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JavaScript Tutorial · JavaScript Toolbox · Lesson 38 of 44

JavaScript Regular Expressions

Match, test, extract and replace text with regular expressions.

Creating a regex

A regular expression describes a text pattern. Write it between slashes, with optional flags after: /pattern/flags.

Flag Meaning
g global — find all matches
i case-insensitive
m ^ and $ match at line breaks

Common tokens

Token Matches
\d \w \s digit, word character, whitespace
. any character except newline
+ * ? {2,4} one or more, zero or more, optional, 2–4 times
[abc] [^abc] any of / none of these characters
^ $ start / end of the string
( ) (?<name> ) capture group / named group

Testing and matching

Example 1Try it yourself
const zip = /^\d{5}$/;
console.log(zip.test("90210"), zip.test("9021"));
const text = "Call 555-1234 or 555-9876 today";
console.log(text.match(/\d{3}-\d{4}/g));
const m = "2026-10-12".match(/(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/);
console.log(m.groups.year, m.groups.month, m.groups.day);

Output

true false
[ '555-1234', '555-9876' ]
2026 10 12

Replacing

Example 2Try it yourself
console.log("a  b   c".replace(/\s+/g, " "));
console.log("Hello World".replace(/o/g, "0"));
console.log("2026-10-12".replace(/(\d+)-(\d+)-(\d+)/, "$3/$2/$1"));
const slug = "  Hello, World! JS Tips ".trim().toLowerCase().replace(/[^a-z0-9]+/g, "-");
console.log(slug);

Output

a b c
Hell0 W0rld
12/10/2026
hello-world-js-tips

Test yourself

Which flag makes a regex find every match instead of only the first?

  1. a
  2. g
  3. i
  4. m
Show answer

Answer: g

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