Python Regular Expression's Cheat Sheet (borrowed from pythex)

Special Characters

• \ escape special characters
• . matches any character
• ^ matches beginning of string
• $ matches end of string
• [5b-d] matches any chars '5', 'b', 'c' or 'd'
• [^a-c6] matches any char except 'a', 'b', 'c' or '6'
• R|S matches either regex R or regex S
• () creates a capture group and indicates precedence

Quantifiers

• * 0 or more (append ? for non-greedy)
• + 1 or more (append ? for non-greedy)
• ? 0 or 1 (append ? for non-greedy)
• {m} exactly mm occurrences
• {m, n} from m to n. m defaults to 0, n to infinity
• {m, n}? from m to n, as few as possible

Special sequences

• \A start of string
• \b matches empty string at word boundary (between \w and \W)
• \B matches empty string not at word boundary
• \d digit
• \D non-digit
• \s whitespace: [ \t\n\r\f\v]
• \S non-whitespace
• \w alphanumeric: [0-9a-zA-Z_]
• \W non-alphanumeric
• \Z end of string
• \g<id> matches a previously defined group

Extensions

• (?iLmsux) Matches empty string, sets re.X flags
• (?:...) Non-capturing version of regular parentheses
• (?P<name>...) Creates a named capturing group.
• (?P=name) Matches whatever matched previously named group
• (?#...) A comment; ignored.
• (?=...) Lookahead assertion: Matches without consuming
• (?!...) Negative lookahead assertion
• (?<=...) Lookbehind assertion: Matches if preceded
• (?<!...) Negative lookbehind assertion
• (?(id)yes|no) Match 'yes' if group 'id' matched, else 'no'

(1) . Matches any single character except newline

>>> s = "foo.123.bar"
>>> re.search(".", s)
<re.Match object; span=(0, 1), match='f'>
>>>
>>> re.search('\.', s)
<re.Match object; span=(3, 4), match='.'>
>>>

(2) ^ Anchors a match at the start of a string

>>> s = "foo123bar"
>>> re.search('^f', s)
<re.Match object; span=(0, 1), match='f'>
>>>

(3) $ Anchors a match at the end of a string

>>> s = 'foo123oof'
>>> re.search('f', s)
<re.Match object; span=(0, 1), match='f'>
>>> re.search('f$', s)
<re.Match object; span=(8, 9), match='f'>
>>>

(4) * Matches zero or more repetitions

>>> s = 'foo123'
>>> re.search('fo*', s)
<re.Match object; span=(0, 3), match='foo'>
>>> s = 'f123'
>>> re.search('fo*', s)
<re.Match object; span=(0, 1), match='f'>
>>>

(5) + Matches one or more repetitions

>>> s = 'foo123'
>>> re.search('fo+', s)
<re.Match object; span=(0, 3), match='foo'>
>>> s = 'f123'
>>> re.search('fo+', s)
>>>

(6) {} Matches an explicitly specified number of repetitions

>>> s = 'foo'
>>> re.search('o{1,2}', s)
<re.Match object; span=(1, 3), match='oo'>
>>> re.search('o{1}', s)
<re.Match object; span=(1, 2), match='o'>
>>>

(7) \ Escapes a metacharacter of its special meaning

>>> s = 'foo123bar'
>>>
>>> re.search('\d+', s)
<re.Match object; span=(3, 6), match='123'>
>>> re.search('\D+', s)
<re.Match object; span=(0, 3), match='foo'>
>>>

(8) [] Specifies a character class

>>> s = 'foo123bar'
>>> re.search('[0-9]{3}', s)
<re.Match object; span=(3, 6), match='123'>
>>> re.search('[a-z]*', s)
<re.Match object; span=(0, 3), match='foo'>