Regex Cheatsheet

Regular expressions, more commonly known as regex, is a powerful way of describing patterns in strings.

It can be pretty complicated, so I wrote this quick guide as a form of note-taking while learning about regex myself.

Usage in JavaScript

// Instantiating a new regular expression
let regex = /123/;
let regexAlt = new RegExp('xyz');

// Testing for matches
regex.test('01234'); // -> true
regexAlt.test('abcxyzdef'); // -> true

// Extracting matches
regex.match('01234'); // -> ['123']
regexAlt.match('abcxyzdef'); // -> ['xyz']

Characters

These special characters match specific character groups when used in a regular expression.

As illustrated, the uppercase counterparts of each special character have opposite meanings to the lowercase characters.

An example use would be expressing one’s birth date in the format DD-MMM-YYYY:

// A regular expression for matching a birth date
let regex = /\d\d-\w\w\w-\d\d\d\d/;
regex.test('01-Apr-1993'); // -> true

Quantifiers

Repeating multiple special characters is pretty tedious. Thankfully, regex offers quantifiers as pattern suffixes to specify how many times a pattern should be matched.

We can now simplify the regexp for a birth date as such:

let regex = /\d{1,2}-\w{3}-\d{4}/;
regex.test('01-Apr-1993'); // -> true
regex.test('1-Apr-1993'); // -> true

Character Classes

Suppose we want to match any one of multiple patterns in a string. Character classes help with this.