PK
['"]).*?(?P=quote)`` (i.e. matching a string quoted with either single or double quotes): +---------------------------------------+----------------------------------+ | Context of reference to group "quote" | Ways to reference it | +=======================================+==================================+ | in the same pattern itself | * ``(?P=quote)`` (as shown) | | | * ``\1`` | +---------------------------------------+----------------------------------+ | when processing match object ``m`` | * ``m.group('quote')`` | | | * ``m.end('quote')`` (etc.) | +---------------------------------------+----------------------------------+ | in a string passed to the ``repl`` | * ``\g`` | | argument of ``re.sub()`` | * ``\g<1>`` | | | * ``\1`` | +---------------------------------------+----------------------------------+ ``(?P=name)`` A backreference to a named group; it 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'``. ``(?!...)`` Matches if ``...`` doesn't match next. This is a negative lookahead assertion. 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. Group references are not supported even if they match strings of some fixed length. Note that patterns which start with positive lookbehind assertions will not 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: >>> import re >>> m = re.search('(?<=abc)def', 'abcdef') >>> m.group(0) 'def' This example looks for a word following a hyphen: >>> m = re.search('(?<=-)\w+', 'spam-egg') >>> m.group(0) 'egg' ``(?)`` is a poor email matching pattern, which will match with ``''`` as well as ``'user@host.com'``, but not with ``'``. When a line contains a ``#`` that is not in a character class and is not preceded by an unescaped backslash, all characters from the leftmost such ``#`` through the end of the line are ignored. This 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=0) Scan through *string* looking for the first 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=0) 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 that even in :const:`MULTILINE` mode, :func:`re.match` will only match at the beginning of the string and not at the beginning of each line. If you want to locate a match anywhere in *string*, use :func:`search` instead (see also :ref:`search-vs-match`). .. function:: split(pattern, string, maxsplit=0, flags=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.) >>> 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.'] >>> re.split('[a-f]+', '0a3B9', flags=re.IGNORECASE) ['0', '3', '9'] 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'] .. versionchanged:: 2.7 Added the optional flags argument. .. function:: findall(pattern, string, flags=0) 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. .. note:: Due to the limitation of the current implementation the character following an empty match is not included in a next match, so ``findall(r'^|\w+', 'two words')`` returns ``['', 'wo', 'words']`` (note missed "t"). This is changed in Python 3.7. .. versionadded:: 1.5.2 .. versionchanged:: 2.4 Added the optional flags argument. .. function:: finditer(pattern, string, flags=0) 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. See also the note about :func:`findall`. .. versionadded:: 2.2 .. versionchanged:: 2.4 Added the optional flags argument. .. function:: sub(pattern, repl, string, count=0, flags=0) 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 carriage return, 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: >>> 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: >>> def dashrepl(matchobj): ... if matchobj.group(0) == '-': return ' ' ... else: return '-' >>> re.sub('-{1,2}', dashrepl, 'pro----gram-files') 'pro--gram files' >>> re.sub(r'\sAND\s', ' & ', 'Baked Beans And Spam', flags=re.IGNORECASE) 'Baked Beans & Spam' The pattern may be a string or an RE object. The optional argument *count* is the maximum number of pattern occurrences to be replaced; *count* must be a non-negative integer. If omitted or zero, all occurrences will be replaced. Empty matches for the pattern are replaced only when not adjacent to a previous match, so ``sub('x*', '-', 'abc')`` returns ``'-a-b-c-'``. In string-type *repl* arguments, in addition to the character escapes and backreferences described above, ``\g`` will use the substring matched by the group named ``name``, as defined by the ``(?P...)