
+   one group.  Empty matches are included in the result unless they touch the
+   beginning of another match.
 
    .. versionadded:: 1.5.2
 
    .. versionchanged:: 2.4
       Added the optional flags argument.
 
 
 .. function:: finditer(pattern, string[, flags])
 
-   Return an iterator over all non-overlapping matches for the RE *pattern* in
-   *string*.  For each match, the iterator returns a match object.  Empty matches
-   are included in the result unless they touch the beginning of another match.
+   Return an :term:`iterator` yielding :class:`MatchObject` instances over all
+   non-overlapping matches for the RE *pattern* in *string*.  The *string* is
+   scanned left-to-right, and matches are returned in the order found.  Empty
+   matches are included in the result unless they touch the beginning of another
+   match.
 
    .. versionadded:: 2.2
 
    .. versionchanged:: 2.4
       Added the optional flags argument.
 
 
 .. function:: sub(pattern, repl, string[, count])
 
    Return the string obtained by replacing the leftmost non-overlapping occurrences
    of *pattern* in *string* by the replacement *repl*.  If the pattern isn't found,
    *string* is returned unchanged.  *repl* can be a string or a function; if it 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 linefeed, and
    so forth.  Unknown escapes such as ``\j`` are left alone.  Backreferences, such
    as ``\6``, are replaced with the substring matched by group 6 in the pattern.
-   For example::
+   For example:
 
       >>> re.sub(r'def\s+([a-zA-Z_][a-zA-Z_0-9]*)\s*\(\s*\):',
       ...        r'static PyObject*\npy_\1(void)\n{',
       ...        'def myfunc():')
       'static PyObject*\npy_myfunc(void)\n{'
 
    If *repl* is a function, it is called for every non-overlapping occurrence of
    *pattern*.  The function takes a single match object argument, and returns the
-   replacement string.  For example::
+   replacement string.  For example:
 
       >>> def dashrepl(matchobj):
       ...     if matchobj.group(0) == '-': return ' '
       ...     else: return '-'
       >>> re.sub('-{1,2}', dashrepl, 'pro----gram-files')
       'pro--gram files'
 
    The pattern may be a string or an RE object; if you need to specify regular
    expression flags, you must use a RE object, or use embedded modifiers in a
    pattern; for example, ``sub("(?i)b+", "x", "bbbb BBBB")`` returns ``'x x'``.
 
    The optional argument *count* is the maximum number of pattern occurrences to be
@@ -636,100 +677,113 @@
 attributes:
 
 
 .. method:: RegexObject.match(string[, pos[, endpos]])
 
    If zero or more characters at the beginning of *string* match this regular
    expression, return a corresponding :class:`MatchObject` instance.  Return
    ``None`` if the string does not match the pattern; note that this is different
    from a zero-length match.
 
    .. note::
 
-      If you want to locate a match anywhere in *string*, use :meth:`search` instead.
+      If you want to locate a match anywhere in *string*, use :meth:`search`
+      instead.
 
    The optional second parameter *pos* gives an index in the string where the
    search is to start; it defaults to ``0``.  This is not completely equivalent to
    slicing the string; the ``'^'`` pattern character matches at the real beginning
    of the string and at positions just after a newline, but not necessarily at the
    index where the search is to start.
 
    The optional parameter *endpos* limits how far the string will be searched; it
    will be as if the string is *endpos* characters long, so only the characters
    from *pos* to ``endpos - 1`` will be searched for a match.  If *endpos* is less
    than *pos*, no match will be found, otherwise, if *rx* is a compiled regular
    expression object, ``rx.match(string, 0, 50)`` is equivalent to
    ``rx.match(string[:50], 0)``.
 
+      >>> pattern = re.compile("o")
+      >>> pattern.match("dog")      # No match as "o" is not at the start of "dog."
+      >>> pattern.match("dog", 1)   # Match as "o" is the 2nd character of "dog".
+      <_sre.SRE_Match object at ...>
+
 
 .. method:: RegexObject.search(string[, pos[, endpos]])
 
    Scan through *string* looking for a location where this regular expression
    produces a match, and return a corresponding :class:`MatchObject` instance.
    Return ``None`` if no position in the string matches the pattern; note that this
    is different from finding a zero-length match at some point in the string.
 
