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:mod:`re` --- Regular expression operations
===========================================
.. module:: re
:synopsis: Regular expression operations.
.. moduleauthor:: Fredrik Lundh <fredrik@pythonware.com>
.. sectionauthor:: Andrew M. Kuchling <amk@amk.ca>
This module provides regular expression matching operations similar to
those found in Perl. 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 :class:`RegexObject` methods. The functions are
shortcuts that don't require you to compile a regex object first, but miss some
fine-tuning parameters.
.. seealso::
Mastering Regular Expressions
Book on regular expressions by Jeffrey Friedl, published by O'Reilly. The
second edition of the book no longer covers Python at all, but the first
edition covered writing good regular expression patterns in great detail.
.. _re-syntax:
Regular Expression Syntax
-------------------------
A regular expression (or RE) specifies a set of strings that matches it; the
functions in this module let you check if a particular string matches a given
regular expression (or if a given regular expression matches a particular
string, which comes down to the same thing).
Regular expressions can be concatenated to form new regular expressions; if *A*
and *B* are both regular expressions, then *AB* is also a regular expression.
In general, if a string *p* matches *A* and another string *q* matches *B*, the
string *pq* will match AB. This holds unless *A* or *B* contain low precedence
operations; boundary conditions between *A* and *B*; or have numbered group
references. Thus, complex expressions can easily be constructed from simpler
primitive expressions like the ones described here. For details of the theory
and implementation of regular expressions, consult the Friedl book referenced
above, or almost any textbook about compiler construction.
A brief explanation of the format of regular expressions follows. For further
information and a gentler presentation, consult the :ref:`regex-howto`.
Regular expressions can contain both special and ordinary characters. Most
ordinary characters, like ``'A'``, ``'a'``, or ``'0'``, are the simplest regular
expressions; they simply match themselves. You can concatenate ordinary
characters, so ``last`` matches the string ``'last'``. (In the rest of this
section, we'll write RE's in ``this special style``, usually without quotes, and
strings to be matched ``'in single quotes'``.)
Some characters, like ``'|'`` or ``'('``, are special. Special
characters either stand for classes of ordinary characters, or affect
how the regular expressions around them are interpreted. 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 :const:`DOTALL` flag has been specified, this matches any character
including a newline.
``'^'``
(Caret.) Matches the start of the string, and in :const:`MULTILINE` mode also
matches immediately after each newline.
``'$'``
Matches the end of the string or just before the newline at the end of the
string, and in :const:`MULTILINE` mode also matches before a newline. ``foo``
matches both 'foo' and 'foobar', while the regular expression ``foo$`` matches
only 'foo'. More interestingly, searching for ``foo.$`` in ``'foo1\nfoo2\n'``
matches 'foo2' normally, but 'foo1' in :const:`MULTILINE` mode; searching for
a single ``$`` 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 :dfn:`greedy`; they match
as much text as possible. Sometimes this behaviour isn't desired; if the RE
``<.*>`` is matched against ``'<H1>title</H1>'``, it will match the entire
string, and not just ``'<H1>'``. Adding ``'?'`` after the qualifier makes it
perform the match in :dfn:`non-greedy` or :dfn:`minimal` fashion; as *few*
characters as possible will be matched. Using ``.*?`` in the previous
expression will match only ``'<H1>'``.
``{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 :const:`LOCALE`
or :const:`UNICODE` mode is in force. If you want to include a
``']'`` or a ``'-'`` inside a set, precede it with a backslash, or
place it as the first character. The pattern ``[]]`` will match
``']'``, for example.
You can match the characters not within a range by :dfn:`complementing` the set.
This is indicated by including a ``'^'`` as the first character of the set;
``'^'`` elsewhere will simply match the ``'^'`` character. For example,
``[^5]`` will match any character except ``'5'``, and ``[^^]`` will match any
character except ``'^'``.
Note that inside ``[]`` the special forms and special characters lose
their meanings and only the syntaxes described here are valid. For
example, ``+``, ``*``, ``(``, ``)``, and so on are treated as
literals inside ``[]``, and backreferences cannot be used inside
``[]``.
