=========================
re -- Regular Expressions
=========================
.. module:: re
:synopsis: Searching within and changing text using formal patterns.
:Purpose: Searching within and changing text using formal patterns.
:Python Version: 1.5 and later
*Regular expressions* are text matching patterns described with a
formal syntax. The patterns are interpreted as a set of instructions,
which are then executed with a string as input to produce a matching
subset or modified version of the original. The term "regular
expressions" is frequently shortened to as "regex" or "regexp" in
conversation. Expressions can include literal text matching,
repetition, pattern-composition, branching, and other sophisticated
rules. A large number of parsing problems are easier to solve with a
regular expression than by creating a special-purpose lexer and
parser.
Regular expressions are typically used in applications that involve a
lot of text processing. For example, they are commonly used as search
patterns in text editing programs used by developers, including vi,
emacs, and modern IDEs. They are also an integral part of Unix
command line utilities such as sed, grep, and awk. Many programming
languages include support for regular expressions in the language
syntax (Perl, Ruby, Awk, and Tcl). Other languages, such as C, C++,
and Python supports regular expressions through extension libraries.
There are multiple open source implementations of regular expressions,
each sharing a common core syntax but with different extensions or
modifications to their advanced features. The syntax used in Python's
:mod:`re` module is based on the syntax used for regular expressions
in Perl, with a few Python-specific enhancements.
.. note::
Although the formal definition of "regular expression" is limited
to expressions that describe regular languages, some of the
extensions supported by :mod:`re` go beyond describing regular
languages. The term "regular expression" is used here in a more
general sense to mean any expression that can be evaluated by
Python's :mod:`re` module.
Finding Patterns in Text
========================
The most common use for :mod:`re` is to search for patterns in text.
This example looks for two literal strings, ``'this'`` and ``'that'``,
in a text string.
.. include:: re_simple.py
:literal:
:start-after: #end_pymotw_header
:func:`search` takes the pattern and text to scan, and returns a
:class:`Match` object when the pattern is found. If the pattern is
not found, :func:`search` returns ``None``.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_simple.py'))
.. }}}
::
$ python re_simple.py
Looking for "this" in "Does this text match the pattern?" -> found a match!
Looking for "that" in "Does this text match the pattern?" -> no match
.. {{{end}}}
The :class:`Match` object returned by :func:`search` holds information
about the nature of the match, including the original input string,
the regular expression used, and the location within the original
string where the pattern occurs.
.. include:: re_simple_match.py
:literal:
:start-after: #end_pymotw_header
The :func:`start` and :func:`end` methods give the integer indexes
into the string showing where the text matched by the pattern occurs.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_simple_match.py'))
.. }}}
::
$ python re_simple_match.py
Found "this" in "Does this text match the pattern?" from 5 to 9 ("this")
.. {{{end}}}
Compiling Expressions
=====================
:mod:`re` includes module-level functions for working with regular
expressions as text strings, but it is usually more efficient to
*compile* the expressions your program uses frequently. The
:func:`compile` function converts an expression string into a
:class:`RegexObject`.
.. include:: re_simple_compiled.py
:literal:
:start-after: #end_pymotw_header
The module-level functions maintain a cache of compiled expressions,
but the size of the cache is limited and using compiled expressions
directly means you can avoid the cache lookup overhead. By
pre-compiling any expressions your module uses when the module is
loaded you shift the compilation work to application startup time,
instead of a point where the program is responding to a user action.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_simple_compiled.py'))
.. }}}
::
$ python re_simple_compiled.py
Looking for "this" in "Does this text match the pattern?" -> found a match!
Looking for "that" in "Does this text match the pattern?" -> no match
.. {{{end}}}
Multiple Matches
================
So far the example patterns have all used :func:`search` to look for
single instances of literal text strings. The :func:`findall`
function returns all of the substrings of the input that match the
pattern without overlapping.
.. include:: re_findall.py
:literal:
:start-after: #end_pymotw_header
There are two instances of ``ab`` in the input string.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_findall.py'))
.. }}}
::
$ python re_findall.py
Found "ab"
Found "ab"
.. {{{end}}}
:func:`finditer` returns an iterator that produces :class:`Match`
instances instead of the strings returned by :func:`findall`.
.. include:: re_finditer.py
:literal:
:start-after: #end_pymotw_header
This example finds the same two occurrences of ``ab``, and the
:class:`Match` instance shows where they are in the original input.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_finditer.py'))
.. }}}
::
$ python re_finditer.py
Found "ab" at 0:2
Found "ab" at 5:7
.. {{{end}}}
Pattern Syntax
==============
Regular expressions support more powerful patterns than simple literal
text strings. Patterns can repeat, can be anchored to different
logical locations within the input, and can be expressed in compact
forms that don't require every literal character be present in the
pattern. All of these features are used by combining literal text
values with *metacharacters* that are part of the regular expression
pattern syntax implemented by :mod:`re`. The following examples will
use this test program to explore variations in patterns.
.. include:: re_test_patterns.py
:literal:
:start-after: #end_pymotw_header
The output of :func:`test_patterns` shows the input text, including
the character positions, as well as the substring range from each
portion of the input that matches the pattern.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_test_patterns.py'))
.. }}}
::
$ python re_test_patterns.py
11111
012345678901234
abbaaabbbbaaaaa
Matching "ab"
0 : 1 = "ab"
5 : 6 = "ab"
.. {{{end}}}
Repetition
----------
There are five ways to express repetition in a pattern. A pattern
followed by the metacharacter ``*`` is repeated zero or more times
(allowing a pattern to repeat zero times means it does not need to
appear at all to match). Replace the ``*`` with ``+`` and the pattern
must appear at least once. Using ``?`` means the pattern appears zero
or one time. For a specific number of occurrences, use ``{m}`` after
the pattern, where *m* is replaced with the number of times the
pattern should repeat. And finally, to allow a variable but limited
number of repetitions, use ``{m,n}`` where *m* is the minimum number
of repetitions and *n* is the maximum. Leaving out *n* (``{m,}``)
means the value appears at least *m* times, with no maximum.
