Regular Expressions

Note

Unless noted, all of the following is written with recent versions of JavaScript in mind. Different environments may support different regex features!

General

default to using the v (widely supported from September 2023 onwards) or u (for older browsers) flags to avoid bugs related to Unicode handling

Match superscript and subscript numbers

\p{No}

Groups


(?:...)	non-capturing group
(?<name>)	named capturing group (the angle brackets are part of the syntax!)

Lookarounds

look for characters around the match, without adding those characters to the match
break them them down into parts:
- ? marks a "special" group (like the ones above)
- < for behind (optional)
- = for positive or ! for negative


(?=...)	positive lookahead
(?!...)	negative lookahead
(?<=...)	positive lookbehind
(?<!...)	negative lookbehind

Lazy quantifiers

Add ? after any quantifier to make it lazy (match as few characters as possible).

For example, given the text: buffalo buffalo buffalo

/buffalo.+buffalo/ matches: buffalo buffalo buffalo

/buffalo.+?buffalo/ matches: buffalo buffalo buffalo

Substitutions


$&	contents of match
$`	contents before match
$'	contents after match
$1	contents of capture group 1

Change case (VSCode)

Warning

These work in VSCode, but not in JavaScript!


\U	start uppercasing
\L	start lowercasing
\E	stop changing case

ex. Title case Markdown headers:

Find:
(#\s*.)(.+)$

Replace:
\U$1\L$2

JavaScript

regexp.test(str) - returns true/false
str.search(regexp) - returns match index (-1 if not found)
regexp.exec(str) - returns [match, ...captureGroups]
- array has extra properties: index, input, groups (named capturing groups), indices (start/end indices of each capture group, only if d flag is used)
str.match(regexp)
- if g flag is not used, same as regexp.exec
- if g flag is used, returns array of all matches
str.matchAll(regexp) - returns iterator of RegExp.exec results
- can turn it into an array using [...str.matchAll(regexp)]