    The optional *pos* and *endpos* parameters have the same meaning as for the
    :meth:`match` method.
 
 
-.. method:: RegexObject.split(string[, maxsplit\ ``= 0``])
+.. method:: RegexObject.split(string[, maxsplit=0])
 
    Identical to the :func:`split` function, using the compiled pattern.
 
 
 .. method:: RegexObject.findall(string[, pos[, endpos]])
 
    Identical to the :func:`findall` function, using the compiled pattern.
 
 
 .. method:: RegexObject.finditer(string[, pos[, endpos]])
 
    Identical to the :func:`finditer` function, using the compiled pattern.
 
 
-.. method:: RegexObject.sub(repl, string[, count\ ``= 0``])
+.. method:: RegexObject.sub(repl, string[, count=0])
 
    Identical to the :func:`sub` function, using the compiled pattern.
 
 
-.. method:: RegexObject.subn(repl, string[, count\ ``= 0``])
+.. method:: RegexObject.subn(repl, string[, count=0])
 
    Identical to the :func:`subn` function, using the compiled pattern.
 
 
 .. attribute:: RegexObject.flags
 
    The flags argument used when the RE object was compiled, or ``0`` if no flags
    were provided.
 
 
+.. attribute:: RegexObject.groups
+
+   The number of capturing groups in the pattern.
+
+
 .. attribute:: RegexObject.groupindex
 
    A dictionary mapping any symbolic group names defined by ``(?P<id>)`` to group
    numbers.  The dictionary is empty if no symbolic groups were used in the
    pattern.
 
 
 .. attribute:: RegexObject.pattern
 
    The pattern string from which the RE object was compiled.
 
 
 .. _match-objects:
 
 Match Objects
 -------------
 
-:class:`MatchObject` instances support the following methods and attributes:
+Match objects always have a boolean value of :const:`True`, so that you can test
+whether e.g. :func:`match` resulted in a match with a simple if statement.  They
+support the following methods and attributes:
 
 
 .. method:: MatchObject.expand(template)
 
    Return the string obtained by doing backslash substitution on the template
    string *template*, as done by the :meth:`sub` method. Escapes such as ``\n`` are
    converted to the appropriate characters, and numeric backreferences (``\1``,
    ``\2``) and named backreferences (``\g<1>``, ``\g<name>``) are replaced by the
    contents of the corresponding group.
 
 
 .. method:: MatchObject.group([group1, ...])
@@ -737,76 +791,127 @@
    Returns one or more subgroups of the match.  If there is a single argument, the
    result is a single string; if there are multiple arguments, the result is a
    tuple with one item per argument. Without arguments, *group1* defaults to zero
    (the whole match is returned). If a *groupN* argument is zero, the corresponding
    return value is the entire matching string; if it is in the inclusive range
    [1..99], it is the string matching the corresponding parenthesized group.  If a
    group number is negative or larger than the number of groups defined in the
    pattern, an :exc:`IndexError` exception is raised. If a group is contained in a
    part of the pattern that did not match, the corresponding result is ``None``.
    If a group is contained in a part of the pattern that matched multiple times,
    the last match is returned.
 
+      >>> m = re.match(r"(\w+) (\w+)", "Isaac Newton, physicist")
+      >>> m.group(0)       # The entire match
+      'Isaac Newton'
+      >>> m.group(1)       # The first parenthesized subgroup.
+      'Isaac'
+      >>> m.group(2)       # The second parenthesized subgroup.
+      'Newton'
+      >>> m.group(1, 2)    # Multiple arguments give us a tuple.
+      ('Isaac', 'Newton')
+
    If the regular expression uses the ``(?P<name>...)`` syntax, the *groupN*
    arguments may also be strings identifying groups by their group name.  If a
    string argument is not used as a group name in the pattern, an :exc:`IndexError`
    exception is raised.
 