``'|'``
``A|B``, where A and B can be arbitrary REs, creates a regular expression that
will match either A or B. An arbitrary number of REs can be separated by the
``'|'`` in this way. This can be used inside groups (see below) as well. As
the target string is scanned, REs separated by ``'|'`` are tried from left to
right. When one pattern completely matches, that branch is accepted. This means
that once ``A`` matches, ``B`` will not be tested further, even if it would
produce a longer overall match. In other words, the ``'|'`` operator is never
greedy. To match a literal ``'|'``, use ``\|``, or enclose it inside a
character class, as in ``[|]``.
``(...)``
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<name>...)`` is the only exception to this rule. Following are the
currently supported extensions.
``(?iLmsux)``
(One or more letters from the set ``'i'``, ``'L'``, ``'m'``, ``'s'``,
``'u'``, ``'x'``.) The group matches the empty string; the letters
set the corresponding flags: :const:`re.I` (ignore case),
:const:`re.L` (locale dependent), :const:`re.M` (multi-line),
:const:`re.S` (dot matches all), :const:`re.U` (Unicode dependent),
and :const:`re.X` (verbose), for the entire regular expression. (The
flags are described in :ref:`contents-of-module-re`.) This
is useful if you wish to include the flags as part of the regular
expression, instead of passing a *flag* argument to the
:func:`re.compile` function.
Note that the ``(?x)`` flag changes how the expression is parsed. It should be
used first in the expression string, or after one or more whitespace characters.
If there are non-whitespace characters before the flag, the results are
undefined.
``(?:...)``
A non-grouping version of regular parentheses. Matches whatever regular
expression is inside the parentheses, but the substring matched by the group
*cannot* be retrieved after performing a match or referenced later in the
pattern.
``(?P<name>...)``
Similar to regular parentheses, but the substring matched by the group is
accessible within the rest of the regular expression via the symbolic group
name *name*. Group names must be valid Python identifiers, and each group
name must be defined only once within a regular expression. A symbolic group
is also a numbered group, just as if the group were not named. So the group
named ``id`` in the example below can also be referenced as the numbered group
``1``.
For example, if the pattern is ``(?P<id>[a-zA-Z_]\w*)``, the group can be
referenced by its name in arguments to methods of match objects, such as
``m.group('id')`` or ``m.end('id')``, and also by name in the regular
expression itself (using ``(?P=id)``) and replacement text given to
``.sub()`` (using ``\g<id>``).
``(?P=name)``
Matches whatever text was matched by the earlier group named *name*.
``(?#...)``
A comment; the contents of the parentheses are simply ignored.
``(?=...)``
Matches if ``...`` matches next, but doesn't consume any of the string. This is
called a lookahead assertion. For example, ``Isaac (?=Asimov)`` will match
``'Isaac '`` only if it's followed by ``'Asimov'``.
``(?!...)``
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. Note that
patterns which start with positive lookbehind assertions will never match at the
beginning of the string being searched; you will most likely want to use the
:func:`search` function rather than the :func:`match` function:
>>> 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'
``(?<!...)``
Matches if the current position in the string is not preceded by a match for
``...``. This is called a :dfn:`negative lookbehind assertion`. Similar to
positive lookbehind assertions, the contained pattern must only match strings of
some fixed length. Patterns which start with negative lookbehind assertions may
match at the beginning of the string being searched.
``(?(id/name)yes-pattern|no-pattern)``
Will try to match with ``yes-pattern`` if the group with given *id* or *name*
exists, and with ``no-pattern`` if it doesn't. ``no-pattern`` is optional and
can be omitted. For example, ``(<)?(\w+@\w+(?:\.\w+)+)(?(1)>)`` is a poor email
matching pattern, which will match with ``'<user@host.com>'`` as well as
``'user@host.com'``, but not with ``'<user@host.com'``.
.. versionadded:: 2.4
The special sequences consist of ``'\'`` and a character from the list below.
If the ordinary character is not on the list, then the resulting RE will match
the second character. For example, ``\$`` matches the character ``'$'``.
``\number``
Matches the contents of the group of the same number. Groups are numbered
starting from 1. For example, ``(.+) \1`` matches ``'the the'`` or ``'55 55'``,
but not ``'the end'`` (note the space after the group). This special sequence
can only be used to match one of the first 99 groups. If the first digit of
*number* is 0, or *number* is 3 octal digits long, it will not be interpreted as
a group match, but as the character with octal value *number*. Inside the
``'['`` and ``']'`` of a character class, all numeric escapes are treated as
characters.