.. include:: re_repetition.py
:literal:
:start-after: #end_pymotw_header
Notice how many more matches there are for ``ab*`` and ``ab?`` than
``ab+``.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_repetition.py'))
.. }}}
::
$ python re_repetition.py
11111
012345678901234
abbaaabbbbaaaaa
Matching "ab*"
0 : 2 = "abb"
3 : 3 = "a"
4 : 4 = "a"
5 : 9 = "abbbb"
10 : 10 = "a"
11 : 11 = "a"
12 : 12 = "a"
13 : 13 = "a"
14 : 14 = "a"
Matching "ab+"
0 : 2 = "abb"
5 : 9 = "abbbb"
Matching "ab?"
0 : 1 = "ab"
3 : 3 = "a"
4 : 4 = "a"
5 : 6 = "ab"
10 : 10 = "a"
11 : 11 = "a"
12 : 12 = "a"
13 : 13 = "a"
14 : 14 = "a"
Matching "ab{3}"
5 : 8 = "abbb"
Matching "ab{2,3}"
0 : 2 = "abb"
5 : 8 = "abbb"
.. {{{end}}}
The normal processing for a repetition instruction is to consume as
much of the input as possible while matching the pattern. This
so-called *greedy* behavior may result in fewer individual matches, or
the matches may include more of the input text than intended.
Greediness can be turned off by following the repetition instruction
with ``?``.
.. include:: re_repetition_non_greedy.py
:literal:
:start-after: #end_pymotw_header
Disabling greedy consumption of the input for any of the patterns
where zero occurences of ``b`` are allowed means the matched substring
does not include any ``b`` characters.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_repetition_non_greedy.py'))
.. }}}
::
$ python re_repetition_non_greedy.py
11111
012345678901234
abbaaabbbbaaaaa
Matching "ab*?"
0 : 0 = "a"
3 : 3 = "a"
4 : 4 = "a"
5 : 5 = "a"
10 : 10 = "a"
11 : 11 = "a"
12 : 12 = "a"
13 : 13 = "a"
14 : 14 = "a"
Matching "ab+?"
0 : 1 = "ab"
5 : 6 = "ab"
Matching "ab??"
0 : 0 = "a"
3 : 3 = "a"
4 : 4 = "a"
5 : 5 = "a"
10 : 10 = "a"
11 : 11 = "a"
12 : 12 = "a"
13 : 13 = "a"
14 : 14 = "a"
Matching "ab{3}?"
5 : 8 = "abbb"
Matching "ab{2,3}?"
0 : 2 = "abb"
5 : 7 = "abb"
.. {{{end}}}
Character Sets
--------------
A *character set* is a group of characters, any one of which can match
at that point in the pattern. For example, ``[ab]`` would match
either ``a`` or ``b``.
.. include:: re_charset.py
:literal:
:start-after: #end_pymotw_header
The greedy form of the expression, ``a[ab]+``, consumes the entire
string because the first letter is ``a`` and every subsequent
character is either ``a`` or ``b``.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_charset.py'))
.. }}}
::
$ python re_charset.py
11111
012345678901234
abbaaabbbbaaaaa
Matching "[ab]"
0 : 0 = "a"
1 : 1 = "b"
2 : 2 = "b"
3 : 3 = "a"
4 : 4 = "a"
5 : 5 = "a"
6 : 6 = "b"
7 : 7 = "b"
8 : 8 = "b"
9 : 9 = "b"
10 : 10 = "a"
11 : 11 = "a"
12 : 12 = "a"
13 : 13 = "a"
14 : 14 = "a"
Matching "a[ab]+"
0 : 14 = "abbaaabbbbaaaaa"
Matching "a[ab]+?"
0 : 1 = "ab"
3 : 4 = "aa"
5 : 6 = "ab"
10 : 11 = "aa"
12 : 13 = "aa"
.. {{{end}}}
A character set can also be used to exclude specific characters. The
special marker ``^`` means to look for characters not in the set
following.
.. include:: re_charset_exclude.py
:literal:
:start-after: #end_pymotw_header
This pattern finds all of the substrings that do not contain the
characters ``-``, ``.``, or a space.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_charset_exclude.py'))
.. }}}
::
$ python re_charset_exclude.py
1111111111222222222233333333
01234567890123456789012345678901234567
This is some text -- with punctuation.
Matching "[^-. ]+"
0 : 3 = "This"
5 : 6 = "is"
8 : 11 = "some"
13 : 16 = "text"
21 : 24 = "with"
26 : 36 = "punctuation"
.. {{{end}}}
As character sets grow larger, typing every character that should (or
should not) match becomes tedious. A more compact format using
*character ranges* lets you define a character set to include all of
the contiguous characters between a start and stop point.
.. include:: re_charset_ranges.py
:literal:
:start-after: #end_pymotw_header
Here the range ``a-z`` includes the lower case ASCII letters, and the
range ``A-Z`` includes the upper case ASCII letters. The ranges can
also be combined into a single character set.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_charset_ranges.py'))
.. }}}
::
$ python re_charset_ranges.py
1111111111222222222233333333
01234567890123456789012345678901234567
This is some text -- with punctuation.