`` syntax. ``\g`` uses the corresponding group number; ``\g<2>`` is therefore equivalent to ``\2``, but isn't ambiguous in a replacement 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 backreference ``\g<0>`` substitutes in the entire substring matched by the RE. .. versionchanged:: 2.7 Added the optional flags argument. .. function:: subn(pattern, repl, string, count=0, flags=0) Perform the same operation as :func:`sub`, but return a tuple ``(new_string, number_of_subs_made)``. .. versionchanged:: 2.7 Added the optional flags argument. .. function:: escape(pattern) Escape all the characters in *pattern* except ASCII letters and numbers. This is useful if you want to match an arbitrary literal string that may have regular expression metacharacters in it. For example:: >>> print re.escape('python.exe') python\.exe >>> legal_chars = string.ascii_lowercase + string.digits + "!#$%&'*+-.^_`|~:" >>> print '[%s]+' % re.escape(legal_chars) [abcdefghijklmnopqrstuvwxyz0123456789\!\#\$\%\&\'\*\+\-\.\^\_\`\|\~\:]+ >>> operators = ['+', '-', '*', '/', '**'] >>> print '|'.join(map(re.escape, sorted(operators, reverse=True))) \/|\-|\+|\*\*|\* .. function:: purge() Clear the regular expression cache. .. exception:: error Exception raised when a string passed to one of the functions here is not a valid regular expression (for example, it might contain unmatched parentheses) or when some other error occurs during compilation or matching. It is never an error if a string contains no match for a pattern. .. _re-objects: Regular Expression Objects -------------------------- .. class:: RegexObject The :class:`RegexObject` class supports the following methods and attributes: .. 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 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.search(string, 0, 50)`` is equivalent to ``rx.search(string[:50], 0)``. >>> pattern = re.compile("d") >>> pattern.search("dog") # Match at index 0 <_sre.SRE_Match object at ...> >>> pattern.search("dog", 1) # No match; search doesn't include the "d" .. 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. The optional *pos* and *endpos* parameters have the same meaning as for the :meth:`~RegexObject.search` method. >>> 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 ...> If you want to locate a match anywhere in *string*, use :meth:`~RegexObject.search` instead (see also :ref:`search-vs-match`). .. method:: RegexObject.split(string, maxsplit=0) Identical to the :func:`split` function, using the compiled pattern. .. method:: RegexObject.findall(string[, pos[, endpos]]) Similar to the :func:`findall` function, using the compiled pattern, but also accepts optional *pos* and *endpos* parameters that limit the search region like for :meth:`match`. .. method:: RegexObject.finditer(string[, pos[, endpos]]) Similar to the :func:`finditer` function, using the compiled pattern, but also accepts optional *pos* and *endpos* parameters that limit the search region like for :meth:`match`. .. method:: RegexObject.sub(repl, string, count=0) Identical to the :func:`sub` function, using the compiled pattern. .. method:: RegexObject.subn(repl, string, count=0) Identical to the :func:`subn` function, using the compiled pattern. .. attribute:: RegexObject.flags The regex matching flags. This is a combination of the flags given to :func:`.compile` and any ``(?...)`` inline flags in the pattern. .. attribute:: RegexObject.groups The number of capturing groups in the pattern. .. attribute:: RegexObject.groupindex A dictionary mapping any symbolic group names defined by ``(?P)`` 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 Match objects always have a boolean value of ``True``. Since :meth:`~regex.match` and :meth:`~regex.search` return ``None`` when there is no match, you can test whether there was a match with a simple ``if`` statement:: match = re.search(pattern, string) if match: process(match) Match objects 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:`~RegexObject.sub` method. Escapes such as ``\n`` are converted to the appropriate characters, and numeric backreferences (``\1``, ``\2``) and named backreferences (``\g<1>``, ``\g``) are replaced by the contents of the corresponding group. .. method:: MatchObject.group([group1, ...]) 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...)`` 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\w+) (?P\w+)", "Malcolm Reynolds") >>> m.group('first_name') 'Malcolm' >>> m.group('last_name') 'Reynolds' Named groups can also be referred to by their index: >>> m.group(1) 'Malcolm' >>> 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``. For example: >>> m = re.match(r"(?P\w+) (?P\w+)", "Malcolm Reynolds") >>> m.groupdict() {'first_name': 'Malcolm', 'last_name': 'Reynolds'} .. method:: MatchObject.start([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)``. *group* defaults to zero, the entire match. .. attribute:: MatchObject.pos The value of *pos* which was passed to the :meth:`~RegexObject.search` or :meth:`~RegexObject.