-   A moderately complicated example::
-
-      m = re.match(r"(?P<int>\d+)\.(\d*)", '3.14')
+   A moderately complicated example:
 
-   After performing this match, ``m.group(1)`` is ``'3'``, as is
-   ``m.group('int')``, and ``m.group(2)`` is ``'14'``.
+      >>> m = re.match(r"(?P<first_name>\w+) (?P<last_name>\w+)", "Malcom Reynolds")
+      >>> m.group('first_name')
+      'Malcom'
+      >>> m.group('last_name')
+      'Reynolds'
+
+   Named groups can also be referred to by their index:
+
+      >>> m.group(1)
+      'Malcom'
+      >>> m.group(2)
+      'Reynolds'
+
+   If a group matches multiple times, only the last match is accessible:
+
+      >>> m = re.match(r"(..)+", "a1b2c3")  # Matches 3 times.
+      >>> m.group(1)                        # Returns only the last match.
+      'c3'
 
 
 .. method:: MatchObject.groups([default])
 
    Return a tuple containing all the subgroups of the match, from 1 up to however
    many groups are in the pattern.  The *default* argument is used for groups that
    did not participate in the match; it defaults to ``None``.  (Incompatibility
    note: in the original Python 1.5 release, if the tuple was one element long, a
    string would be returned instead.  In later versions (from 1.5.1 on), a
    singleton tuple is returned in such cases.)
 
+   For example:
+
+      >>> m = re.match(r"(\d+)\.(\d+)", "24.1632")
+      >>> m.groups()
+      ('24', '1632')
+
+   If we make the decimal place and everything after it optional, not all groups
+   might participate in the match.  These groups will default to ``None`` unless
+   the *default* argument is given:
+
+      >>> m = re.match(r"(\d+)\.?(\d+)?", "24")
+      >>> m.groups()      # Second group defaults to None.
+      ('24', None)
+      >>> m.groups('0')   # Now, the second group defaults to '0'.
+      ('24', '0')
+
 
 .. method:: MatchObject.groupdict([default])
 
    Return a dictionary containing all the *named* subgroups of the match, keyed by
    the subgroup name.  The *default* argument is used for groups that did not
-   participate in the match; it defaults to ``None``.
+   participate in the match; it defaults to ``None``.  For example:
+
+      >>> m = re.match(r"(?P<first_name>\w+) (?P<last_name>\w+)", "Malcom Reynolds")
+      >>> m.groupdict()
+      {'first_name': 'Malcom', 'last_name': 'Reynolds'}
 
 
 .. method:: MatchObject.start([group])
-            XXX Class.end([group])
+            MatchObject.end([group])
 
    Return the indices of the start and end of the substring matched by *group*;
    *group* defaults to zero (meaning the whole matched substring). Return ``-1`` if
    *group* exists but did not contribute to the match.  For a match object *m*, and
    a group *g* that did contribute to the match, the substring matched by group *g*
    (equivalent to ``m.group(g)``) is ::
 
       m.string[m.start(g):m.end(g)]
 
    Note that ``m.start(group)`` will equal ``m.end(group)`` if *group* matched a
    null string.  For example, after ``m = re.search('b(c?)', 'cba')``,
    ``m.start(0)`` is 1, ``m.end(0)`` is 2, ``m.start(1)`` and ``m.end(1)`` are both
    2, and ``m.start(2)`` raises an :exc:`IndexError` exception.
 
+   An example that will remove *remove_this* from email addresses:
+
+      >>> email = "tony@tiremove_thisger.net"
+      >>> m = re.search("remove_this", email)
+      >>> email[:m.start()] + email[m.end():]
+      'tony@tiger.net'
+
 
 .. method:: MatchObject.span([group])
 
    For :class:`MatchObject` *m*, return the 2-tuple ``(m.start(group),
    m.end(group))``. Note that if *group* did not contribute to the match, this is
-   ``(-1, -1)``.  Again, *group* defaults to zero.
+   ``(-1, -1)``.  *group* defaults to zero, the entire match.
 