Matching "[a-z]+"
1 : 3 = "his"
5 : 6 = "is"
8 : 11 = "some"
13 : 16 = "text"
21 : 24 = "with"
26 : 36 = "punctuation"
Matching "[A-Z]+"
0 : 0 = "T"
Matching "[a-zA-Z]+"
0 : 3 = "This"
5 : 6 = "is"
8 : 11 = "some"
13 : 16 = "text"
21 : 24 = "with"
26 : 36 = "punctuation"
Matching "[A-Z][a-z]+"
0 : 3 = "This"
.. {{{end}}}
As a special case of a character set the metacharacter dot, or period
(``.``), indicates that the pattern should match any single character
in that position.
.. include:: re_charset_dot.py
:literal:
:start-after: #end_pymotw_header
Combining dot with repetition can result in very long matches, unless
the non-greedy form is used.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_charset_dot.py'))
.. }}}
::
$ python re_charset_dot.py
11111
012345678901234
abbaaabbbbaaaaa
Matching "a."
0 : 1 = "ab"
3 : 4 = "aa"
5 : 6 = "ab"
10 : 11 = "aa"
12 : 13 = "aa"
Matching "b."
1 : 2 = "bb"
6 : 7 = "bb"
8 : 9 = "bb"
Matching "a.*b"
0 : 9 = "abbaaabbbb"
Matching "a.*?b"
0 : 1 = "ab"
3 : 6 = "aaab"
.. {{{end}}}
Escape Codes
------------
An even more compact representation uses escape codes for several
pre-defined character sets. The escape codes recognized by :mod:`re`
are:
====== =======
Code Meaning
====== =======
``\d`` a digit
``\D`` a non-digit
``\s`` whitespace (tab, space, newline, etc.)
``\S`` non-whitespace
``\w`` alphanumeric
``\W`` non-alphanumeric
====== =======
.. note::
Escapes are indicated by prefixing the character with a backslash
(``\``). Unfortunately, a backslash must itself be escaped in
normal Python strings, and that results in expressions that are
difficult to read. Using *raw* strings, created by prefixing the
literal value with ``r``, for creating regular expressions
eliminates this problem and maintains readability.
.. include:: re_escape_codes.py
:literal:
:start-after: #end_pymotw_header
These sample expressions combine escape codes with repetition to find
sequences of like characters in the input string.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_escape_codes.py'))
.. }}}
::
$ python re_escape_codes.py
11111111112222222
012345678901234567890123456
This is a prime #1 example!
Matching "\d+"
17 : 17 = "1"
Matching "\D+"
0 : 16 = "This is a prime #"
18 : 26 = " example!"
Matching "\s+"
4 : 4 = " "
7 : 7 = " "
9 : 9 = " "
15 : 15 = " "
18 : 18 = " "
Matching "\S+"
0 : 3 = "This"
5 : 6 = "is"
8 : 8 = "a"
10 : 14 = "prime"
16 : 17 = "#1"
19 : 26 = "example!"
Matching "\w+"
0 : 3 = "This"
5 : 6 = "is"
8 : 8 = "a"
10 : 14 = "prime"
17 : 17 = "1"
19 : 25 = "example"
Matching "\W+"
4 : 4 = " "
7 : 7 = " "
9 : 9 = " "
15 : 16 = " #"
18 : 18 = " "
26 : 26 = "!"
.. {{{end}}}
To match the characters that are part of the regular expression
syntax, escape the characters in the search pattern.
.. include:: re_escape_escapes.py
:literal:
:start-after: #end_pymotw_header
These patterns escape the backslash and plus characters, since as
metacharacters both have special meaning in a regular expression.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_escape_escapes.py'))
.. }}}
::
$ python re_escape_escapes.py
1111111111222
01234567890123456789012
\d+ \D+ \s+ \S+ \w+ \W+
Matching "\\d\+"
0 : 2 = "\d+"
Matching "\\D\+"
4 : 6 = "\D+"
Matching "\\s\+"
8 : 10 = "\s+"
Matching "\\S\+"
12 : 14 = "\S+"
Matching "\\w\+"
16 : 18 = "\w+"
Matching "\\W\+"
20 : 22 = "\W+"
.. {{{end}}}
Anchoring
---------
In addition to describing the content of a pattern to match, you can
also specify the relative location in the input text where the pattern
should appear using *anchoring* instructions.
====== =======
Code Meaning
====== =======
``^`` start of string, or line
``$`` end of string, or line
``\A`` start of string
``\Z`` end of string
``\b`` empty string at the beginning or end of a word
``\B`` empty string not at the beginning or end of a word
====== =======
.. include:: re_anchoring.py
:literal:
:start-after: #end_pymotw_header
The patterns in the example for matching words at the beginning and
end of the string are different because the word at the end of the
string is followed by punctuation to terminate the sentence. The
pattern ``\w+$`` would not match, since ``.`` is not considered an
alphanumeric character.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_anchoring.py'))
.. }}}
::
$ python re_anchoring.py
1111111111222222222233333333
01234567890123456789012345678901234567
This is some text -- with punctuation.
Matching "^\w+"
0 : 3 = "This"
Matching "\A\w+"
0 : 3 = "This"
Matching "\w+\S*$"
26 : 37 = "punctuation."
Matching "\w+\S*\Z"
26 : 37 = "punctuation."
