The tables below are a reference to basic regex. While reading the rest of the site, when in doubt, you can always come back and look here. (It you want a bookmark, here's a direct link to the regex reference tables). I encourage you to print the tables so you have a cheat sheet on your desk for quick reference.

Pythex is a real-time regular expression editor for Python, a quick way to test your regular expressions. Python 2.7 Regular Expressions Special characters: escapes special characters. Matches any character ^ matches start of the string (or line if MULTILINE) $ matches end of the string (or line if MULTILINE) 5b-d matches any chars '5', 'b', 'c' or 'd' ^a-c6 matches any char except 'a', 'b', 'c' or '6'. The match function matches the Python RegEx pattern to the string with optional flags. Syntax: re.match(pattern, string, flags=0) Where ‘pattern’ is a regular expression to be matched, and the second parameter is a Python String that will be searched to match the pattern at the starting of the string.

The tables are not exhaustive, for two reasons. First, every regex flavor is different, and I didn't want to crowd the page with overly exotic syntax. For a full reference to the particular regex flavors you'll be using, it's always best to go straight to the source. In fact, for some regex engines (such as Perl, PCRE, Java and .NET) you may want to check once a year, as their creators often introduce new features.

1. 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 This blog post gives an overview and examples of regular expression syntax as implemented by the re built-in module (Python 3.8+).
2. A truly Pythonic cheat sheet about Python programming language. Types': Type, String, RegularExp. That offer the same functionality as lambda expressions.

The other reason the tables are not exhaustive is that I wanted them to serve as a quick introduction to regex. If you are a complete beginner, you should get a firm grasp of basic regex syntax just by reading the examples in the tables. I tried to introduce features in a logical order and to keep out oddities that I've never seen in actual use, such as the 'bell character'. With these tables as a jumping board, you will be able to advance to mastery by exploring the other pages on the site.

How to use the tables

The tables are meant to serve as an accelerated regex course, and they are meant to be read slowly, one line at a time. On each line, in the leftmost column, you will find a new element of regex syntax. The next column, 'Legend', explains what the element means (or encodes) in the regex syntax. The next two columns work hand in hand: the 'Example' column gives a valid regular expression that uses the element, and the 'Sample Match' column presents a text string that could be matched by the regular expression.
You can read the tables online, of course, but if you suffer from even the mildest case of online-ADD (attention deficit disorder), like most of us… Well then, I highly recommend you print them out. You'll be able to study them slowly, and to use them as a cheat sheet later, when you are reading the rest of the site or experimenting with your own regular expressions.
Enjoy!
If you overdose, make sure not to miss the next page, which comes back down to Earth and talks about some really cool stuff: The 1001 ways to use Regex.

Regular Expression Syntax

Regex Accelerated Course and Cheat Sheet

For easy navigation, here are some jumping points to various sections of the page:
✽ Characters
✽ Quantifiers
✽ More Characters
✽ Logic
✽ More White-Space
✽ More Quantifiers
✽ Character Classes
✽ Anchors and Boundaries
✽ POSIX Classes
✽ Inline Modifiers
✽ Lookarounds
✽ Character Class Operations
✽ Other Syntax
(direct link)

Characters

Character	Legend	Example	Sample Match
d	Most engines: one digit from 0 to 9	file_dd	file_25
d	.NET, Python 3: one Unicode digit in any script	file_dd	file_9੩
w	Most engines: 'word character': ASCII letter, digit or underscore	w-www	A-b_1
w	.Python 3: 'word character': Unicode letter, ideogram, digit, or underscore	w-www	字-ま_۳
w	.NET: 'word character': Unicode letter, ideogram, digit, or connector	w-www	字-ま‿۳
s	Most engines: 'whitespace character': space, tab, newline, carriage return, vertical tab	asbsc	a b c
s	.NET, Python 3, JavaScript: 'whitespace character': any Unicode separator	asbsc	a b c
D	One character that is not a digit as defined by your engine's d	DDD	ABC
W	One character that is not a word character as defined by your engine's w	WWWWW	*-+=)
S	One character that is not a whitespace character as defined by your engine's s	SSSS	Yoyo

(direct link)

Quantifiers

Quantifier	Legend	Example	Sample Match
+	One or more	Version w-w+	Version A-b1_1
{3}	Exactly three times	D{3}	ABC
{2,4}	Two to four times	d{2,4}	156
{3,}	Three or more times	w{3,}	regex_tutorial
*	Zero or more times	A*B*C*	AAACC
?	Once or none	plurals?	plural

(direct link)

More Characters

Character	Legend	Example	Sample Match
.	Any character except line break	a.c	abc
.	Any character except line break	.*	whatever, man.
.	A period (special character: needs to be escaped by a )	a.c	a.c
	Escapes a special character	.*+? $^/	.*+? $^/
	Escapes a special character	[{()}]	[{()}]

(direct link)