``\A``
Matches only at the start of the string.
``\b``
Matches the empty string, but only at the beginning or end of a word. A word is
defined as a sequence of alphanumeric or underscore characters, so the end of a
word is indicated by whitespace or a non-alphanumeric, non-underscore character.
Note that ``\b`` is defined as the boundary between ``\w`` and ``\W``, so the
precise set of characters deemed to be alphanumeric depends on the values of the
``UNICODE`` and ``LOCALE`` flags. Inside a character range, ``\b`` represents
the backspace character, for compatibility with Python's string literals.
``\B``
Matches the empty string, but only when it is *not* at the beginning or end of a
word. This is just the opposite of ``\b``, so is also subject to the settings
of ``LOCALE`` and ``UNICODE``.
``\d``
When the :const:`UNICODE` flag is not specified, matches any decimal digit; this
is equivalent to the set ``[0-9]``. With :const:`UNICODE`, it will match
whatever is classified as a digit in the Unicode character properties database.
``\D``
When the :const:`UNICODE` flag is not specified, matches any non-digit
character; this is equivalent to the set ``[^0-9]``. With :const:`UNICODE`, it
will match anything other than character marked as digits in the Unicode
character properties database.
``\s``
When the :const:`LOCALE` and :const:`UNICODE` flags are not specified, matches
any whitespace character; this is equivalent to the set ``[ \t\n\r\f\v]``. With
:const:`LOCALE`, it will match this set plus whatever characters are defined as
space for the current locale. If :const:`UNICODE` is set, this will match the
characters ``[ \t\n\r\f\v]`` plus whatever is classified as space in the Unicode
character properties database.
``\S``
When the :const:`LOCALE` and :const:`UNICODE` flags are not specified, matches
any non-whitespace character; this is equivalent to the set ``[^ \t\n\r\f\v]``
With :const:`LOCALE`, it will match any character not in this set, and not
defined as space in the current locale. If :const:`UNICODE` is set, this will
match anything other than ``[ \t\n\r\f\v]`` and characters marked as space in
the Unicode character properties database.
``\w``
When the :const:`LOCALE` and :const:`UNICODE` flags are not specified, matches
any alphanumeric character and the underscore; this is equivalent to the set
``[a-zA-Z0-9_]``. With :const:`LOCALE`, it will match the set ``[0-9_]`` plus
whatever characters are defined as alphanumeric for the current locale. If
:const:`UNICODE` is set, this will match the characters ``[0-9_]`` plus whatever
is classified as alphanumeric in the Unicode character properties database.
``\W``
When the :const:`LOCALE` and :const:`UNICODE` flags are not specified, matches
any non-alphanumeric character; this is equivalent to the set ``[^a-zA-Z0-9_]``.
With :const:`LOCALE`, it will match any character not in the set ``[0-9_]``, and
not defined as alphanumeric for the current locale. If :const:`UNICODE` is set,
this will match anything other than ``[0-9_]`` and characters marked as
alphanumeric in the Unicode character properties database.
``\Z``
Matches only at the end of the string.
Most of the standard escapes supported by Python string literals are also
accepted by the regular expression parser::
\a \b \f \n
\r \t \v \x
\\
Octal escapes are included in a limited form: If the first digit is a 0, or if
there are three octal digits, it is considered an octal escape. Otherwise, it is
a group reference. As for string literals, octal escapes are always at most
three digits in length.
.. _matching-searching:
Matching vs Searching
---------------------
.. sectionauthor:: Fred L. Drake, Jr. <fdrake@acm.org>
Python offers two different primitive operations based on regular expressions:
**match** checks for a match only at the beginning of the string, while
**search** checks for a match anywhere in the string (this is what Perl does
by default).
Note that match may differ from search even when using a regular expression
beginning with ``'^'``: ``'^'`` matches only at the start of the string, or in
:const:`MULTILINE` mode also immediately following a newline. The "match"
operation succeeds only if the pattern matches at the start of the string
regardless of mode, or at the starting position given by the optional *pos*
argument regardless of whether a newline precedes it.
>>> re.match("c", "abcdef") # No match
>>> re.search("c", "abcdef") # Match
<_sre.SRE_Match object at ...>
.. _contents-of-module-re:
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.
