JS RegExp

JS RegExp

Regular Expressions (RegExp) are patterns used to match character combinations in strings.

const pattern = /js/i;
console.log(pattern.test("JavaScript")); // true

JS RegExp Flags

Flags modify how a pattern behaves.

/g  // global
/i  // case-insensitive
/m  // multiline
/s  // dotAll (dot matches newline)
/u  // unicode
/y  // sticky

const text = "JS js Js";
console.log(text.match(/js/gi));

JS RegExp Classes

Character classes match sets of characters.

/[abc]/      // a or b or c
/[0-9]/      // digits
/[A-Z]/      // uppercase letters
/d/         // digit
/w/         // word character
/s/         // whitespace

JS RegExp Metachars

Metacharacters have special meaning.

.   // any character except newline
^   // start of string
$   // end of string
|   // OR
()  // grouping
[]  // character set
{}  // quantifier

JS RegExp Assertions

Assertions check positions without consuming characters.

^Hello     // starts with Hello
world$     // ends with world

Lookahead and lookbehind:

/d(?=px)/      // digit followed by px
/(?<=$)d+/    // digits after $

JS RegExp Quantifiers

Quantifiers define how many times a pattern must appear.

a+   // one or more
a*   // zero or more
a?   // zero or one
a{3} // exactly 3
a{2,5} // between 2 and 5

JS RegExp Patterns

Common practical examples:

Remove extra whitespace:

const text = "Hello     JS";
console.log(text.replace(/s+/g, " "));

Slug normalization:

function slugify(str) {
  return str
    .toLowerCase()
    .replace(/s+/g, "-")
    .replace(/[^a-z0-9-]/g, "");
}

console.log(slugify("JS RegExp Guide!"));

Named capture groups:

const date = "2026-02-13";
const match = date.match(/(?<year>d{4})-(?<month>d{2})-(?<day>d{2})/);

console.log(match.groups.year);

JS RegExp Objects

You can create regex using literal or constructor syntax.

const r1 = /js/i;
const r2 = new RegExp("js", "i");

JS RegExp Methods

RegExp and String both provide useful methods.

const text2 = "Learn JS easily";

console.log(/JS/.test(text2));
console.log(text2.search(/JS/));
console.log(text2.match(/JS/));
console.log(text2.replace(/JS/, "JavaScript"));

matchAll() returns all matches including capture groups.

const str = "1 apple, 2 oranges";
const matches = str.matchAll(/d+/g);

for (const m of matches) {
  console.log(m[0]);
}

Next Step

Continue with JS Data Types to understand primitive and reference values in JavaScript.