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 :meth:`~RegexObject.search` or :meth:`~RegexObject.match` method of the :class:`RegexObject`. This is the index into the string beyond which the RE engine will not go. .. attribute:: MatchObject.lastindex The integer index of the last matched capturing group, or ``None`` if no group was matched at all. For example, the expressions ``(a)b``, ``((a)(b))``, and ``((ab))`` will have ``lastindex == 1`` if applied to the string ``'ab'``, while the expression ``(a)(b)`` will have ``lastindex == 2``, if applied to the same string. .. attribute:: MatchObject.lastgroup The name of the last matched capturing group, or ``None`` if the group didn't have a name, or if no group was matched at all. .. attribute:: MatchObject.re The regular expression object whose :meth:`~RegexObject.match` or :meth:`~RegexObject.search` method produced this :class:`MatchObject` instance. .. attribute:: MatchObject.string The string passed to :meth:`~RegexObject.match` or :meth:`~RegexObject.search`. Examples -------- 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.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, "t" for 10, and "2" 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"^[a2-9tjqk]{5}$") >>> displaymatch(valid.match("akt5q")) # Valid. "" >>> displaymatch(valid.match("akt5e")) # Invalid. >>> displaymatch(valid.match("akt")) # Invalid. >>> displaymatch(valid.match("727ak")) # Valid. "" 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. "" >>> displaymatch(pair.match("718ak")) # No pairs. >>> displaymatch(pair.match("354aa")) # Pair of aces. "" To find out what card the pair consists of, one could use the :meth:`~MatchObject.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 "", line 1, in 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 :c:func:`scanf`. Regular expressions are generally more powerful, though also more verbose, than :c:func:`scanf` format strings. The table below offers some more-or-less equivalent mappings between :c:func:`scanf` format tokens and regular expressions. +--------------------------------+---------------------------------------------+ | :c:func:`scanf` Token | Regular Expression | +================================+=============================================+ | ``%c`` | ``.`` | +--------------------------------+---------------------------------------------+ | ``%5c`` | ``.{5}`` | +--------------------------------+---------------------------------------------+ | ``%d`` | ``[-+]?\d+`` | +--------------------------------+---------------------------------------------+ | ``%e``, ``%E``, ``%f``, ``%g`` | ``[-+]?(\d+(\.\d*)?|\.\d+)([eE][-+]?\d+)?`` | +--------------------------------+---------------------------------------------+ | ``%i`` | ``[-+]?(0[xX][\dA-Fa-f]+|0[0-7]*|\d+)`` | +--------------------------------+---------------------------------------------+ | ``%o`` | ``[-+]?[0-7]+`` | +--------------------------------+---------------------------------------------+ | ``%s`` | ``\S+`` | +--------------------------------+---------------------------------------------+ | ``%u`` | ``\d+`` | +--------------------------------+---------------------------------------------+ | ``%x``, ``%X`` | ``[-+]?(0[xX])?[\dA-Fa-f]+`` | +--------------------------------+---------------------------------------------+ To extract the filename and numbers from a string like :: /usr/sbin/sendmail - 0 errors, 4 warnings you would use a :c:func:`scanf` format like :: %s - %d errors, %d warnings The equivalent regular expression would be :: (\S+) - (\d+) errors, (\d+) warnings .. _search-vs-match: search() vs. match() ^^^^^^^^^^^^^^^^^^^^ .. sectionauthor:: Fred L. Drake, Jr. Python offers two different primitive operations based on regular expressions: :func:`re.match` checks for a match only at the beginning of the string, while :func:`re.search` checks for a match anywhere in the string (this is what Perl does by default). For example:: >>> re.match("c", "abcdef") # No match >>> re.search("c", "abcdef") # Match <_sre.SRE_Match object at ...> Regular expressions beginning with ``'^'`` can be used with :func:`search` to restrict the match at the beginning of the string:: >>> re.match("c", "abcdef") # No match >>> re.search("^c", "abcdef") # No match >>> re.search("^a", "abcdef") # Match <_sre.SRE_Match object at ...> Note however that in :const:`MULTILINE` mode :func:`match` only matches at the beginning of the string, whereas using :func:`search` with a regular expression beginning with ``'^'`` will match at the beginning of each line. >>> re.match('X', 'A\nB\nX', re.MULTILINE) # No match >>> re.search('^X', 'A\nB\nX', re.MULTILINE) # Match <_sre.SRE_Match object at ...> 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: >>> text = """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+", text) >>> 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(r"(\w)(\w+)(\w)", repl, text) 'Poefsrosr Aealmlobdk, pslaee reorpt your abnseces plmrptoy.' >>> re.sub(r"(\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 a writer wanted to find all of the adverbs in some text, they 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 a writer wanted to find all of the adverbs *and their positions* in some text, they 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 ...>