Logic

Regular Expression In Python Cheat Sheet

Logic	Legend	Example	Sample Match
|	Alternation / OR operand	22|33	33
( … )	Capturing group	A(nt|pple)	Apple (captures 'pple')
1	Contents of Group 1	r(w)g1x	regex
2	Contents of Group 2	(dd)+(dd)=2+1	12+65=65+12
(?: … )	Non-capturing group	A(?:nt|pple)	Apple

(direct link)

More White-Space

Character	Legend	Example	Sample Match
t	Tab	Ttw{2}	T ab
r	Carriage return character	see below
n	Line feed character	see below
rn	Line separator on Windows	ABrnCD	AB CD
N	Perl, PCRE (C, PHP, R…): one character that is not a line break	N+	ABC
h	Perl, PCRE (C, PHP, R…), Java: one horizontal whitespace character: tab or Unicode space separator
H	One character that is not a horizontal whitespace
v	.NET, JavaScript, Python, Ruby: vertical tab
v	Perl, PCRE (C, PHP, R…), Java: one vertical whitespace character: line feed, carriage return, vertical tab, form feed, paragraph or line separator
V	Perl, PCRE (C, PHP, R…), Java: any character that is not a vertical whitespace
R	Perl, PCRE (C, PHP, R…), Java: one line break (carriage return + line feed pair, and all the characters matched by v)

(direct link)

More Quantifiers

Quantifier	Legend	Example	Sample Match
+	The + (one or more) is 'greedy'	d+	12345
?	Makes quantifiers 'lazy'	d+?	1 in 12345
*	The * (zero or more) is 'greedy'	A*	AAA
?	Makes quantifiers 'lazy'	A*?	empty in AAA
{2,4}	Two to four times, 'greedy'	w{2,4}	abcd
?	Makes quantifiers 'lazy'	w{2,4}?	ab in abcd

(direct link)

Character Classes

Character	Legend	Example	Sample Match
[ … ]	One of the characters in the brackets	[AEIOU]	One uppercase vowel
[ … ]	One of the characters in the brackets	T[ao]p	Tap or Top
-	Range indicator	[a-z]	One lowercase letter
[x-y]	One of the characters in the range from x to y	[A-Z]+	GREAT
[ … ]	One of the characters in the brackets	[AB1-5w-z]	One of either: A,B,1,2,3,4,5,w,x,y,z
[x-y]	One of the characters in the range from x to y	[ -~]+	Characters in the printable section of the ASCII table.
[^x]	One character that is not x	[^a-z]{3}	A1!
[^x-y]	One of the characters not in the range from x to y	[^ -~]+	Characters that are not in the printable section of the ASCII table.
[dD]	One character that is a digit or a non-digit	[dD]+	Any characters, inc- luding new lines, which the regular dot doesn't match
[x41]	Matches the character at hexadecimal position 41 in the ASCII table, i.e. A	[x41-x45]{3}	ABE

(direct link)

Anchors and Boundaries

Anchor	Legend	Example	Sample Match
^	Start of string or start of line depending on multiline mode. (But when [^inside brackets], it means 'not')	^abc .*	abc (line start)
$	End of string or end of line depending on multiline mode. Many engine-dependent subtleties.	.*? the end$	this is the end
A	Beginning of string (all major engines except JS)	Aabc[dD]*	abc (string... ...start)
z	Very end of the string Not available in Python and JS	the endz	this is...n...the end
Z	End of string or (except Python) before final line break Not available in JS	the endZ	this is...n...the endn
G	Beginning of String or End of Previous Match .NET, Java, PCRE (C, PHP, R…), Perl, Ruby
b	Word boundary Most engines: position where one side only is an ASCII letter, digit or underscore	Bob.*bcatb	Bob ate the cat
b	Word boundary .NET, Java, Python 3, Ruby: position where one side only is a Unicode letter, digit or underscore	Bob.*bкошкаb	Bob ate the кошка
B	Not a word boundary	c.*BcatB.*	copycats

(direct link)

POSIX Classes

Character	Legend	Example	Sample Match
[:alpha:]	PCRE (C, PHP, R…): ASCII letters A-Z and a-z	[8[:alpha:]]+	WellDone88
[:alpha:]	Ruby 2: Unicode letter or ideogram	[[:alpha:]d]+	кошка99
[:alnum:]	PCRE (C, PHP, R…): ASCII digits and letters A-Z and a-z	[[:alnum:]]{10}	ABCDE12345
[:alnum:]	Ruby 2: Unicode digit, letter or ideogram	[[:alnum:]]{10}	кошка90210
[:punct:]	PCRE (C, PHP, R…): ASCII punctuation mark	[[:punct:]]+	?!.,:;
[:punct:]	Ruby: Unicode punctuation mark	[[:punct:]]+	‽,:〽⁆

(direct link)

Inline Modifiers

None of these are supported in JavaScript. In Ruby, beware of (?s) and (?m)

Regex Cheat Sheet Pdf

.