.. function:: compile(pattern[, flags])
Compile a regular expression pattern into a regular expression object, which
can be used for matching using its :func:`match` and :func:`search` methods,
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 :func:`re.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
:func:`re.match`, :func:`re.search` or :func:`re.compile` are cached, so
programs that use only a few regular expressions at a time needn't worry
about compiling regular expressions.
.. data:: I
IGNORECASE
Perform case-insensitive matching; expressions like ``[A-Z]`` will match
lowercase letters, too. This is not affected by the current locale.
.. data:: L
LOCALE
Make ``\w``, ``\W``, ``\b``, ``\B``, ``\s`` and ``\S`` dependent on the
current locale.
.. data:: M
MULTILINE
When specified, the pattern character ``'^'`` matches at the beginning of the
string and at the beginning of each line (immediately following each newline);
and the pattern character ``'$'`` matches at the end of the string and at the
end of each line (immediately preceding each newline). By default, ``'^'``
matches only at the beginning of the string, and ``'$'`` only at the end of the
string and immediately before the newline (if any) at the end of the string.
.. data:: S
DOTALL
Make the ``'.'`` special character match any character at all, including a
newline; without this flag, ``'.'`` will match anything *except* a newline.
.. data:: U
UNICODE
Make ``\w``, ``\W``, ``\b``, ``\B``, ``\d``, ``\D``, ``\s`` and ``\S`` dependent
on the Unicode character properties database.
.. versionadded:: 2.0
.. data:: X
VERBOSE
This flag allows you to write regular expressions that look nicer. Whitespace
within the pattern is ignored, except when in a character class or preceded by
an unescaped backslash, and, when a line contains a ``'#'`` neither in a
character class or preceded by an unescaped backslash, all characters from the
leftmost such ``'#'`` through the end of the line are ignored.
That means that the two following regular expression objects that match a
decimal number are functionally equal::
a = re.compile(r"""\d + # the integral part
\. # the decimal point
\d * # some fractional digits""", re.X)
b = re.compile(r"\d+\.\d*")
.. function:: search(pattern, string[, flags])
Scan through *string* looking for a location where the regular expression
*pattern* produces a match, and return a corresponding :class:`MatchObject`
instance. Return ``None`` if no position in the string matches the pattern; note
that this is different from finding a zero-length match at some point in the
string.
.. function:: match(pattern, string[, flags])
If zero or more characters at the beginning of *string* match the regular
expression *pattern*, return a corresponding :class:`MatchObject` instance.
Return ``None`` if the string does not match the pattern; note that this is
different from a zero-length match.
.. note::
If you want to locate a match anywhere in *string*, use :func:`search`
instead.
.. function:: split(pattern, string[, maxsplit=0])
Split *string* by the occurrences of *pattern*. If capturing parentheses are
used in *pattern*, then the text of all groups in the pattern are also returned
as part of the resulting list. If *maxsplit* is nonzero, at most *maxsplit*
splits occur, and the remainder of the string is returned as the final element
of the list. (Incompatibility note: in the original Python 1.5 release,
*maxsplit* was ignored. This has been fixed in later releases.)
>>> re.split('\W+', 'Words, words, words.')
['Words', 'words', 'words', '']
>>> re.split('(\W+)', 'Words, words, words.')
['Words', ', ', 'words', ', ', 'words', '.', '']
>>> re.split('\W+', 'Words, words, words.', 1)
['Words', 'words, words.']
If there are capturing groups in the separator and it matches at the start of
the string, the result will start with an empty string. The same holds for
the end of the string:
>>> re.split('(\W+)', '...words, words...')
['', '...', 'words', ', ', 'words', '...', '']
That way, separator components are always found at the same relative
indices within the result list (e.g., if there's one capturing group
in the separator, the 0th, the 2nd and so forth).
Note that *split* will never split a string on an empty pattern match.
For example:
>>> re.split('x*', 'foo')
['foo']
>>> re.split("(?m)^$", "foo\n\nbar\n")
['foo\n\nbar\n']
.. function:: findall(pattern, string[, flags])
Return all non-overlapping matches of *pattern* in *string*, as a list of
strings. The *string* is scanned left-to-right, and matches are returned in
the order found. If one or more groups are present in the pattern, return a
list of groups; this will be a list of tuples if the pattern has more than
one group. Empty matches are included in the result unless they touch the
beginning of another match.
