4.2.1 Regular Expression Syntax Python Library Reference

4.2.1 Regular Expression Syntax Python Library Reference
Python Library Reference
4.2.1 Regular Expression Syntax
A regular expression (or RE) specifies a set of strings that matches it; the functions in this module let you check
if a particular string matches a given regular expression (or if a given regular expression matches a particular
string, which comes down to the same thing).
Regular expressions can be concatenated to form new regular expressions; if A and B are both regular
expressions, then AB is also a regular expression. In general, if a string p matches A and another string q matches
B, the string pq will match AB. This holds unless A or B contain low precedence operations; boundary conditions
between A and B; or have numbered group references. Thus, complex expressions can easily be constructed from
simpler primitive expressions like the ones described here. For details of the theory and implementation of
regular expressions, consult the Friedl book referenced above, or almost any textbook about compiler
construction.
A brief explanation of the format of regular expressions follows. For further information and a gentler
presentation, consult the Regular Expression HOWTO, accessible from http://www.python.org/doc/howto/.
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.
The special characters are:
"."
(Dot.) In the default mode, this matches any character except a newline. If the DOTALL flag has been
specified, this matches any character including a newline.
"^"
(Caret.) Matches the start of the string, and in MULTILINE mode also matches immediately after each
newline.
"$"
Matches the end of the string or just before the newline at the end of the string, and in MULTILINE mode
also matches before a newline. foo matches both 'foo' and 'foobar', while the regular expression foo$
matches only 'foo'. More interestingly, searching for foo.$ in 'foo1\nfoo2\n' matches 'foo2' normally, but
'foo1' in MULTILINE mode.
"*"
Causes the resulting RE to match 0 or more repetitions of the preceding RE, as many repetitions as are
possible. ab* will match 'a', 'ab', or 'a' followed by any number of 'b's.
"+"
Causes the resulting RE to match 1 or more repetitions of the preceding RE. ab+ will match 'a' followed
by any non-zero number of 'b's; it will not match just 'a'.
"?"
Causes the resulting RE to match 0 or 1 repetitions of the preceding RE. ab? will match either 'a' or 'ab'.
*?, +?, ??
The "*", "+", and "?" qualifiers are all greedy; they match as much text as possible. Sometimes this
behaviour isn't desired; if the RE <.*> is matched against '<H1>title</H1>', it will match the entire
string, and not just '<H1>'. Adding "?" after the qualifier makes it perform the match in non-greedy or
minimal fashion; as few characters as possible will be matched. Using .*? in the previous expression will
match only '<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. If you want to include a "]" or a "-" inside a set,
precede it with a backslash, or place it as the first character. The pattern []] will match ']', for example.
You can match the characters not within a range by complementing the set. This is indicated by including a
"^" as the first character of the set; "^" elsewhere will simply match the "^" character. For example, [^5]
will match any character except "5", and [^^] will match any character except "^".
"|"
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 (re.I, re.L, re.M, re.S, re.U, re.X) for the entire regular
expression. This is useful if you wish to include the flags as part of the regular expression, instead of
passing a flag argument to the compile() function.
Note that the (?x)
flag changes how the expression is parsed. It should be used first in the expression string, or after one or
more whitespace characters. If there are non-whitespace characters before the flag, the results are
undefined.
(?:...)
A non-grouping version of regular parentheses. Matches whatever regular expression is inside the
parentheses, but the substring matched by the group cannot be retrieved after performing a match or
referenced later in the pattern.
(?P<name>...)
Similar to regular parentheses, but the substring matched by the group is accessible via the symbolic group
name name. Group names must be valid Python identifiers, and each group name must be defined only
once within a regular expression. A symbolic group is also a numbered group, just as if the group were not
named. So the group named 'id' in the example above can also be referenced as the numbered group 1.
For example, if the pattern is (?P<id>[a-zA-Z_]\w*), the group can be referenced by its name in
arguments to methods of match objects, such as m.group('id') or m.end('id'), and also by name
in pattern text (for example, (?P=id)) and replacement text (such as \g<id>).
(?P=name)
Matches whatever text was matched by the earlier group named name.
(?#...)
A comment; the contents of the parentheses are simply ignored.
(?=...)
Matches if ...
matches next, but doesn't consume any of the string. This is called a lookahead assertion. For example,
Isaac (?=Asimov) will match 'Isaac ' only if it's followed by 'Asimov'.
(?!...)
Matches if ... doesn't match next. This is a negative lookahead assertion. For example, Isaac
(?!Asimov) will match 'Isaac ' only if it's not followed by 'Asimov'.
(?<=...)
Matches if the current position in the string is preceded by a match for ... that ends at the current
position. This is called a positive lookbehind assertion. (?<=abc)def will find a match in "abcdef",
since the lookbehind will back up 3 characters and check if the contained pattern matches. The contained
pattern must only match strings of some fixed length, meaning that abc or a|b are allowed, but a* and
a{3,4}
are not. Note that patterns which start with positive lookbehind assertions will never match at the
beginning of the string being searched; you will most likely want to use the search() function rather
than the match() function:
>>> import re
>>> m = re.search('(?<=abc)def', 'abcdef')
>>> m.group(0)
'def'
This example looks for a word following a hyphen:
>>> m = re.search('(?<=-)\w+', 'spam-egg')
>>> m.group(0)
'egg'
(?<!...)
Matches if the current position in the string is not preceded by a match for .... This is called a 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,
(<)?(\[email protected]\w+(?:\.\w+)+)(?(1)>) is a poor email matching pattern, which will match with
'<[email protected]>' as well as '[email protected]', but not with '<[email protected]'. New in
version 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 UNICODE flag is not specified, matches any decimal digit; this is equivalent to the set [0-9].
With UNICODE, it will match whatever is classified as a digit in the Unicode character properties database.
\D
When the UNICODE flag is not specified, matches any non-digit character; this is equivalent to the set
[^0-9]. With UNICODE, it will match anything other than character marked as digits in the Unicode
character properties database.
\s
When the LOCALE and UNICODE
flags are not specified, matches any whitespace character; this is equivalent to the set [ \t\n\r\f\v].
With LOCALE, it will match this set plus whatever characters are defined as space for the current locale. If
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 LOCALE and UNICODE
flags are not specified, matches any non-whitespace character; this is equivalent to the set [^
\t\n\r\f\v] With LOCALE, it will match any character not in this set, and not defined as space in the
current locale. If 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 LOCALE and UNICODE
flags are not specified, matches any alphanumeric character and the underscore; this is equivalent to the set
[a-zA-Z0-9_]. With LOCALE, it will match the set [0-9_] plus whatever characters are defined as
alphanumeric for the current locale. If 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 LOCALE and UNICODE
flags are not specified, matches any non-alphanumeric character; this is equivalent to the set
[^a-zA-Z0-9_]. With LOCALE, it will match any character not in the set [0-9_], and not defined as
alphanumeric for the current locale. If 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
\r
\b
\t
\f
\v
\n
\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.
Release 2.5, documentation updated on 19th September, 2006.
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