JavaScript Regex Cheat Sheet

Covers regular expression syntax, flags, common validation patterns, named capture groups, and the string and RegExp methods used to apply them.

1 PageIntermediateApr 15, 2026

Regex Syntax Basics

Core building blocks of a pattern.

javascript

/abc/          // Literal match/a|b/          // Alternation: a OR b/ab?/          // ? = 0 or 1 of preceding (matches "a" or "ab")/ab*/          // * = 0 or more/ab+/          // + = 1 or more/a{2,4}/       // Between 2 and 4 of preceding/^abc$/        // ^ start, $ end of string (or line, with /m flag)/[abc]/        // Character class: a, b, or c/[^abc]/       // Negated class: anything but a, b, c/./            // Any character except newline (unless /s flag)/\d \w \s/     // Digit, word char, whitespace ( \D \W \S = negated)/(abc)/        // Capturing group/(?:abc)/      // Non-capturing group/(?<year>\d{4})/  // Named capturing group

Testing & Matching

RegExp and String methods for applying patterns.

javascript

const re = /\d+/;re.test("order 42");         // true -- does it match at all?"order 42".match(re);        // ["42", index: 6, ...] -- first match detailsconst global = /\d+/g;"a1 b22 c333".match(global);        // ["1", "22", "333"] -- all matches, no groups[..."a1 b22 c333".matchAll(/\d+/g)]; // Array of full match objects (needs /g flag)"2024-01-15".replace(/(\d+)-(\d+)-(\d+)/, "$2/$3/$1");// "01/15/2024" -- $1, $2, $3 reference captured groups"a,b, c ,d".split(/\s*,\s*/);   // ["a", "b", "c", "d"] -- split on pattern

Common Patterns

Pragmatic patterns for everyday validation.

javascript

const email = /^[\w.+-]+@[\w-]+\.[a-zA-Z]{2,}$/;const url = /^https?:\/\/[\w.-]+\.[a-zA-Z]{2,}(\/\S*)?$/;const digitsOnly = /^\d+$/;const whitespaceCollapse = /\s+/g;         // "a   b" -> "a b" via replace(re, " ")const usPhone = /^\(?\d{3}\)?[-.\s]?\d{3}[-.\s]?\d{4}$/;"  hello  world  ".replace(whitespaceCollapse, " ").trim();// "hello world"// Note: these are pragmatic, not RFC-perfect (e.g. real email validation is far more complex)

Named Groups & Lookaround

Extract structured data and match with context.

javascript

const match = "2024-01-15".match(/(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/);match.groups.year;    // "2024"match.groups.month;   // "01"// Lookahead: match "foo" only if followed by "bar"/foo(?=bar)/.test("foobar");    // true/foo(?=bar)/.test("foobaz");    // false// Negative lookahead: match "foo" NOT followed by "bar"/foo(?!bar)/.test("foobaz");    // true// Lookbehind: match digits preceded by "$""$100".match(/(?<=\$)\d+/)[0];  // "100"

Flags & Methods Reference

Flags change how the whole pattern behaves.

g- Global: find all matches, not just the first
i- Case-insensitive matching
m- Multiline: ^ and $ match the start/end of each line
s- Dotall: . also matches newline characters
u- Unicode: enables full Unicode code point matching
y- Sticky: matches only from lastIndex, no scanning ahead
str.search(re)- Returns the index of the first match, or -1
re.exec(str)- Returns the next match; with /g, advances lastIndex on repeated calls

Pro Tip

A /g regex object is stateful -- it remembers lastIndex between calls to .exec() or .test(), which causes intermittent bugs if you reuse the same regex instance across unrelated strings; create a fresh literal or reset lastIndex = 0 first.

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