Regular Expressions 101: A Beginner's Complete Guide

Table of Contents

• What is a Regular Expression?
• Literal Characters
• Character Classes
• Anchors
• Quantifiers
• Groups and Alternation
• Flags

What is a Regular Expression?

A regular expression (regex) is a pattern that describes a set of strings. It's used to search, validate, and transform text.

const pattern = /hello/;
pattern.test("hello world"); // true
pattern.test("goodbye");     // false

Literal Characters

The simplest regex matches literal text: /cat/ matches the string 'cat' anywhere in the input.

Special characters that need escaping: . * + ? ^ $ { } [ ] | ( ) \

Character Classes

[abc]      - matches a, b, or c
[a-z]      - any lowercase letter
[0-9]      - any digit
[^abc]     - NOT a, b, or c
\d         - digit [0-9]
\w         - word character [a-zA-Z0-9_]
\s         - whitespace
.          - any character (except newline)

Anchors

^     - start of string
$     - end of string
\b    - word boundary

/^hello/.test('hello world') // true
/world$/.test('hello world') // true
/\bcat\b/.test('cats')       // false — not a whole word

Quantifiers

*      - 0 or more
+      - 1 or more
?      - 0 or 1 (optional)
{n}    - exactly n times
{n,m}  - between n and m times

Groups and Alternation

/(cat|dog)/  // matches 'cat' or 'dog'

const match = '2025-02-22'.match(/(\d{4})-(\d{2})-(\d{2})/);
match[1] // '2025'
match[2] // '02'

Flags

i  - case-insensitive
g  - global (find all matches)
m  - multiline (^ and $ match line boundaries)

Practice with the Regex Tester — with real-time match highlighting.