``re`` --- Regular expression operations
****************************************
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.
It is important to note that most regular expression operations are
available as module-level functions and ``RegexObject`` methods. The
functions are shortcuts that don't require you to compile a regex
object first, but miss some fine-tuning parameters.
See also:
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 edition covered writing good
regular expression patterns in great detail.
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*.
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. 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:
``'.'``
(Dot.) In the default mode, this matches any character except a
newline. If the ``DOTALL`` flag has been specified, this matches
any character including a newline.
``'^'``
(Caret.) Matches the start of the string, and in ``MULTILINE``
mode also matches immediately after each newline.
``'$'``
Matches the end of the string or just before the newline at the end
of the string, and in ``MULTILINE`` mode also matches before a
newline. ``foo`` matches both 'foo' and 'foobar', while the
regular expression ``foo$`` matches only 'foo'. More
interestingly, searching for ``foo.$`` in ``'foo1\nfoo2\n'``
matches 'foo2' normally, but 'foo1' in ``MULTILINE`` mode;
searching for a single ``$`` 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'.
``'?'``
Causes the resulting RE to match 0 or 1 repetitions of the
preceding RE. ``ab?`` will match either 'a' or 'ab'.
``*?``, ``+?``, ``??``
The ``'*'``, ``'+'``, and ``'?'`` qualifiers are all *greedy*; they
match as much text as possible. Sometimes this behaviour isn't
desired; if the RE ``<.*>`` is matched against
``'title
'``, it will match the entire string, and not just
``''``. Adding ``'?'`` after the qualifier makes it perform
the match in *non-greedy* or *minimal* fashion; as *few* characters
as possible will be matched. Using ``.*?`` in the previous
expression will match only ``''``.
``{m}``
Specifies that exactly *m* copies of the previous RE should be
matched; fewer matches cause the entire RE not to match. For
example, ``a{6}`` will match exactly six ``'a'`` characters, but
not five.
``{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.
``'\'``
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, although the characters they match depends on whether
``LOCALE`` or ``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 *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 ``[|]``.
``(...)``
Matches whatever regular expression is inside the parentheses, and
indicates the start and end of a group; the contents of a group can
be retrieved after a match 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...)`` 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: ``re.I`` (ignore case),
``re.L`` (locale dependent), ``re.M`` (multi-line), ``re.S`` (dot
matches all), ``re.U`` (Unicode dependent), and ``re.X`` (verbose),
for the entire regular expression. (The flags are described in
*Module Contents*.) This is useful if you wish to include the flags
as part of the regular expression, instead of passing a *flag*
argument to the ``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...)``
Similar to regular parentheses, but the substring matched by the
group is 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[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 the regular expression itself (using ``(?P=id)``) and
replacement text given to ``.sub()`` (using ``\g``).
``(?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'``.
``(?!...)``
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 *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
``search()`` function rather than the ``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 ``'>> re.match("c", "abcdef") # No match
>>> re.search("c", "abcdef") # Match
<_sre.SRE_Match object at ...>
Module Contents
===============
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.
re.compile(pattern[, flags])
Compile a regular expression pattern into a regular expression
object, which can be used for matching using its ``match()`` and
``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(pattern)
result = prog.match(string)
is equivalent to
result = re.match(pattern, string)
but using ``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.
Note: The compiled versions of the most recent patterns passed to
``re.match()``, ``re.search()`` or ``re.compile()`` are cached,
so programs that use only a few regular expressions at a time
needn't worry about compiling regular expressions.
re.I
re.IGNORECASE
Perform case-insensitive matching; expressions like ``[A-Z]`` will
match lowercase letters, too. This is not affected by the current
locale.
re.L
re.LOCALE
Make ``\w``, ``\W``, ``\b``, ``\B``, ``\s`` and ``\S`` dependent on
the current locale.
re.M
re.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.
re.S
re.DOTALL
Make the ``'.'`` special character match any character at all,
including a newline; without this flag, ``'.'`` will match anything
*except* a newline.
re.U
re.UNICODE
Make ``\w``, ``\W``, ``\b``, ``\B``, ``\d``, ``\D``, ``\s`` and
``\S`` dependent on the Unicode character properties database.
New in version 2.0.
re.X
re.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.
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*")
re.search(pattern, string[, flags])
Scan through *string* looking for a location where the regular
expression *pattern* produces a match, and return a corresponding
``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.
re.match(pattern, string[, flags])
If zero or more characters at the beginning of *string* match the
regular expression *pattern*, return a corresponding
``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
``search()`` instead.
re.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.)
>>> 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']
re.findall(pattern, string[, flags])
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.
New in version 1.5.2.
