Regexp

Tools

Regexper: Visulization tool for Regexp
Regex101: Write, test and debug Regexp

Syntax

Syntax	Description
Special Characters
\n	New line.
\f	Form feed.
\r	Return.
\s	Space.
\t	Tab.
\v	Vertical tab.
[\b]	Backspace. As \b refers to boundary(see below), [] is used to differentiate backspace and boundary.
Repeats
?	Zero or one occurrence.
*	Zero or more occurrences.
{x}	X times.
{min, max}	min to max times. If max not given, then it means at least min times.
Margins
\b	Boundary. Matches a pseudo position between word character \w and non-word character \W. Often used to extract single word. Match /\bcat\b/ in The cat scattered his food will not match cat in scattered as cat in scattered is surrounded by word character s and t.
\B	Non-boundary.
^	Start of string.
$	End of string.
Modifiers
/m	Multiple line.
/i	Ignore case.
/g	Global.
Sub-expression
Sub-expression	Expression within brackets ( and ) is a sub-expression.
Backreference	Use \1, \2 in the back to reference the first, second expression in the front. Example: Use \b(\w+)\s\1 to match two continuous same words in Hello what what is the first thing, and I am am Tom..
Non-capturing group	Use (?:) to avoid being captured. Example: Using (Chendongti)(?:an) to match Chendongtian will only return one group Chendongti, an is not captured.
Lookahead	Use (?=)/(?!) to add a must-have/must-not-have suffix. Example: Using happ(?=ily) to match happ happily will only return happ in happily, and happ(?!ily) will only return happ in happy.
Lookbehind	Use (?<=)/(?<!) to add must-have/must-not-have prefix. Example: Using (?<=ap)ple to match apple people will only return ple in apple, and (?<!ap)ple will only return ple in people.
Logical operators
NOT	^. [^abc].
OR	|. (a|b).