Modifier	Legend	Example	Sample Match
(?i)	Case-insensitive mode (except JavaScript)	(?i)Monday	monDAY
(?s)	DOTALL mode (except JS and Ruby). The dot (.) matches new line characters (rn). Also known as 'single-line mode' because the dot treats the entire input as a single line	(?s)From A.*to Z	From A to Z
(?m)	Multiline mode (except Ruby and JS) ^ and $ match at the beginning and end of every line	(?m)1rn^2$rn^3$	1 2 3
(?m)	In Ruby: the same as (?s) in other engines, i.e. DOTALL mode, i.e. dot matches line breaks	(?m)From A.*to Z	From A to Z
(?x)	Free-Spacing Mode mode (except JavaScript). Also known as comment mode or whitespace mode	(?x) # this is a # comment abc # write on multiple # lines [ ]d # spaces must be # in brackets	abc d
(?n)	.NET, PCRE 10.30+: named capture only	Turns all (parentheses) into non-capture groups. To capture, use named groups.
(?d)	Java: Unix linebreaks only	The dot and the ^ and $ anchors are only affected by n
(?^)	PCRE 10.32+: unset modifiers	Unsets ismnx modifiers

(direct link)

Lookarounds

Lookaround	Legend	Example	Sample Match
(?=…)	Positive lookahead	(?=d{10})d{5}	01234 in 0123456789
(?<=…)	Positive lookbehind	(?<=d)cat	cat in 1cat
(?!…)	Negative lookahead	(?!theatre)thew+	theme
(?<!…)	Negative lookbehind	w{3}(?<!mon)ster	Munster

(direct link)

Character Class Operations

Class Operation	Legend	Example	Sample Match
[…-[…]]	.NET: character class subtraction. One character that is in those on the left, but not in the subtracted class.	[a-z-[aeiou]]	Any lowercase consonant
[…-[…]]	.NET: character class subtraction.	[p{IsArabic}-[D]]	An Arabic character that is not a non-digit, i.e., an Arabic digit
[…&&[…]]	Java, Ruby 2+: character class intersection. One character that is both in those on the left and in the && class.	[S&&[D]]	An non-whitespace character that is a non-digit.
[…&&[…]]	Java, Ruby 2+: character class intersection.	[S&&[D]&&[^a-zA-Z]]	An non-whitespace character that a non-digit and not a letter.
[…&&[^…]]	Java, Ruby 2+: character class subtraction is obtained by intersecting a class with a negated class	[a-z&&[^aeiou]]	An English lowercase letter that is not a vowel.
[…&&[^…]]	Java, Ruby 2+: character class subtraction	[p{InArabic}&&[^p{L}p{N}]]	An Arabic character that is not a letter or a number

(direct link)

Other Syntax

Syntax	Legend	Example	Sample Match
K	Keep Out Perl, PCRE (C, PHP, R…), Python's alternate regex engine, Ruby 2+: drop everything that was matched so far from the overall match to be returned	prefixKd+	12
Q…E	Perl, PCRE (C, PHP, R…), Java: treat anything between the delimiters as a literal string. Useful to escape metacharacters.	Q(C++ ?)E	(C++ ?)

The 1001 ways to use Regex

Python Regular Expressions Pdf

Above visualization is a screenshot created usingdebuggexfor the patternr'bpar(en|ro)?tb'

From docs.python: re:

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

This blog post gives an overview and examples of regular expression syntax as implemented by the re built-in module (Python 3.8+). Assume ASCII character set unless otherwise specified. This post is an excerpt from my Python re(gex)? book.

Elements that define a regular expression🔗

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.

Feature	Description
|	multiple RE combined as conditional OR
each alternative can have independent anchors
(RE)	group pattern(s), also a capturing group
a(b|c)d is same as abd|acd
(?:RE)	non-capturing group
(?P<name>pat)	named capture group
.	Match any character except the newline character n
[]	Character class, matches one character among many

Greedy 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

Greedy here means that the above 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. Quantifiers can be applied to literal characters, groups, backreferences and character classes.

Character class	Description
[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 [ tnrfv]
S	Match non-whitespace character, same as [^ tnrfv] or [^s]

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	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)

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 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

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:

Regular expression examples🔗

As a good practice, always use raw strings to construct RE, unless other formats are required. This will avoid clash of special meaning of backslash character between RE and normal quoted strings.

• examples for re.search

• difference between string and line anchors

• examples for re.findall

• examples for re.split

• backreferencing within search pattern

• working with matched portions

• examples for re.finditer

• examples for re.sub

• backreferencing in replacement section

• using functions in replacement section of re.sub

• examples for lookarounds

• examples for re.compile

Regular expressions can be compiled using re.compile function, which gives back a re.Pattern object. The top level re module functions are all available as methods for this object. Compiling a regular expression helps if the RE has to be used in multiple places or called upon multiple times inside a loop (speed benefit). By default, Python maintains a small list of recently used RE, so the speed benefit doesn't apply for trivial use cases.

Python re(gex)? book🔗

Visit my repo Python re(gex)? for details about the book I wrote on Python regular expressions. The ebook uses plenty of examples to explain the concepts from the very beginning and step by step introduces more advanced concepts. The book also covers the third party module regex. The cheatsheet and examples presented in this post are based on contents of this book.

Use this leanpub link for a discounted price.