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sandbox/morph / Doc / library / re.rst

:mod:`re` --- Regular expression operations

This module provides regular expression matching operations similar to those found in Perl. Both patterns and strings to be searched can be Unicode strings as well as 8-bit strings.

Regular expressions use the backslash character ('\') to indicate special forms or to allow special characters to be used without invoking their special meaning. This collides with Python's usage of the same character for the same purpose in string literals; for example, to match a literal backslash, one might have to write '\\\\' as the pattern string, because the regular expression must be \\, and each backslash must be expressed as \\ inside a regular Python string literal.

The solution is to use Python's raw string notation for regular expression patterns; backslashes are not handled in any special way in a string literal prefixed with 'r'. So r"\n" is a two-character string containing '\' and 'n', while "\n" is a one-character string containing a newline. Usually patterns will be expressed in Python code using this raw string notation.

It is important to note that most regular expression operations are available as module-level functions and :class:`RegexObject` methods. The functions are shortcuts that don't require you to compile a regex object first, but miss some fine-tuning parameters.

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. In a set:

  • Characters can be listed individually, e.g. [amk] will match 'a', 'm', or 'k'.
  • Ranges of characters can be indicated by giving two characters and separating them by a '-', for example [a-z] will match any lowercase ASCII letter, [0-5][0-9] will match all the two-digits numbers from 00 to 59, and [0-9A-Fa-f] will match any hexadecimal digit. If - is escaped (e.g. [a\-z]) or if it's placed as the first or last character (e.g. [a-]), it will match a literal '-'.
  • Special characters lose their special meaning inside sets. For example, [(+*)] will match any of the literal characters '(', '+', '*', or ')'.
  • Character classes such as \w or \S (defined below) are also accepted inside a set, although the characters they match depends on whether :const:`LOCALE` or :const:`UNICODE` mode is in force.
  • Characters that are not within a range can be matched by :dfn:`complementing` the set. If the first character of the set is '^', all the characters that are not in the set will be matched. For example, [^5] will match any character except '5', and [^^] will match any character except '^'. ^ has no special meaning if it's not the first character in the set.
  • To match a literal ']' inside a set, precede it with a backslash, or place it at the beginning of the set. For example, both [()[\]{}] and []()[{}] will both match a parenthesis.
'|'
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-capturing 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'.

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 decimal 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 vs Searching

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 ...>

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.

Regular Expression Objects

The :class:`RegexObject` class supports the following methods and attributes:

Match Objects

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:

Examples

Checking For a Pair

In this example, we'll use the following helper function to display match objects a little more gracefully:

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, "t" for 10, and "2" 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"^[a2-9tjqk]{5}$")
>>> displaymatch(valid.match("akt5q"))  # Valid.
"<Match: 'akt5q', groups=()>"
>>> displaymatch(valid.match("akt5e"))  # Invalid.
>>> displaymatch(valid.match("akt"))    # 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:

Simulating scanf()

Python does not currently have an equivalent to :c:func:`scanf`. Regular expressions are generally more powerful, though also more verbose, than :c:func:`scanf` format strings. The table below offers some more-or-less equivalent mappings between :c:func:`scanf` format tokens and regular expressions.

:c:func:`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 :c:func:`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:

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:

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:

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 ...>