Matching "\w*t\w*"
13 : 16 = "text"
21 : 24 = "with"
26 : 36 = "punctuation"
Matching "\bt\w+"
13 : 16 = "text"
Matching "\w+t\b"
13 : 16 = "text"
Matching "\Bt\B"
23 : 23 = "t"
30 : 30 = "t"
33 : 33 = "t"
.. {{{end}}}
Constraining the Search
=======================
In situations where you know in advance that only a subset of the full
input should be searched, you can further constrain the regular
expression match by telling :mod:`re` to limit the search range. For
example, if your pattern must appear at the front of the input, then
using :func:`match` instead of :func:`search` will anchor the search
without having to explicitly include an anchor in the search pattern.
.. include:: re_match.py
:literal:
:start-after: #end_pymotw_header
Since the literal text ``is`` does not appear at the start of the
input text, it is not found using :func:`match`. The sequence appears
two other times in the text, though, so :func:`search` finds it.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_match.py'))
.. }}}
::
$ python re_match.py
Text : This is some text -- with punctuation.
Pattern: is
Match : None
Search : <_sre.SRE_Match object at 0x1002d2ed0>
.. {{{end}}}
The :func:`search` method of a compiled regular expression accepts
optional *start* and *end* position parameters to limit the search to
a substring of the input.
.. include:: re_search_substring.py
:literal:
:start-after: #end_pymotw_header
This example implements a less efficient form of :func:`iterall`.
Each time a match is found, the end position of that match is used for
the next search.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_search_substring.py'))
.. }}}
::
$ python re_search_substring.py
Text: This is some text -- with punctuation.
0 : 3 = "This"
5 : 6 = "is"
.. {{{end}}}
Dissecting Matches with Groups
==============================
Searching for pattern matches is the basis of the powerful
capabilities provided by regular expressions. Adding *groups* to a
pattern lets you isolate parts of the matching text, expanding those
capabilities to create a parser. Groups are defined by enclosing
patterns in parentheses (``(`` and ``)``).
.. include:: re_groups.py
:literal:
:start-after: #end_pymotw_header
Any complete regular expression can be converted to a group and nested
within a larger expression. All of the repetition modifiers can be
applied to a group as a whole, requiring the entire group pattern to
repeat.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_groups.py'))
.. }}}
::
$ python re_groups.py
11111
012345678901234
abbaaabbbbaaaaa
Matching "a(ab)"
4 : 6 = "aab"
Matching "a(a*b*)"
0 : 2 = "abb"
3 : 9 = "aaabbbb"
10 : 14 = "aaaaa"
Matching "a(ab)*"
0 : 0 = "a"
3 : 3 = "a"
4 : 6 = "aab"
10 : 10 = "a"
11 : 11 = "a"
12 : 12 = "a"
13 : 13 = "a"
14 : 14 = "a"
Matching "a(ab)+"
4 : 6 = "aab"
.. {{{end}}}
To access the substrings matched by the individual groups within a
pattern, use the :func:`groups` method of the :class:`Match` object.
.. include:: re_groups_match.py
:literal:
:start-after: #end_pymotw_header
:func:`Match.groups` returns a sequence of strings in the order of the
group within the expression that matches the string.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_groups_match.py'))
.. }}}
::
$ python re_groups_match.py
This is some text -- with punctuation.
Matching "^(\w+)"
('This',)
Matching "(\w+)\S*$"
('punctuation',)
Matching "(\bt\w+)\W+(\w+)"
('text', 'with')
Matching "(\w+t)\b"
('text',)
.. {{{end}}}
If you are using grouping to find parts of the string, but you don't
need all of the parts matched by groups, you can ask for the match of
only a single group with :func:`group`.
.. include:: re_groups_individual.py
:literal:
:start-after: #end_pymotw_header
Group ``0`` represents the string matched by the entire expression,
and sub-groups are numbered starting with ``1`` in the order their
left parenthesis appears in the expression.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_groups_individual.py'))
.. }}}
::
$ python re_groups_individual.py
Input text : This is some text -- with punctuation.
Pattern : (\bt\w+)\W+(\w+)
Entire match : text -- with
Word starting with "t": text
Word after "t" word : with
.. {{{end}}}
Python extends the basic grouping syntax to add *named groups*. Using
names to refer to groups makes it easier to modify the pattern over
time, without having to also modify the code using the match results.
To set the name of a group, use the syntax ``(P?<name>pattern)``.
.. include:: re_groups_named.py
:literal:
:start-after: #end_pymotw_header
Use :func:`groupdict` to retrieve the dictionary mapping group names
to substrings from the match. Named patterns are included in the
ordered sequence returned by :func:`groups`, as well.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_groups_named.py'))
.. }}}
::
$ python re_groups_named.py
This is some text -- with punctuation.
Matching "^(?P<first_word>\w+)"
('This',)
{'first_word': 'This'}
Matching "(?P<last_word>\w+)\S*$"
('punctuation',)
{'last_word': 'punctuation'}
Matching "(?P<t_word>\bt\w+)\W+(?P<other_word>\w+)"
('text', 'with')
{'other_word': 'with', 't_word': 'text'}
Matching "(?P<ends_with_t>\w+t)\b"
('text',)
{'ends_with_t': 'text'}
.. {{{end}}}
An updated version of :func:`test_patterns` that shows the numbered
and named groups matched by a pattern will make the following examples
easier to follow.
.. include:: re_test_patterns_groups.py
:literal:
:start-after: #end_pymotw_header
Since a group is itself a complete regular expression, groups can be
nested within other groups to build even more complicated expressions.