`` | | argument of ``re.sub()`` | * ``\g<1>`` | | | * ``\1`` | +---------------------------------------+----------------------------------+ ``(?P=name)`` A backreference to a named group; it 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'``. ``(?!...)`` Matches if ``...`` doesn't match next. This is a negative lookahead assertion. 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. Group references are not supported even if they match strings of some fixed length. Note that patterns which start with positive lookbehind assertions will not 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: >>> import re >>> m = re.search('(?<=abc)def', 'abcdef') >>> m.group(0) 'def' This example looks for a word following a hyphen: >>> m = re.search('(?<=-)\w+', 'spam-egg') >>> m.group(0) 'egg' ``(?)`` is a poor email matching pattern, which will match with ``''`` as well as ``'user@host.com'``, but not with ``'``. When a line contains a ``#`` that is not in a character class and is not preceded by an unescaped backslash, all characters from the leftmost such ``#`` through the end of the line are ignored. This 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=0) Scan through *string* looking for the first 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=0) 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 that even in :const:`MULTILINE` mode, :func:`re.match` will only match at the beginning of the string and not at the beginning of each line. If you want to locate a match anywhere in *string*, use :func:`search` instead (see also :ref:`search-vs-match`). .. function:: split(pattern, string, maxsplit=0, flags=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.) >>> 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.'] >>> re.split('[a-f]+', '0a3B9', flags=re.IGNORECASE) ['0', '3', '9'] 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'] .. versionchanged:: 2.7 Added the optional flags argument. .. function:: findall(pattern, string, flags=0) 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. .. note:: Due to the limitation of the current implementation the character following an empty match is not included in a next match, so ``findall(r'^|\w+', 'two words')`` returns ``['', 'wo', 'words']`` (note missed "t"). This is changed in Python 3.7. .. versionadded:: 1.5.2 .. versionchanged:: 2.4 Added the optional flags argument. .. function:: finditer(pattern, string, flags=0) 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. See also the note about :func:`findall`. .. versionadded:: 2.2 .. versionchanged:: 2.4 Added the optional flags argument. .. function:: sub(pattern, repl, string, count=0, flags=0) 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 carriage return, 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: >>> 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: >>> def dashrepl(matchobj): ... if matchobj.group(0) == '-': return ' ' ... else: return '-' >>> re.sub('-{1,2}', dashrepl, 'pro----gram-files') 'pro--gram files' >>> re.sub(r'\sAND\s', ' & ', 'Baked Beans And Spam', flags=re.IGNORECASE) 'Baked Beans & Spam' The pattern may be a string or an RE object. The optional argument *count* is the maximum number of pattern occurrences to be replaced; *count* must be a non-negative integer. If omitted or zero, all occurrences will be replaced. Empty matches for the pattern are replaced only when not adjacent to a previous match, so ``sub('x*', '-', 'abc')`` returns ``'-a-b-c-'``. In string-type *repl* arguments, in addition to the character escapes and backreferences described above, ``\g`` will use the substring matched by the group named ``name``, as defined by the ``(?P...)`` syntax. ``\g`` uses the corresponding group number; ``\g<2>`` is therefore equivalent to ``\2``, but isn't ambiguous in a replacement 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 backreference ``\g<0>`` substitutes in the entire substring matched by the RE. .. versionchanged:: 2.7 Added the optional flags argument. .. function:: subn(pattern, repl, string, count=0, flags=0) Perform the same operation as :func:`sub`, but return a tuple ``(new_string, number_of_subs_made)``. .. versionchanged:: 2.7 Added the optional flags argument. .. function:: escape(pattern) Escape all the characters in *pattern* except ASCII letters and numbers. This is useful if you want to match an arbitrary literal string that may have regular expression metacharacters in it. For example:: >>> print re.escape('python.exe') python\.exe >>> legal_chars = string.ascii_lowercase + string.digits + "!