Regular Expressions (RegEx) in JavaScript

A regular expression is a pattern used to match, search, and manipulate strings. They are extremely powerful for validation, parsing, and text processing.

Creation of Regular Expressions

There are two ways to create a RegEx in JavaScript:

Using a Regex Literal (preferred for static patterns):

const regex = /abc/;

Using the RegExp Constructor (useful for dynamic patterns):

const regex = new RegExp("abc");

Flags

Flag	Meaning
g	Global search (find all matches)
i	Case-insensitive search
m	Multi-line search
s	Dot matches newline
u	Unicode mode
y	Sticky search (match from index)

const regex = /hello/gi;

Basic Patterns

Pattern	Description	Example
.	Any single character	/h.t/ → matches "hat", "hit"
\\d	Digit (0-9)	/\\d/ → matches "3"
\\D	Non-digit	/\\D/ → matches "a"
\\w	Word character (a-z, A-Z, 0-9, _)	/\\w/ → matches "a"
\\W	Non-word character	/\\W/ → matches "!"
\\s	Whitespace	/\\s/ → matches " "
\\S	Non-whitespace	/\\S/ → matches "a"
^	Start of string	/^a/ → "apple"
$	End of string	/a$/ → "pizza"
[]	Character set	/[aeiou]/ → matches vowels
`	`	OR
?	Optional (0 or 1)	/colou?r/ → matches "color" or "colour"
*	0 or more times	/a*/ → matches "", "a", "aa"
+	1 or more times	/a+/ → matches "a", "aa"
{n}	Exactly n times	/a{2}/ → matches "aa"
{n,}	n or more times	/a{2,}/ → "aa", "aaa"
{n,m}	Between n and m times	/a{2,3}/ → "aa" or "aaa"

Methods Using RegEx

Method	Description
test()	Returns true if pattern exists in string
exec()	Returns matched result or null
match()	String method → returns matches
matchAll()	Returns all matches with capture groups (ES2020+)
replace()	Replaces matches with a replacement string
search()	Returns index of first match
split()	Splits string by pattern

Examples:

const str = "Hello123";

// test()
const regex = /\d+/;
console.log(regex.test(str)); // true

// exec()
console.log(regex.exec(str)); // ["123"]

// match()
console.log(str.match(/\d+/)); // ["123"]

// replace()
console.log(str.replace(/\d+/, "456")); // "Hello456"

// search()
console.log(str.search(/\d+/)); // 5

// split()
console.log(str.split(/\d+/)); // ["Hello", ""]