.. include:: re_groups_nested.py
:literal:
:start-after: #end_pymotw_header
In this case, the group ``(a*)`` matches an empty string, so the
return value from :func:`groups` includes that empty string as the
matched value.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_groups_nested.py'))
.. }}}
::
$ python re_groups_nested.py
11111
012345678901234
abbaaabbbbaaaaa
Matching "a((a*)(b*))"
0 : 2 = "abb"
Groups: ('bb', '', 'bb')
3 : 9 = "aaabbbb"
Groups: ('aabbbb', 'aa', 'bbbb')
10 : 14 = "aaaaa"
Groups: ('aaaa', 'aaaa', '')
.. {{{end}}}
Groups are also useful for specifying alternative patterns. Use ``|``
to indicate that one pattern or another should match. Consider the
placement of the ``|`` carefully, though. The first expression in
this example matches a sequence of ``a`` followed by a sequence
consisting entirely of a single letter, ``a`` or ``b``. The second
pattern matches ``a`` followed by a sequence that may include *either*
``a`` or ``b``. The patterns are similar, but the resulting matches
are completely different.
.. include:: re_groups_alternative.py
:literal:
:start-after: #end_pymotw_header
When an alternative group is not matched, but the entire pattern does
match, the return value of :func:`groups` includes a ``None`` value at
the point in the sequence where the alternative group should appear.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_groups_alternative.py'))
.. }}}
::
$ python re_groups_alternative.py
11111
012345678901234
abbaaabbbbaaaaa
Matching "a((a+)|(b+))"
0 : 2 = "abb"
Groups: ('bb', None, 'bb')
3 : 5 = "aaa"
Groups: ('aa', 'aa', None)
10 : 14 = "aaaaa"
Groups: ('aaaa', 'aaaa', None)
Matching "a((a|b)+)"
0 : 14 = "abbaaabbbbaaaaa"
Groups: ('bbaaabbbbaaaaa', 'a')
.. {{{end}}}
Defining a group containing a sub-pattern is also useful in cases
where the string matching the sub-pattern is not part of what you want
to extract from the full text. These groups are called
*non-capturing*. To create a non-capturing group, use the syntax
``(?:pattern)``.
.. include:: re_groups_non_capturing.py
:literal:
:start-after: #end_pymotw_header
Compare the groups returned for the capturing and non-capturing forms
of a pattern that matches the same results.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_groups_non_capturing.py'))
.. }}}
::
$ python re_groups_non_capturing.py
11111
012345678901234
abbaaabbbbaaaaa
Matching "a((a+)|(b+))"
0 : 2 = "abb"
Groups: ('bb', None, 'bb')
3 : 5 = "aaa"
Groups: ('aa', 'aa', None)
10 : 14 = "aaaaa"
Groups: ('aaaa', 'aaaa', None)
Matching "a((?:a+)|(?:b+))"
0 : 2 = "abb"
Groups: ('bb',)
3 : 5 = "aaa"
Groups: ('aa',)
10 : 14 = "aaaaa"
Groups: ('aaaa',)
.. {{{end}}}
Search Options
==============
You can change the way the matching engine processes an expression
using option flags. The flags can be combined using a bitwise or
operation, and passed to :func:`compile`, :func:`search`,
:func:`match`, and other functions that accept a pattern for
searching.
Case-insensitive Matching
-------------------------
:const:`IGNORECASE` causes literal characters and character ranges in
the pattern to match both upper and lower case characters.
.. include:: re_flags_ignorecase.py
:literal:
:start-after: #end_pymotw_header
Since the pattern includes the literal ``T``, without setting
:const:`IGNORECASE` the only match is the word ``This``. When case is
ignored, ``text`` also matches.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_flags_ignorecase.py'))
.. }}}
::
$ python re_flags_ignorecase.py
Text : This is some text -- with punctuation.
Pattern : \bT\w+
Case-sensitive : ['This']
Case-insensitive: ['This', 'text']
.. {{{end}}}
Input with Multiple Lines
-------------------------
There are two flags that effect how searching in multi-line input
works. The :const:`MULTILINE` flag controls how the pattern matching
code processes anchoring instructions for text containing newline
characters. When multiline mode is turned on, the anchor rules for
``^`` and ``$`` apply at the beginning and end of each line, in
addition to the entire string.
.. include:: re_flags_multiline.py
:literal:
:start-after: #end_pymotw_header
The pattern in the example matches the first or last word of the
input. It matches ``line.`` at the end of the string, even though
there is no newline.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_flags_multiline.py'))
.. }}}
::
$ python re_flags_multiline.py
Text : 'This is some text -- with punctuation.\nAnd a second line.'
Pattern : (^\w+)|(\w+\S*$)
Single Line : [('This', ''), ('', 'line.')]
Multline : [('This', ''), ('', 'punctuation.'), ('And', ''), ('', 'line.')]
.. {{{end}}}
:const:`DOTALL` is the other flag related to multiline text. Normally
the dot character ``.`` matches everything in the input text except a
newline character. The flag allows dot to match newlines as well.
.. include:: re_flags_dotall.py
:literal:
:start-after: #end_pymotw_header
Without the flag, each line of the input text matches the pattern
separately. Adding the flag causes the entire string to be consumed.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_flags_dotall.py'))
.. }}}
::
$ python re_flags_dotall.py
Text : 'This is some text -- with punctuation.\nAnd a second line.'
Pattern : .+
No newlines : ['This is some text -- with punctuation.', 'And a second line.']
Dotall : ['This is some text -- with punctuation.\nAnd a second line.']
.. {{{end}}}
Unicode
-------
Under Python 2, :class:`str` objects use the ASCII character set, and
regular expression processing assumes that the pattern and input text
are both ASCII. The escape codes described earlier are defined in
terms of ASCII by default. Those assumptions mean that the pattern
``\w+`` will match the word "French" but not "Français", since the
``ç`` is not part of the ASCII character set. To enable Unicode
matching in Python 2, add the :const:`UNICODE` flag when compiling the
pattern.