#$%&'*+-.^_`|~:" >>> print '[%s]+' % re.escape(legal_chars) [abcdefghijklmnopqrstuvwxyz0123456789\!\#\$\%\&\'\*\+\-\.\^\_\`\|\~\:]+ >>> operators = ['+', '-', '*', '/', '**'] >>> print '|'.join(map(re.escape, sorted(operators, reverse=True))) \/|\-|\+|\*\*|\* .. function:: purge() Clear the regular expression cache. .. exception:: error Exception raised when a string passed to one of the functions here is not a valid regular expression (for example, it might contain unmatched parentheses) or when some other error occurs during compilation or matching. It is never an error if a string contains no match for a pattern. .. _re-objects: Regular Expression Objects -------------------------- .. class:: RegexObject The :class:`RegexObject` class supports the following methods and attributes: .. 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 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.search(string, 0, 50)`` is equivalent to ``rx.search(string[:50], 0)``. >>> pattern = re.compile("d") >>> pattern.search("dog") # Match at index 0 <_sre.SRE_Match object at ...> >>> pattern.search("dog", 1) # No match; search doesn't include the "d" .. 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. The optional *pos* and *endpos* parameters have the same meaning as for the :meth:`~RegexObject.search` method. >>> 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 ...> If you want to locate a match anywhere in *string*, use :meth:`~RegexObject.search` instead (see also :ref:`search-vs-match`). .. method:: RegexObject.split(string, maxsplit=0) Identical to the :func:`split` function, using the compiled pattern. .. method:: RegexObject.findall(string[, pos[, endpos]]) Similar to the :func:`findall` function, using the compiled pattern, but also accepts optional *pos* and *endpos* parameters that limit the search region like for :meth:`match`. .. method:: RegexObject.finditer(string[, pos[, endpos]]) Similar to the :func:`finditer` function, using the compiled pattern, but also accepts optional *pos* and *endpos* parameters that limit the search region like for :meth:`match`. .. method:: RegexObject.sub(repl, string, count=0) Identical to the :func:`sub` function, using the compiled pattern. .. method:: RegexObject.subn(repl, string, count=0) Identical to the :func:`subn` function, using the compiled pattern. .. attribute:: RegexObject.flags The regex matching flags. This is a combination of the flags given to :func:`.compile` and any ``(?...)`` inline flags in the pattern. .. attribute:: RegexObject.groups The number of capturing groups in the pattern. .. attribute:: RegexObject.groupindex A dictionary mapping any symbolic group names defined by ``(?P)`` 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 Match objects always have a boolean value of ``True``. Since :meth:`~regex.match` and :meth:`~regex.search` return ``None`` when there is no match, you can test whether there was a match with a simple ``if`` statement:: match = re.search(pattern, string) if match: process(match) Match objects 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:`~RegexObject.sub` method. Escapes such as ``\n`` are converted to the appropriate characters, and numeric backreferences (``\1``, ``\2``) and named backreferences (``\g<1>``, ``\g``) are replaced by the contents of the corresponding group. .. method:: MatchObject.group([group1, ...]) 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...)`` 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\w+) (?P\w+)", "Malcolm Reynolds") >>> m.group('first_name') 'Malcolm' >>> m.group('last_name') 'Reynolds' Named groups can also be referred to by their index: >>> m.group(1) 'Malcolm' >>> 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``. For example: >>> m = re.match(r"(?P\w+) (?P\w+)", "Malcolm Reynolds") >>> m.groupdict() {'first_name': 'Malcolm', 'last_name': 'Reynolds'} .. method:: MatchObject.start([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)``. *group* defaults to zero, the entire match. .. attribute:: MatchObject.pos The value of *pos* which was passed to the :meth:`~RegexObject.search` or :meth:`~RegexObject.