.. versionadded:: 1.5.2
.. versionchanged:: 2.4
Added the optional flags argument.
.. function:: finditer(pattern, string[, flags])
Return an :term:`iterator` yielding :class:`MatchObject` instances over all
non-overlapping matches for the RE *pattern* in *string*. The *string* is
scanned left-to-right, and matches are returned in the order found. Empty
matches are included in the result unless they touch the beginning of another
match.
.. versionadded:: 2.2
.. versionchanged:: 2.4
Added the optional flags argument.
.. function:: sub(pattern, repl, string[, count])
Return the string obtained by replacing the leftmost non-overlapping occurrences
of *pattern* in *string* by the replacement *repl*. If the pattern isn't found,
*string* is returned unchanged. *repl* can be a string or a function; if it is
a string, any backslash escapes in it are processed. That is, ``\n`` is
converted to a single newline character, ``\r`` is converted to a linefeed, and
so forth. Unknown escapes such as ``\j`` are left alone. Backreferences, such
as ``\6``, are replaced with the substring matched by group 6 in the pattern.
For example:
>>> 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<name>`` will use the substring matched by the group named ``name``, as
defined by the ``(?P<name>...)`` syntax. ``\g<number>`` 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.
.. function:: subn(pattern, repl, string[, count])
Perform the same operation as :func:`sub`, but return a tuple ``(new_string,
number_of_subs_made)``.
.. function:: 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.
.. 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.
.. note::
If you want to locate a match anywhere in *string*, use
:meth:`~RegexObject.search` instead.
>>> pattern = re.compile("o")
>>> pattern.match("dog") # No match as "o" is not at the start of "dog".
>>> pattern.match("dog", 1) # Match as "o" is the 2nd character of "dog".
<_sre.SRE_Match object at ...>
.. method:: RegexObject.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 flags argument used when the RE object was compiled, or ``0`` if no flags
were provided.
.. attribute:: RegexObject.groups
The number of capturing groups in the pattern.
.. attribute:: RegexObject.groupindex
A dictionary mapping any symbolic group names defined by ``(?P<id>)`` to group
numbers. The dictionary is empty if no symbolic groups were used in the
pattern.
.. attribute:: RegexObject.pattern
The pattern string from which the RE object was compiled.
.. _match-objects:
Match Objects
-------------
.. class:: MatchObject
Match Objects always have a boolean value of :const:`True`, so that you can test
whether e.g. :func:`match` resulted in a match with a simple if statement. They
support the following methods and attributes:
.. method:: MatchObject.expand(template)
Return the string obtained by doing backslash substitution on the template
string *template*, as done by the :meth:`~RegexObject.sub` method. Escapes
such as ``\n`` are converted to the appropriate characters, and numeric
backreferences (``\1``, ``\2``) and named backreferences (``\g<1>``,
``\g<name>``) are replaced by the contents of the corresponding group.
.. method:: MatchObject.group([group1, ...])
Returns one or more subgroups of the match. If there is a single argument, the
result is a single string; if there are multiple arguments, the result is a
tuple with one item per argument. Without arguments, *group1* defaults to zero
(the whole match is returned). If a *groupN* argument is zero, the corresponding
return value is the entire matching string; if it is in the inclusive range
[1..99], it is the string matching the corresponding parenthesized group. If a
group number is negative or larger than the number of groups defined in the
pattern, an :exc:`IndexError` exception is raised. If a group is contained in a
part of the pattern that did not match, the corresponding result is ``None``.
If a group is contained in a part of the pattern that matched multiple times,
the last match is returned.
>>> m = re.match(r"(\w+) (\w+)", "Isaac Newton, physicist")
>>> m.group(0) # The entire match
'Isaac Newton'
>>> m.group(1) # The first parenthesized subgroup.
'Isaac'
>>> m.group(2) # The second parenthesized subgroup.
'Newton'
>>> m.group(1, 2) # Multiple arguments give us a tuple.
('Isaac', 'Newton')
If the regular expression uses the ``(?P<name>...)`` syntax, the *groupN*
arguments may also be strings identifying groups by their group name. If a
string argument is not used as a group name in the pattern, an :exc:`IndexError`
exception is raised.
