JS: Regular Expression Tutorial

What is regular expression

Regular Expression (or regex in short) is a character sequence that represent a pattern of text. For example, you can use it to find all numbers in a file.

Regex is used by many functions to check if a string contains certain pattern, or extract it, or replace it with other text.

Regex pattern and flags

Regular Expression has 2 parts:

• regex → specify a text pattern.
• flags → flags is used to tweak regex syntax meaning or how the regex function behaves. e.g. ignore letter case.

• JS: Regex Syntax
• JS: Regex Flags

Create regex object

• JS: Create Regex Object

Example. get index of first match

// find index of first digit occurrence
console.log(
 "there are 36 cats".search(/\d/),
);
// 10
// return the start position of match

// if not found, returns -1
console.log(
 "no cat".search(/\d/),
);
// -1

• JS: String.prototype.search

Example. find pattern

console.log(
 "394 cats and 98 dogs.".match(/\d+/g),
);
// [ "394", "98" ]

• JS: String.prototype.match

Example. regex replace

console.log(
 "year 2026-09-08".replaceAll(/(\d\d\d\d)-\d\d-\d\d/g, `$1`),
);
// year 2026

• JS: String.prototype.replaceAll