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 :meth:`~RegexObject.search` or :meth:`~RegexObject.match` method of the :class:`RegexObject`. This is the index into the string beyond which the RE engine will not go. .. attribute:: MatchObject.lastindex The integer index of the last matched capturing group, or ``None`` if no group was matched at all. For example, the expressions ``(a)b``, ``((a)(b))``, and ``((ab))`` will have ``lastindex == 1`` if applied to the string ``'ab'``, while the expression ``(a)(b)`` will have ``lastindex == 2``, if applied to the same string. .. attribute:: MatchObject.lastgroup The name of the last matched capturing group, or ``None`` if the group didn't have a name, or if no group was matched at all. .. attribute:: MatchObject.re The regular expression object whose :meth:`~RegexObject.match` or :meth:`~RegexObject.search` method produced this :class:`MatchObject` instance. .. attribute:: MatchObject.string The string passed to :meth:`~RegexObject.match` or :meth:`~RegexObject.search`. Examples -------- 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.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, "t" for 10, and "2" 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"^[a2-9tjqk]{5}$") >>> displaymatch(valid.match("akt5q")) # Valid. "" >>> displaymatch(valid.match("akt5e")) # Invalid. >>> displaymatch(valid.match("akt")) # Invalid. >>> displaymatch(valid.match("727ak")) # Valid. "" 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. "" >>> displaymatch(pair.match("718ak")) # No pairs. >>> displaymatch(pair.match("354aa")) # Pair of aces. "" To find out what card the pair consists of, one could use the :meth:`~MatchObject.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 "", line 1, in 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 :c:func:`scanf`. Regular expressions are generally more powerful, though also more verbose, than :c:func:`scanf` format strings. The table below offers some more-or-less equivalent mappings between :c:func:`scanf` format tokens and regular expressions. +--------------------------------+---------------------------------------------+ | :c:func:`scanf` Token | Regular Expression | +================================+=============================================+ | ``%c`` | ``.`` | +--------------------------------+---------------------------------------------+ | ``%5c`` | ``.{5}`` | +--------------------------------+---------------------------------------------+ | ``%d`` | ``[-+]?\d+`` | +--------------------------------+---------------------------------------------+ | ``%e``, ``%E``, ``%f``, ``%g`` | ``[-+]?(\d+(\.\d*)?|\.\d+)([eE][-+]?\d+)?`` | +--------------------------------+---------------------------------------------+ | ``%i`` | ``[-+]?(0[xX][\dA-Fa-f]+|0[0-7]*|\d+)`` | +--------------------------------+---------------------------------------------+ | ``%o`` | ``[-+]?[0-7]+`` | +--------------------------------+---------------------------------------------+ | ``%s`` | ``\S+`` | +--------------------------------+---------------------------------------------+ | ``%u`` | ``\d+`` | +--------------------------------+---------------------------------------------+ | ``%x``, ``%X`` | ``[-+]?(0[xX])?[\dA-Fa-f]+`` | +--------------------------------+---------------------------------------------+ To extract the filename and numbers from a string like :: /usr/sbin/sendmail - 0 errors, 4 warnings you would use a :c:func:`scanf` format like :: %s - %d errors, %d warnings The equivalent regular expression would be :: (\S+) - (\d+) errors, (\d+) warnings .. _search-vs-match: search() vs. match() ^^^^^^^^^^^^^^^^^^^^ .. sectionauthor:: Fred L. Drake, Jr. Python offers two different primitive operations based on regular expressions: :func:`re.match` checks for a match only at the beginning of the string, while :func:`re.search` checks for a match anywhere in the string (this is what Perl does by default). For example:: >>> re.match("c", "abcdef") # No match >>> re.search("c", "abcdef") # Match <_sre.SRE_Match object at ...> Regular expressions beginning with ``'^'`` can be used with :func:`search` to restrict the match at the beginning of the string:: >>> re.match("c", "abcdef") # No match >>> re.search("^c", "abcdef") # No match >>> re.search("^a", "abcdef") # Match <_sre.SRE_Match object at ...> Note however that in :const:`MULTILINE` mode :func:`match` only matches at the beginning of the string, whereas using :func:`search` with a regular expression beginning with ``'^'`` will match at the beginning of each line. >>> re.match('X', 'A\nB\nX', re.MULTILINE) # No match >>> re.search('^X', 'A\nB\nX', re.MULTILINE) # Match <_sre.SRE_Match object at ...> 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: >>> text = """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+", text) >>> 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(r"(\w)(\w+)(\w)", repl, text) 'Poefsrosr Aealmlobdk, pslaee reorpt your abnseces plmrptoy.' >>> re.sub(r"(\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 a writer wanted to find all of the adverbs in some text, they 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 a writer wanted to find all of the adverbs *and their positions* in some text, they 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 ...>