.. include:: re_flags_unicode.py
:literal:
:start-after: #end_pymotw_header
The other escape sequences (``\W``, ``\b``, ``\B``, ``\d``, ``\D``,
``\s``, and ``\S``) are also processed differently for Unicode text.
Instead of assuming the members of the character set identified by the
escape sequence, the regular expression engine consults the Unicode
database to find the properties of each character.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_flags_unicode.py'))
.. }}}
::
$ python re_flags_unicode.py
Text : Français złoty Österreich
Pattern : \w+
ASCII : Fran, ais, z, oty, sterreich
Unicode : Français, złoty, Österreich
.. {{{end}}}
.. note::
Python 3 uses Unicode for all strings by default, so the flag is not
necessary.
Verbose Expression Syntax
-------------------------
The compact format of regular expression syntax can become a hindrance
as expressions grow more complicated. As the number of groups in your
expression increases, you will have trouble keeping track of why each
element is needed and how exactly the parts of the expression
interact. Using named groups helps mitigate these issues, but a
better solution is to use *verbose mode* expressions, which allow you
to add comments and extra whitespace.
A pattern to validate email addresses will illustrate how verbose mode
makes working with regular expressions easier. The first version
recognizes addresses that end in one of three top-level domains,
``.com``, ``.org``, and ``.edu``.
.. include:: re_email_compact.py
:literal:
:start-after: #end_pymotw_header
This expression is already complex. There are several character
classes, groups, and repetition expressions.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_email_compact.py'))
.. }}}
::
$ python re_email_compact.py
Candidate: first.last@example.com
Matches
Candidate: first.last+category@gmail.com
Matches
Candidate: valid-address@mail.example.com
Matches
Candidate: not-valid@example.foo
No match
.. {{{end}}}
Converting the expression to a more verbose format will make it easier
to extend.
.. include:: re_email_verbose.py
:literal:
:start-after: #end_pymotw_header
The expression matches the same inputs, but in this extended format it
is easier to read. The comments also help identify different parts of
the pattern so that it can be expanded to match more inputs.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_email_verbose.py'))
.. }}}
::
$ python re_email_verbose.py
Candidate: first.last@example.com
Matches
Candidate: first.last+category@gmail.com
Matches
Candidate: valid-address@mail.example.com
Matches
Candidate: not-valid@example.foo
No match
.. {{{end}}}
This expanded version parses inputs that include a person's name and
email address, as might appear in an email header. The name comes
first and stands on its own, and the email address follows surrounded
by angle brackets (``<`` and ``>``).
.. include:: re_email_with_name.py
:literal:
:start-after: #end_pymotw_header
As with other programming languages, the ability to insert comments
into verbose regular expressions helps with their maintainability.
This final version includes implementation notes to future maintainers
and whitespace to separate the groups from each other and highlight
their nesting level.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_email_with_name.py'))
.. }}}
::
$ python re_email_with_name.py
Candidate: first.last@example.com
Match name : None
Match email: first.last@example.com
Candidate: first.last+category@gmail.com
Match name : None
Match email: first.last+category@gmail.com
Candidate: valid-address@mail.example.com
Match name : None
Match email: valid-address@mail.example.com
Candidate: not-valid@example.foo
No match
Candidate: First Last <first.last@example.com>
Match name : First Last
Match email: first.last@example.com
Candidate: No Brackets first.last@example.com
Match name : None
Match email: first.last@example.com
Candidate: First Last
No match
Candidate: First Middle Last <first.last@example.com>
Match name : First Middle Last
Match email: first.last@example.com
Candidate: First M. Last <first.last@example.com>
Match name : First M. Last
Match email: first.last@example.com
Candidate: <first.last@example.com>
Match name : None
Match email: first.last@example.com
.. {{{end}}}
Embedding Flags in Patterns
---------------------------
In situations where you cannot add flags when compiling an expression,
such as when you are passing a pattern to a library function that will
compile it later, you can embed the flags inside the expression string
itself. For example, to turn case-insensitive matching on, add
``(?i)`` to the beginning of the expression.
.. include:: re_flags_embedded.py
:literal:
:start-after: #end_pymotw_header
Because the options control the way the entire expression is evaluated
or parsed, they should always come at the beginning of the expression.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_flags_embedded.py'))
.. }}}
::
$ python re_flags_embedded.py
Text : This is some text -- with punctuation.
Pattern : (?i)\bT\w+
Matches : ['This', 'text']
.. {{{end}}}
The abbreviations for all of the flags are:
==================== ============
Flag Abbreviation
==================== ============
:const:`IGNORECASE` ``i``
:const:`MULTILINE` ``m``
:const:`DOTALL` ``s``
:const:`UNICODE` ``u``
:const:`VERBOSE` ``x``
==================== ============
Embedded flags can be combined by placing them within the same group.
For example, ``(?imu)`` turns on case-insensitive matching for
multiline Unicode strings.
Looking Ahead, or Behind
========================
There are many cases where it is useful to match a part of a pattern
only if some other part will also match. For example, in the email
parsing expression the angle brackets were each marked as optional.
Really, though, the brackets should be paired, and the expression
should only match if both are present, or neither are. This modified
version of the expression uses a *positive look ahead* assertion to
match the pair. The look ahead assertion syntax is ``(?=pattern)``.
.. include:: re_look_ahead.py
:literal:
:start-after: #end_pymotw_header
There are several important changes in this version of the expression.