Changed in version 2.4: Added the optional flags argument.
re.finditer(pattern, string[, flags])
Return an *iterator* yielding ``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.
New in version 2.2.
Changed in version 2.4: Added the optional flags argument.
re.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:
>>> 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'
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 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 addition to character escapes and backreferences as 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.
re.subn(pattern, repl, string[, count])
Perform the same operation as ``sub()``, but return a tuple
``(new_string, number_of_subs_made)``.
re.escape(string)
Return *string* with all non-alphanumerics backslashed; this is
useful if you want to match an arbitrary literal string that may
have regular expression metacharacters in it.
exception exception re.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.
Regular Expression Objects
==========================
Compiled regular expression objects support the following methods and
attributes:
RegexObject.match(string[, pos[, endpos]])
If zero or more characters at the beginning of *string* match this
regular expression, return a corresponding ``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
``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 ...>
RegexObject.search(string[, pos[, endpos]])
Scan through *string* looking for a location where this regular
expression produces a match, and return a corresponding
``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 ``match()`` method.
RegexObject.split(string[, maxsplit=0])
Identical to the ``split()`` function, using the compiled pattern.
RegexObject.findall(string[, pos[, endpos]])
Identical to the ``findall()`` function, using the compiled
pattern.
RegexObject.finditer(string[, pos[, endpos]])
Identical to the ``finditer()`` function, using the compiled
pattern.
RegexObject.sub(repl, string[, count=0])
Identical to the ``sub()`` function, using the compiled pattern.
RegexObject.subn(repl, string[, count=0])
Identical to the ``subn()`` function, using the compiled pattern.
RegexObject.flags
The flags argument used when the RE object was compiled, or ``0``
if no flags were provided.
RegexObject.groups
The number of capturing groups in the pattern.
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.
RegexObject.pattern
The pattern string from which the RE object was compiled.
Match Objects
=============
Match objects always have a boolean value of ``True``, so that you can
test whether e.g. ``match()`` resulted in a match with a simple if
statement. They support the following methods and attributes:
MatchObject.expand(template)
Return the string obtained by doing backslash substitution on the
template string *template*, as done by the ``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.
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 ``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 ``IndexError`` exception is raised.
A moderately complicated example:
>>> m = re.match(r"(?P\w+) (?P\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'
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')
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+)", "Malcom Reynolds")
>>> m.groupdict()
{'first_name': 'Malcom', 'last_name': 'Reynolds'}
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
``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'
MatchObject.span([group])
For ``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.
MatchObject.pos
The value of *pos* which was passed to the ``search()`` or
``match()`` method of the ``RegexObject``. This is the index into
the string at which the RE engine started looking for a match.
MatchObject.endpos
The value of *endpos* which was passed to the ``search()`` or
``match()`` method of the ``RegexObject``. This is the index into
the string beyond which the RE engine will not go.
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.
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.
MatchObject.re
The regular expression object whose ``match()`` or ``search()``
method produced this ``MatchObject`` instance.
MatchObject.string
The string passed to ``match()`` or ``search()``.
Examples
========
Checking For a Pair
-------------------
In this example, we'll use the following helper function to display
match objects a little more gracefully:
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, "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.
""
>>> displaymatch(valid.match("ak05e")) # Invalid.
>>> displaymatch(valid.match("ak0")) # 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
``group()`` method of ``MatchObject`` in the following manner:
>>> 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()
------------------
Python does not currently have an equivalent to ``scanf()``. Regular
expressions are generally more powerful, though also more verbose,
than ``scanf()`` format strings. The table below offers some more-or-
less equivalent mappings between ``scanf()`` format tokens and regular
expressions.
+----------------------------------+-----------------------------------------------+
| ``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[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 ``scanf()`` format like
%s - %d errors, %d warnings
The equivalent regular expression would be
(\S+) - (\d+) errors, (\d+) warnings
Avoiding recursion
------------------
If you create regular expressions that require the engine to perform a
lot of recursion, you may encounter a ``RuntimeError`` exception with
the message ``maximum recursion limit`` exceeded. For example,
>>> s = 'Begin ' + 1000*'a very long string ' + 'end'
>>> re.match('Begin (\w| )*? end', s).end()
Traceback (most recent call last):
File "", 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, ``match()`` only attempts to match a pattern at the
beginning of a string where ``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)``.
``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
------------------
``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:
>>> 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
``split()`` because the address has spaces, our splitting pattern, in
it:
>>> [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:
>>> [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
------------
``sub()`` replaces every occurrence of a pattern with a string or the
result of a function. This example demonstrates using ``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
-------------------
``findall()`` matches *all* occurrences of a pattern, not just the
first one as ``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
``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, ``finditer()`` is useful as it provides instances of
``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 ``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 ...>