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.
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
- Matches any decimal digit; this is equivalent to the set [0-9].
\D
- Matches any non-digit character; this is equivalent to the set [^0-9].
\s
- Matches any whitespace character; this is equivalent to the set [
\t\n\r\f\v].
\S
- Matches any non-whitespace character; this is equivalent to the set [^
\t\n\r\f\v].
\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 \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.
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