 
 .. attribute:: MatchObject.pos
 
    The value of *pos* which was passed to the :func:`search` or :func:`match`
    method of the :class:`RegexObject`.  This is the index into the string at which
    the RE engine started looking for a match.
 
 
 .. attribute:: MatchObject.endpos
 
    The value of *endpos* which was passed to the :func:`search` or :func:`match`
@@ -834,25 +939,84 @@
    The regular expression object whose :meth:`match` or :meth:`search` method
    produced this :class:`MatchObject` instance.
 
 
 .. attribute:: MatchObject.string
 
    The string passed to :func:`match` or :func:`search`.
 
 
 Examples
 --------
 
-**Simulating scanf()**
+
+Checking For a Pair
+^^^^^^^^^^^^^^^^^^^
+
+In this example, we'll use the following helper function to display match
+objects a little more gracefully:
+
+.. testcode::
+
+   def displaymatch(match):
+       if match is None:
+           return None
+       return '<Match: %r, groups=%r>' % (match.group(), match.groups())
+
+Suppose you are writing a poker program where a player's hand is represented as
+a 5-character string with each character representing a card, "a" for ace, "k"
+for king, "q" for queen, j for jack, "0" for 10, and "1" through "9"
+representing the card with that value.
+
+To see if a given string is a valid hand, one could do the following:
+
+   >>> valid = re.compile(r"[0-9akqj]{5}$")
+   >>> displaymatch(valid.match("ak05q"))  # Valid.
+   "<Match: 'ak05q', groups=()>"
+   >>> displaymatch(valid.match("ak05e"))  # Invalid.
+   >>> displaymatch(valid.match("ak0"))    # Invalid.
+   >>> displaymatch(valid.match("727ak"))  # Valid.
+   "<Match: '727ak', groups=()>"
+
+That last hand, ``"727ak"``, contained a pair, or two of the same valued cards.
+To match this with a regular expression, one could use backreferences as such:
+
+   >>> pair = re.compile(r".*(.).*\1")
+   >>> displaymatch(pair.match("717ak"))     # Pair of 7s.
+   "<Match: '717', groups=('7',)>"
+   >>> displaymatch(pair.match("718ak"))     # No pairs.
+   >>> displaymatch(pair.match("354aa"))     # Pair of aces.
+   "<Match: '354aa', groups=('a',)>"
+
+To find out what card the pair consists of, one could use the :func:`group`
+method of :class:`MatchObject` in the following manner:
+
+.. doctest::
+
+   >>> pair.match("717ak").group(1)
+   '7'
+
+   # Error because re.match() returns None, which doesn't have a group() method:
+   >>> pair.match("718ak").group(1)
+   Traceback (most recent call last):
+     File "<pyshell#23>", line 1, in <module>
+       re.match(r".*(.).*\1", "718ak").group(1)
+   AttributeError: 'NoneType' object has no attribute 'group'
+
+   >>> pair.match("354aa").group(1)
+   'a'
+
+
+Simulating scanf()
+^^^^^^^^^^^^^^^^^^
 
 .. index:: single: scanf()
 
 Python does not currently have an equivalent to :cfunc:`scanf`.  Regular
 expressions are generally more powerful, though also more verbose, than
 :cfunc:`scanf` format strings.  The table below offers some more-or-less
 equivalent mappings between :cfunc:`scanf` format tokens and regular
 expressions.
 
 +--------------------------------+---------------------------------------------+
 | :cfunc:`scanf` Token           | Regular Expression                          |
 +================================+=============================================+
@@ -878,34 +1042,197 @@
 To extract the filename and numbers from a string like ::
 
    /usr/sbin/sendmail - 0 errors, 4 warnings
 
 you would use a :cfunc:`scanf` format like ::
 
    %s - %d errors, %d warnings
 
 The equivalent regular expression would be ::
 