A moderately complicated example:
>>> m = re.match(r"(?P<first_name>\w+) (?P<last_name>\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<first_name>\w+) (?P<last_name>\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: %r, groups=%r>' % (match.group(), match.groups())
Suppose you are writing a poker program where a player's hand is represented as
a 5-character string with each character representing a card, "a" for ace, "k"
for king, "q" for queen, j for jack, "0" for 10, and "1" through "9"
representing the card with that value.
To see if a given string is a valid hand, one could do the following:
>>> valid = re.compile(r"[0-9akqj]{5}$")
>>> displaymatch(valid.match("ak05q")) # Valid.
"<Match: 'ak05q', groups=()>"
>>> displaymatch(valid.match("ak05e")) # Invalid.
>>> displaymatch(valid.match("ak0")) # Invalid.
>>> displaymatch(valid.match("727ak")) # Valid.
"<Match: '727ak', groups=()>"
That last hand, ``"727ak"``, contained a pair, or two of the same valued cards.
To match this with a regular expression, one could use backreferences as such:
>>> pair = re.compile(r".*(.).*\1")
>>> displaymatch(pair.match("717ak")) # Pair of 7s.
"<Match: '717', groups=('7',)>"
>>> displaymatch(pair.match("718ak")) # No pairs.
>>> displaymatch(pair.match("354aa")) # Pair of aces.
"<Match: '354aa', groups=('a',)>"
To find out what card the pair consists of, one could use the
: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 "<pyshell#23>", line 1, in <module>
re.match(r".*(.).*\1", "718ak").group(1)
AttributeError: 'NoneType' object has no attribute 'group'
>>> pair.match("354aa").group(1)
'a'
Simulating scanf()
^^^^^^^^^^^^^^^^^^
.. index:: single: scanf()
Python does not currently have an equivalent to :cfunc:`scanf`. Regular
expressions are generally more powerful, though also more verbose, than
:cfunc:`scanf` format strings. The table below offers some more-or-less
equivalent mappings between :cfunc:`scanf` format tokens and regular
expressions.
+--------------------------------+---------------------------------------------+
| :cfunc:`scanf` Token | Regular Expression |
+================================+=============================================+
| ``%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 :cfunc:`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 :exc:`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 "<stdin>", line 1, in ?
File "/usr/local/lib/python2.5/re.py", line 132, in match
return _compile(pattern, flags).match(string)
RuntimeError: maximum recursion limit exceeded
You can often restructure your regular expression to avoid recursion.
Starting with Python 2.3, simple uses of the ``*?`` pattern are special-cased to
avoid recursion. Thus, the above regular expression can avoid recursion by
being recast as ``Begin [a-zA-Z0-9_ ]*?end``. As a further benefit, such
regular expressions will run faster than their recursive equivalents.
search() vs. match()
^^^^^^^^^^^^^^^^^^^^
In a nutshell, :func:`match` only attempts to match a pattern at the beginning
of a string where :func:`search` will match a pattern anywhere in a string.
For example:
>>> re.match("o", "dog") # No match as "o" is not the first letter of "dog".
>>> re.search("o", "dog") # Match as search() looks everywhere in the string.
<_sre.SRE_Match object at ...>
.. note::
The following applies only to regular expression objects like those created
with ``re.compile("pattern")``, not the primitives ``re.match(pattern,
string)`` or ``re.search(pattern, string)``.
:func:`match` has an optional second parameter that gives an index in the string
where the search is to start::
>>> pattern = re.compile("o")
>>> pattern.match("dog") # No match as "o" is not at the start of "dog."
# Equivalent to the above expression as 0 is the default starting index:
>>> pattern.match("dog", 0)
# Match as "o" is the 2nd character of "dog" (index 0 is the first):
>>> pattern.match("dog", 1)
<_sre.SRE_Match object at ...>
>>> pattern.match("dog", 2) # No match as "o" is not the 3rd character of "dog."
Making a Phonebook
^^^^^^^^^^^^^^^^^^
:func:`split` splits a string into a list delimited by the passed pattern. The
method is invaluable for converting textual data into data structures that can be
easily read and modified by Python as demonstrated in the following example that
creates a phonebook.
First, here is the input. Normally it may come from a file, here we are using
triple-quoted string syntax:
>>> input = """Ross McFluff: 834.345.1254 155 Elm Street
...
