Regular Expressions

JavaScript · Reference cheat sheet

📋 Overview

Regular expressions match and extract text patterns. Create with /pattern/flags or new RegExp(string, flags). Use with String methods (match, matchAll, replace, search, split) and RegExp methods (test, exec). Prefer readable patterns; escape user input when building dynamic regexes.

🔧 Core concepts

• Groups: (...) capturing, (?:...) non-capturing, (?<name>...) named.
• Lookaround: (?=...), (?!...), (?<=...), (?<!...).
• Quantifiers: *, +, ?, \{n,m\} — add ? for lazy.
• Classes: \d \w \s, [a-z], [^...].

const re = /\b\w+@\w+\.\w+\b/gi;
re.test("a@b.co"); // true

💡 Examples

// exec loop (mind lastIndex with /g)
const re = /(\d+)/g;
let m;
while ((m = re.exec("a12 b34"))) {
  console.log(m[1], m.index);
}

// matchAll (preferred)
for (const m of "a12 b34".matchAll(/(\d+)/g)) {
  console.log(m[1]);
}

// Named groups + replace
const date = "2026-07-10";
const { groups } = date.match(/(?<y>\d{4})-(?<m>\d{2})-(?<d>\d{2})/);
// groups.y === "2026"

"hello world".replace(/(\w+)\s+(\w+)/, "$2 $1"); // "world hello"
"price: 5".replace(/(?<n>\d+)/, "$$$<n>"); // careful with $

// Escape user input
function escapeRegExp(s) {
  return s.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
}
new RegExp(`^${escapeRegExp(user)}.txt$`);

// Unicode
/\p{Emoji}/u.test("😀"); // true

// Validation
const emailish = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
emailish.test("dev@example.com");

⚠️ Pitfalls

• /g regexes keep lastIndex — reuse can skip matches; reset or use matchAll.
• Dot . does not match newlines unless s flag.
• Catastrophic backtracking on crafted input — avoid nested ambiguous quantifiers on untrusted strings.
• new RegExp("\\d") needs double escaping vs literals.
• replace with a string treats $&, $1, etc. specially.

🔗 Related

• strings.md — string APIs
• template_literals.md — building patterns
• encode.md — encoding vs matching
• error.md — SyntaxError on bad patterns