    (\S+) - (\d+) errors, (\d+) warnings
 
-**Avoiding recursion**
+
+Avoiding recursion
+^^^^^^^^^^^^^^^^^^
 
 If you create regular expressions that require the engine to perform a lot of
 recursion, you may encounter a :exc:`RuntimeError` exception with the message
 ``maximum recursion limit`` exceeded. For example, ::
 
-   >>> import re
    >>> s = 'Begin ' + 1000*'a very long string ' + 'end'
    >>> re.match('Begin (\w| )*? end', s).end()
    Traceback (most recent call last):
      File "<stdin>", line 1, in ?
      File "/usr/local/lib/python2.5/re.py", line 132, in match
        return _compile(pattern, flags).match(string)
    RuntimeError: maximum recursion limit exceeded
 
 You can often restructure your regular expression to avoid recursion.
 
 Starting with Python 2.3, simple uses of the ``*?`` pattern are special-cased to
 avoid recursion.  Thus, the above regular expression can avoid recursion by
 being recast as ``Begin [a-zA-Z0-9_ ]*?end``.  As a further benefit, such
 regular expressions will run faster than their recursive equivalents.
 
+
+search() vs. match()
+^^^^^^^^^^^^^^^^^^^^
+
+In a nutshell, :func:`match` only attempts to match a pattern at the beginning
+of a string where :func:`search` will match a pattern anywhere in a string.
+For example:
+
+   >>> re.match("o", "dog")  # No match as "o" is not the first letter of "dog".
+   >>> re.search("o", "dog") # Match as search() looks everywhere in the string.
+   <_sre.SRE_Match object at ...>
+
+.. note::
+
+   The following applies only to regular expression objects like those created
+   with ``re.compile("pattern")``, not the primitives ``re.match(pattern,
+   string)`` or ``re.search(pattern, string)``.
+
+:func:`match` has an optional second parameter that gives an index in the string
+where the search is to start::
+
+   >>> pattern = re.compile("o")
+   >>> pattern.match("dog")      # No match as "o" is not at the start of "dog."
+
+   # Equivalent to the above expression as 0 is the default starting index:
+   >>> pattern.match("dog", 0)
+
+   # Match as "o" is the 2nd character of "dog" (index 0 is the first):
+   >>> pattern.match("dog", 1)
+   <_sre.SRE_Match object at ...>
+   >>> pattern.match("dog", 2)   # No match as "o" is not the 3rd character of "dog."
+
+
+Making a Phonebook
+^^^^^^^^^^^^^^^^^^
+
+:func:`split` splits a string into a list delimited by the passed pattern.  The
+method is invaluable for converting textual data into data structures that can be
+easily read and modified by Python as demonstrated in the following example that
+creates a phonebook.
+
+First, here is the input.  Normally it may come from a file, here we are using
+triple-quoted string syntax:
+
+   >>> input = """Ross McFluff: 834.345.1254 155 Elm Street
+   ...
+   ... Ronald Heathmore: 892.345.3428 436 Finley Avenue
+   ... Frank Burger: 925.541.7625 662 South Dogwood Way
+   ...
+   ...
+   ... Heather Albrecht: 548.326.4584 919 Park Place"""
+
+The entries are separated by one or more newlines. Now we convert the string
+into a list with each nonempty line having its own entry:
+
+.. doctest::
+   :options: +NORMALIZE_WHITESPACE
+
+   >>> entries = re.split("\n+", input)
+   >>> entries
+   ['Ross McFluff: 834.345.1254 155 Elm Street',
+   'Ronald Heathmore: 892.345.3428 436 Finley Avenue',
+   'Frank Burger: 925.541.7625 662 South Dogwood Way',
+   'Heather Albrecht: 548.326.4584 919 Park Place']
+
+Finally, split each entry into a list with first name, last name, telephone
+number, and address.  We use the ``maxsplit`` parameter of :func:`split`
+because the address has spaces, our splitting pattern, in it:
+
+.. doctest::
+   :options: +NORMALIZE_WHITESPACE