... Ronald Heathmore: 892.345.3428 436 Finley Avenue
... Frank Burger: 925.541.7625 662 South Dogwood Way
...
...
... Heather Albrecht: 548.326.4584 919 Park Place"""
The entries are separated by one or more newlines. Now we convert the string
into a list with each nonempty line having its own entry:
.. doctest::
:options: +NORMALIZE_WHITESPACE
>>> entries = re.split("\n+", input)
>>> entries
['Ross McFluff: 834.345.1254 155 Elm Street',
'Ronald Heathmore: 892.345.3428 436 Finley Avenue',
'Frank Burger: 925.541.7625 662 South Dogwood Way',
'Heather Albrecht: 548.326.4584 919 Park Place']
Finally, split each entry into a list with first name, last name, telephone
number, and address. We use the ``maxsplit`` parameter of :func:`split`
because the address has spaces, our splitting pattern, in it:
.. doctest::
:options: +NORMALIZE_WHITESPACE
>>> [re.split(":? ", entry, 3) for entry in entries]
[['Ross', 'McFluff', '834.345.1254', '155 Elm Street'],
['Ronald', 'Heathmore', '892.345.3428', '436 Finley Avenue'],
['Frank', 'Burger', '925.541.7625', '662 South Dogwood Way'],
['Heather', 'Albrecht', '548.326.4584', '919 Park Place']]
The ``:?`` pattern matches the colon after the last name, so that it does not
occur in the result list. With a ``maxsplit`` of ``4``, we could separate the
house number from the street name:
.. doctest::
:options: +NORMALIZE_WHITESPACE
>>> [re.split(":? ", entry, 4) for entry in entries]
[['Ross', 'McFluff', '834.345.1254', '155', 'Elm Street'],
['Ronald', 'Heathmore', '892.345.3428', '436', 'Finley Avenue'],
['Frank', 'Burger', '925.541.7625', '662', 'South Dogwood Way'],
['Heather', 'Albrecht', '548.326.4584', '919', 'Park Place']]
Text Munging
^^^^^^^^^^^^
:func:`sub` replaces every occurrence of a pattern with a string or the
result of a function. This example demonstrates using :func:`sub` with
a function to "munge" text, or randomize the order of all the characters
in each word of a sentence except for the first and last characters::
>>> def repl(m):
... inner_word = list(m.group(2))
... random.shuffle(inner_word)
... return m.group(1) + "".join(inner_word) + m.group(3)
>>> text = "Professor Abdolmalek, please report your absences promptly."
>>> re.sub(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 one was a writer and wanted to
find all of the adverbs in some text, he or she might use :func:`findall` in
the following manner:
>>> text = "He was carefully disguised but captured quickly by police."
>>> re.findall(r"\w+ly", text)
['carefully', 'quickly']
Finding all Adverbs and their Positions
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
If one wants more information about all matches of a pattern than the matched
text, :func:`finditer` is useful as it provides instances of
:class:`MatchObject` instead of strings. Continuing with the previous example,
if one was a writer who wanted to find all of the adverbs *and their positions*
in some text, he or she would use :func:`finditer` in the following manner:
>>> text = "He was carefully disguised but captured quickly by police."
>>> for m in re.finditer(r"\w+ly", text):
... print '%02d-%02d: %s' % (m.start(), m.end(), m.group(0))
07-16: carefully
40-47: quickly
Raw String Notation
^^^^^^^^^^^^^^^^^^^
Raw string notation (``r"text"``) keeps regular expressions sane. Without it,
every backslash (``'\'``) in a regular expression would have to be prefixed with
another one to escape it. For example, the two following lines of code are
functionally identical:
>>> re.match(r"\W(.)\1\W", " ff ")
<_sre.SRE_Match object at ...>
>>> re.match("\\W(.)\\1\\W", " ff ")
<_sre.SRE_Match object at ...>
When one wants to match a literal backslash, it must be escaped in the regular
expression. With raw string notation, this means ``r"\\"``. Without raw string
notation, one must use ``"\\\\"``, making the following lines of code
functionally identical:
>>> re.match(r"\\", r"\\")
<_sre.SRE_Match object at ...>
>>> re.match("\\\\", r"\\")
<_sre.SRE_Match object at ...>
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