First, the name portion is no longer optional. That means stand-alone
addresses do not match, but it also prevents improperly formatted
name/address combinations from matching. The positive look ahead rule
after the "name" group asserts that the remainder of the string is
either wrapped with a pair of angle brackets, or there is not a
mismatched bracket; the brackets are either both present or neither
is. The look ahead is expressed as a group, but the match for a look
ahead group does not consume any of the input text, so the rest of the
pattern picks up from the same spot after the look ahead matches.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_look_ahead.py'))
.. }}}
::
$ python re_look_ahead.py
Candidate: First Last <first.last@example.com>
Match name : First Last
Match email: first.last@example.com
Candidate: No Brackets first.last@example.com
Match name : No Brackets
Match email: first.last@example.com
Candidate: Open Bracket <first.last@example.com
No match
Candidate: Close Bracket first.last@example.com>
No match
.. {{{end}}}
A *negative look ahead* assertion (``(?!pattern)``) says that the
pattern does not match the text following the current point. For
example, the email recognition pattern could be modified to ignore
``noreply`` mailing addresses commonly used by automated systems.
.. include:: re_negative_look_ahead.py
:literal:
:start-after: #end_pymotw_header
The address starting ``noreply`` does not match the pattern, since the
look ahead assertion fails.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_negative_look_ahead.py'))
.. }}}
::
$ python re_negative_look_ahead.py
Candidate: first.last@example.com
Match: first.last@example.com
Candidate: noreply@example.com
No match
.. {{{end}}}
Instead of looking ahead for ``noreply`` in the username portion of
the email address, the pattern can also be written using a *negative
look behind* assertion after the username is matched using the syntax
``(?<!pattern)``.
.. include:: re_negative_look_behind.py
:literal:
:start-after: #end_pymotw_header
Looking backwards works a little differently than looking ahead, in
that the expression must use a fixed length pattern. Repetitions are
allowed, as long as there is a fixed number (no wildcards or ranges).
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_negative_look_behind.py'))
.. }}}
::
$ python re_negative_look_behind.py
Candidate: first.last@example.com
Match: first.last@example.com
Candidate: noreply@example.com
No match
.. {{{end}}}
A *positive look behind* assertion can be used to find text following
a pattern using the syntax ``(?<=pattern)``. For example, this
expression finds Twitter handles.
.. include:: re_look_behind.py
:literal:
:start-after: #end_pymotw_header
The pattern matches sequences of characters that can make up a Twitter
handle, as long as they are preceded by an ``@``.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_look_behind.py'))
.. }}}
::
$ python re_look_behind.py
This text includes two Twitter handles.
One for @ThePSF, and one for the author, @doughellmann.
Handle: ThePSF
Handle: doughellmann
.. {{{end}}}
Self-referencing Expressions
============================
Matched values can be used in later parts of an expression. For
example, the email example can be updated to match only addresses
composed of the first and last name of the person by including
back-references to those groups. The easiest way to achieve this is
by referring to the previously matched group by id number, using
``\num``.
.. include:: re_refer_to_group.py
:literal:
:start-after: #end_pymotw_header
Although the syntax is simple, creating back-references by numerical
id has a couple of disadvantages. From a practical standpoint, as the
expression changes, you must count the groups again and possibly
update every reference. The other disadvantage is that only 99
references can be made this way, because if the id number is three
digits long it will be interpreted as an octal character value instead
of a group reference. On the other hand, if you have more than 99
groups in your expression you will have more serious maintenance
challenges than not being able to refer to some of the groups in the
expression.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_refer_to_group.py'))
.. }}}
::
$ python re_refer_to_group.py
Candidate: First Last <first.last@example.com>
Match name : First Last
Match email: first.last@example.com
Candidate: Different Name <first.last@example.com>
No match
Candidate: First Middle Last <first.last@example.com>
Match name : First Last
Match email: first.last@example.com
Candidate: First M. Last <first.last@example.com>
Match name : First Last
Match email: first.last@example.com
.. {{{end}}}
Python's expression parser includes an extension that uses
``(?P=name)`` to refer to the value of a named group matched earlier
in the expression.
.. include:: re_refer_to_named_group.py
:literal:
:start-after: #end_pymotw_header
The address expression is compiled with the :const:`IGNORECASE` flag
on, since proper names are normally capitalized but email addresses
are not.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_refer_to_named_group.py'))
.. }}}
::
$ python re_refer_to_named_group.py
Candidate: First Last <first.last@example.com>
Match name : First Last
Match email: first.last@example.com
Candidate: Different Name <first.last@example.com>
No match
Candidate: First Middle Last <first.last@example.com>
Match name : First Last
Match email: first.last@example.com
Candidate: First M. Last <first.last@example.com>
Match name : First Last
Match email: first.last@example.com
.. {{{end}}}
The other mechanism for using back-references in expressions lets you
choose a different pattern based on whether or not a previous group
matched. The email pattern can be corrected so that the angle
brackets are required if a name is present, and not if the email
address is by itself. The syntax for testing to see if a group has
matched is ``(?(id)yes-expression|no-expression)``, where *id* is the
group name or number, *yes-expression* is the pattern to use if the
group has a value and *no-expression* is the pattern to use otherwise.
