Regex - guidence
October 3, 2019

Regex Cheat Sheet

Regex Cheat Sheet

0. Introduction

Regex (short for “regular expression”) is a notation for describing text mode. It is used as a professional language to match some particular modes.

1. Syntax

1
/^hello/i --match--> "Hello regex"

2. Particles

These little parts will show how a regex composed of and their function.

2.1 Meta signs

sign meaning
. any single character
| “or”, means select
\d any number
\D any non-number
\s any space character
\S any non-space character
\w any word
\W any non-word
\X unicode sequence
\p{Prop} unicode property
\P{Prop} unicode property

2.2 Special signs

sign meaning
\ used to represent escape sequences
\f formfeed
\n newline
\r carriage return
\t horizontal tab
\v vertical tab
\0 space
\num octal number
\xnum hexadecimal number
\x{num} hexadecimal number
\unum unicode escape sequence
\Unum unicode escape sequence
\cchar control character

2.3 Character sets

character set meaning
[…] single character of character set
[^…] character set except these characters
[a-z] characters between ‘a’ and ‘z’
[^a-z] characters except ‘a’ to ‘z’
[a-zA-Z] characters between ‘a’ and ‘z’ and between ‘A’ and ‘Z’

2.4 POSIX character sets

character set meaning
[[:alnum:]] any alphabet and number
[[:alpha:]] any alphabet
[[:ascii:]] ascii codes (0-127)
[[:blank:]] space or tab
[[:cntrl:]] control characters
[[:digit:]] digital numbers
[[:graph:]] visible characters except space
[[:lower:]] lower characters
[[:print:]] printable characters
[[:punct:]] punctions
[[:space:]] space characters
[[:upper:]] upper characters
[[:word:]] words, characters, numbers and underscore
[[:xdigit:]] hexadecimal numbers
[[:<:]] the beginning of word
[[:>:]] the end of word

2.5 Anchors

anchor meaning
^ the beginning of strings/lines
\A the beginning of strings/lines, ignore m signs
$ the end of strings/lines
\Z the end of strings/lines, ignore m signs
\z match the end of strings only
\b match the boundary of words
\B match not the boundary of words
\< match the beginning of words
\> match the end of words
\G match the begning location

2.6 Quantitives

quantitive meaning
? repeat 0 or 1 time
?? repeat 0 or 1 time (not greedy match)
* repeat 0 or more times
*? repeat 0 or more times (not greedy match)
+ repeat 1 or more times
+? repeat 1 or more times (not greedy match)
{n} repeat n times (n isn’t negative integer)
{n,} repeat at least n times (n isn’t negative integer)
{n, m} repeat n times at least m times at most (n, m are both non-negative integers)

2.7 Groups

group meaning
(…) capturing group
(?:…) non-capturing group
(?P<name>…) capturing group with name
(?<name>…) capturing group with name
(?’name’…) capturing group with name
(?>…) non-capturing group
(?|…) reset child-mode number
(?#…) comment the group
(?=…) posible lookahead
(?!…) negative lookahead
(?<=…) positive reverse lookahead
(?<!…) negative reverse lookahead

2.8 Replacements

replacement meaning
$1 capture contents in group 1
$foo capture contents in group foo

2.9 Mode prefix signs

mode prefix sign meaning
g global mode
i ignore upper and lower
m multiple lines
s single line
x allow spaces and comments
u unicode characters
U non-greedy match

2.10 Regex engines

3. Websites

You can test your regex and understand it in visual way in these websites.

About this Post

This post is written by Chen Li, licensed under CC BY-NC 4.0.