+
+   >>> [re.split(":? ", entry, 3) for entry in entries]
+   [['Ross', 'McFluff', '834.345.1254', '155 Elm Street'],
+   ['Ronald', 'Heathmore', '892.345.3428', '436 Finley Avenue'],
+   ['Frank', 'Burger', '925.541.7625', '662 South Dogwood Way'],
+   ['Heather', 'Albrecht', '548.326.4584', '919 Park Place']]
+
+The ``:?`` pattern matches the colon after the last name, so that it does not
+occur in the result list.  With a ``maxsplit`` of ``4``, we could separate the
+house number from the street name:
+
+.. doctest::
+   :options: +NORMALIZE_WHITESPACE
+
+   >>> [re.split(":? ", entry, 4) for entry in entries]
+   [['Ross', 'McFluff', '834.345.1254', '155', 'Elm Street'],
+   ['Ronald', 'Heathmore', '892.345.3428', '436', 'Finley Avenue'],
+   ['Frank', 'Burger', '925.541.7625', '662', 'South Dogwood Way'],
+   ['Heather', 'Albrecht', '548.326.4584', '919', 'Park Place']]
+
+
+Text Munging
+^^^^^^^^^^^^
+
+:func:`sub` replaces every occurrence of a pattern with a string or the
+result of a function.  This example demonstrates using :func:`sub` with
+a function to "munge" text, or randomize the order of all the characters
+in each word of a sentence except for the first and last characters::
+
+   >>> def repl(m):
+   ...   inner_word = list(m.group(2))
+   ...   random.shuffle(inner_word)
+   ...   return m.group(1) + "".join(inner_word) + m.group(3)
+   >>> text = "Professor Abdolmalek, please report your absences promptly."
+   >>> re.sub("(\w)(\w+)(\w)", repl, text)
+   'Poefsrosr Aealmlobdk, pslaee reorpt your abnseces plmrptoy.'
+   >>> re.sub("(\w)(\w+)(\w)", repl, text)
+   'Pofsroser Aodlambelk, plasee reoprt yuor asnebces potlmrpy.'
+
+
+Finding all Adverbs
+^^^^^^^^^^^^^^^^^^^
+
+:func:`findall` matches *all* occurrences of a pattern, not just the first
+one as :func:`search` does.  For example, if one was a writer and wanted to
+find all of the adverbs in some text, he or she might use :func:`findall` in
+the following manner:
+
+   >>> text = "He was carefully disguised but captured quickly by police."
+   >>> re.findall(r"\w+ly", text)
+   ['carefully', 'quickly']
+
+
+Finding all Adverbs and their Positions
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If one wants more information about all matches of a pattern than the matched
+text, :func:`finditer` is useful as it provides instances of
+:class:`MatchObject` instead of strings.  Continuing with the previous example,
+if one was a writer who wanted to find all of the adverbs *and their positions*
+in some text, he or she would use :func:`finditer` in the following manner:
+
+   >>> text = "He was carefully disguised but captured quickly by police."
+   >>> for m in re.finditer(r"\w+ly", text):
+   ...     print '%02d-%02d: %s' % (m.start(), m.end(), m.group(0))
+   07-16: carefully
+   40-47: quickly
+
+
+Raw String Notation
+^^^^^^^^^^^^^^^^^^^
+
+Raw string notation (``r"text"``) keeps regular expressions sane.  Without it,
+every backslash (``'\'``) in a regular expression would have to be prefixed with
+another one to escape it.  For example, the two following lines of code are
+functionally identical:
+
+   >>> re.match(r"\W(.)\1\W", " ff ")
+   <_sre.SRE_Match object at ...>
+   >>> re.match("\\W(.)\\1\\W", " ff ")
+   <_sre.SRE_Match object at ...>
+
+When one wants to match a literal backslash, it must be escaped in the regular
+expression.  With raw string notation, this means ``r"\\"``.  Without raw string
+notation, one must use ``"\\\\"``, making the following lines of code
+functionally identical:
+
+   >>> re.match(r"\\", r"\\")
+   <_sre.SRE_Match object at ...>
+   >>> re.match("\\\\", r"\\")
+   <_sre.SRE_Match object at ...>