.. include:: re_id.py
:literal:
:start-after: #end_pymotw_header
This version of the email address parser uses two tests. If the
``name`` group matches, then the look ahead assertion requires both
angle brackets and sets up the ``brackets`` group. If ``name`` is not
matched, the assertion requires the rest of the text not have angle
brackets around it. Later, if the ``brackets`` group is set, the
actual pattern matching code consumes the brackets in the input using
literal patterns, otherwise it consumes any blank space.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_id.py'))
.. }}}
::
$ python re_id.py
Candidate: First Last <first.last@example.com>
Match name : First Last
Match email: first.last@example.com
Candidate: No Brackets first.last@example.com
No match
Candidate: Open Bracket <first.last@example.com
No match
Candidate: Close Bracket first.last@example.com>
No match
Candidate: no.brackets@example.com
Match name : None
Match email: no.brackets@example.com
.. {{{end}}}
Modifying Strings with Patterns
===============================
In addition to searching through text, :mod:`re` also supports
modifying text using regular expressions as the search mechanism, and
the replacements can reference groups matched in the regex as part of
the substitution text. Use :func:`sub` to replace all occurances of a
pattern with another string.
.. include:: re_sub.py
:literal:
:start-after: #end_pymotw_header
References to the text matched by the pattern can be inserted using
the ``\num`` syntax used for back-references above.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_sub.py'))
.. }}}
::
$ python re_sub.py
Text: Make this **bold**. This **too**.
Bold: Make this <b>bold</b>. This <b>too</b>.
.. {{{end}}}
To use named groups in the substitution, use the syntax ``\g<name>``.
.. include:: re_sub_named_groups.py
:literal:
:start-after: #end_pymotw_header
The ``\g<name>`` syntax also works with numbered references, and using
it eliminates any ambiguity between group numbers and surrounding
literal digits.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_sub_named_groups.py'))
.. }}}
::
$ python re_sub_named_groups.py
Text: Make this **bold**. This **too**.
Bold: Make this <b>bold</b>. This <b>too</b>.
.. {{{end}}}
Pass a value to *count* to limit the number of substitutions
performed.
.. include:: re_sub_count.py
:literal:
:start-after: #end_pymotw_header
Only the first substitution is made because *count* is ``1``.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_sub_count.py'))
.. }}}
::
$ python re_sub_count.py
Text: Make this **bold**. This **too**.
Bold: Make this <b>bold</b>. This **too**.
.. {{{end}}}
:func:`subn` works just like :func:`sub` except that it returns both
the modified string and the count of substitutions made.
.. include:: re_subn.py
:literal:
:start-after: #end_pymotw_header
The search pattern matches twice in the example.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_subn.py'))
.. }}}
::
$ python re_subn.py
Text: Make this **bold**. This **too**.
Bold: ('Make this <b>bold</b>. This <b>too</b>.', 2)
.. {{{end}}}
Splitting with Patterns
=======================
:func:`str.split` is one of the most frequently used methods for
breaking apart strings to parse them. It only supports using literal
values as separators, though, and sometimes a regular expression is
necessary if the input is not consistently formatted. For example,
many plain text markup languages define paragraph separators as two or
more newline (``\n``) characters. In this case, :func:`str.split`
cannot be used because of the "or more" part of the definition.
A strategy for identifying paragraphs using :func:`findall` would use
a pattern like ``(.+?)\n{2,}``.
.. include:: re_paragraphs_findall.py
:literal:
:start-after: #end_pymotw_header
That pattern fails for paragraphs at the end of the input text, as
illustrated by the fact that "Paragraph three." is not part of the
output.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_paragraphs_findall.py'))
.. }}}
::
$ python re_paragraphs_findall.py
0 'Paragraph one\non two lines.'
1 'Paragraph two.'
.. {{{end}}}
Extending the pattern to say that a paragraph ends with two or more
newlines, or the end of input, fixes the problem but makes the pattern
more complicated. Converting to :func:`re.split` instead of
:func:`re.findall` handles the boundary condition automatically and
keeps the pattern simple.
.. include:: re_split.py
:literal:
:start-after: #end_pymotw_header
The pattern argument to :func:`split` expresses the markup
specification more precisely: Two or more newline characters mark a
separator point between paragraphs in the input string.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_split.py'))
.. }}}
::
$ python re_split.py
With findall:
0 ('Paragraph one\non two lines.', '\n\n')
1 ('Paragraph two.', '\n\n\n')
2 ('Paragraph three.', '')
With split:
0 'Paragraph one\non two lines.'
1 'Paragraph two.'
2 'Paragraph three.'
.. {{{end}}}
Enclosing the expression in parentheses to define a group causes
:func:`split` to work more like :func:`str.partition`, so it returns
the separator values as well as the other parts of the string.
.. include:: re_split_groups.py
:literal:
:start-after: #end_pymotw_header
The output now includes each paragraph, as well as the sequence of
newlines separating them.
.. {{{cog
.. cog.out(run_script(cog.inFile, 're_split_groups.py'))
.. }}}
::
$ python re_split_groups.py
With split:
0 'Paragraph one\non two lines.'
1 '\n\n'
2 'Paragraph two.'
3 '\n\n\n'
4 'Paragraph three.'
.. {{{end}}}
.. seealso::
`re <http://docs.python.org/library/re.html>`_
The standard library documentation for this module.
`Regular Expression HOWTO <http://docs.python.org/howto/regex.html>`__
Andrew Kuchling's introduction to regular expressions for Python developers.
`Kodos <http://kodos.sourceforge.net/>`_
An interactive regular expression testing tool by Phil Schwartz.
`Python Regular Expression Testing Tool <http://www.pythonregex.com/>`_
A web-based tool for testing regular expressions created by
David Naffziger at BrandVerity.com. Inspired by Kodos.
`Wikipedia: Regular expression <http://en.wikipedia.org/wiki/Regular_expressions>`__
General introduction to regular expression concepts and techniques.
:mod:`locale`
Use the :mod:`locale` module to set your language
configuration when working with Unicode text.
:mod:`unicodedata`
Programmatic access to the Unicode character property database.