cheatsheet.md

Python Regular Expressions Cheatsheet

From https://docs.python.org/3/library/re.html:

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

An overview of regular expression syntax as implemented by the re built-in module is shown below. Assume ASCII character set unless otherwise specified.

The cheatsheet and examples presented in this app are based on the contents of my Understanding Python re(gex)? ebook. Visit my GitHub repo https://github.com/learnbyexample/py_regular_expressions for details.

Anchors

Anchors	Description
\A	restricts the match to the start of string
\Z	restricts the match to the end of string
^	restricts the match to the start of line
$	restricts the match to the end of line
\n	newline character is used as line separator
re.MULTILINE or re.M	flag to treat input as multiline string
\b	restricts the match to the start/end of words
	word characters: alphabets, digits, underscore
\B	matches wherever \b doesn't match

^, $ and \ are metacharacters in the above table, as these characters have special meaning. Prefix a \ character to remove the special meaning and match such characters literally. For example, \^ will match a ^ character instead of acting as an anchor.

Alternation and Grouping

Feature	Description
|	multiple RE combined as conditional OR
	each alternative can have independent anchors
(pat)	group pattern(s), also a capturing group
	a(b|c)d is same as abd|acd
(?:pat)	non-capturing group
(?P<name>pat)	named capture group

Quantifiers

Quantifiers	Description
*	Match zero or more times
+	Match one or more times
?	Match zero or one times
{m,n}	Match m to n times (inclusive)
{m,}	Match at least m times
{,n}	Match up to n times (including 0 times)
{n}	Match exactly n times
pat1.*pat2	any number of characters between pat1 and pat2
pat1.*pat2|pat2.*pat1	match both pat1 and pat2 in any order

The above table is for greedy quantifiers. Greedy here means that these quantifiers will match as much as possible that'll also honor the overall RE. Appending a ? to greedy quantifiers makes them non-greedy, i.e. match as minimally as possible. Appending a + to greedy quantifiers makes them possessive, which prevents backtracking. You can also use (?>pat) atomic grouping to safeguard from backtracking. Quantifiers can be applied to literal characters, groups, backreferences and character classes.

Dot metacharacter and Character class

Class	Description
.	Match any character except the newline character \n
[]	Character class, matches one character among many
[aeiou]	Match any vowel
[^aeiou]	^ inverts selection, so this matches any consonant
[a-f]	- defines a range, so this matches any of abcdef characters
\d	Match a digit, same as [0-9]
\D	Match non-digit, same as [^0-9] or [^\d]
\w	Match word character, same as [a-zA-Z0-9_]
\W	Match non-word character, same as [^a-zA-Z0-9_] or [^\w]
\s	Match whitespace character, same as [\ \t\n\r\f\v]
\S	Match non-whitespace character, same as [^\ \t\n\r\f\v] or [^\s]

Lookarounds

Lookarounds	Description
lookarounds	custom assertions, zero-width like anchors
(?!pat)	negative lookahead assertion
(?<!pat)	negative lookbehind assertion
(?=pat)	positive lookahead assertion
(?<=pat)	positive lookbehind assertion
(?!pat1)(?=pat2)	multiple assertions can be specified in any order
	as they mark a matching location without consuming characters
((?!pat).)*	Negate a grouping, similar to negated character class

Flags

Flags	Description
re.IGNORECASE or re.I	flag to ignore case
re.DOTALL or re.S	allow . metacharacter to match newline character
flags=re.S|re.I	multiple flags can be combined using | operator
re.MULTILINE or re.M	allow ^ and $ anchors to match line wise
re.VERBOSE or re.X	allows to use literal whitespaces for aligning purposes
	and to add comments after the # character
	escape spaces and # if needed as part of actual RE
re.ASCII or re.A	match only ASCII characters for \b, \w, \d, \s
	and their opposites, applicable only for Unicode patterns
re.LOCALE or re.L	use locale settings for byte patterns and 8-bit locales
(?#comment)	another way to add comments, not a flag
(?flags:pat)	inline flags only for this pat, overrides flags argument
	flags is i for re.I, s for re.S, etc, except L for re.L
(?-flags:pat)	negate flags only for this pat
(?flags-flags:pat)	apply and negate particular flags only for this pat
(?flags)	apply flags for whole RE, can be used only at start of RE
	anchors if any, should be specified after (?flags)

Working with matched portions

Matched portion	Description
re.Match object	details like matched portions, location, etc
m[0] or m.group(0)	entire matched portion of re.Match object m
m[n] or m.group(n)	matched portion of nth capture group
m.groups()	tuple of all the capture groups' matched portions
m.span()	start and end+1 index of entire matched portion
	pass a number to get span of that particular capture group
	can also use m.start() and m.end()
\N	backreference, gives matched portion of Nth capture group
	applies to both search and replacement sections
	possible values: \1, \2 up to \99 provided no more digits
\g<N>	backreference, gives matched portion of Nth capture group
	possible values: \g<0>, \g<1>, etc (not limited to 99)
	\g<0> refers to the entire matched portion
(?P<name>pat)	named capture group
	refer as 'name' in re.Match object
	refer as (?P=name) in search section
	refer as \g<name> in replacement section
groupdict()	method applied on a re.Match object
	gives named capture group portions as a dict

Note: \0 and \100 onwards are considered as octal values, hence cannot be used as backreferences.

re module functions

Function	Description
re.search	Check if given pattern is present anywhere in input string
	Output is a re.Match object, usable in conditional expressions
	r-strings preferred to define RE
	Use byte pattern for byte input
	Python also maintains a small cache of recent RE
re.fullmatch	ensures pattern matches the entire input string
re.compile	Compile a pattern for reuse, outputs re.Pattern object
re.sub	search and replace
re.sub(r'pat', f, s)	function f with re.Match object as argument
re.escape	automatically escape all metacharacters
re.split	split a string based on RE
	text matched by the groups will be part of the output
	portion matched by pattern outside group won't be in output
re.findall	returns all the matches as a list
	if 1 capture group is used, only its matches are returned
	1+, each element will be tuple of capture groups
	portion matched by pattern outside group won't be in output
re.finditer	iterator with re.Match object for each match
re.subn	gives tuple of modified string and number of substitutions

The function definitions are given below:

re.search(pattern, string, flags=0)
re.fullmatch(pattern, string, flags=0)
re.compile(pattern, flags=0)
re.sub(pattern, repl, string, count=0, flags=0)
re.escape(pattern)
re.split(pattern, string, maxsplit=0, flags=0)
re.findall(pattern, string, flags=0)
re.finditer(pattern, string, flags=0)
re.subn(pattern, repl, string, count=0, flags=0)