Regular Expressions (Regex): A Beginner's Guide

What is Regular Expression?

Regular expressions (regex) are patterns used to match character combinations in text. They're incredibly powerful for validation, search, and text manipulation.

Basic Regex Syntax

Literal Characters

Most characters match themselves literally:

Pattern: hello
Matches: "hello" in "hello world"

Special Characters

Character	Meaning	Example
`.`	Any character	`h.t` → hat, hot, hit
`\d`	Any digit	`\d\d\d` → 123
`\w`	Word character	`\w+` → hello
`\s`	Whitespace	`hello\sworld`
`^`	Start of string	`^Hello`
`$`	End of string	`end$`

Quantifiers

Quantifier	Meaning	Example
`*`	0 or more	`ab*c` → ac, abc, abbc
`+`	1 or more	`ab+c` → abc, abbc
`?`	0 or 1	`colou?r` → color, colour
`{n}`	Exactly n	`\d{4}` → 2024
`{n,m}`	Between n and m	`\d{2,4}` → 12, 123, 1234

💡 Pro Tip: Use our Regex Tester to experiment with patterns in real-time and see matches highlighted.

Common Regex Patterns

Email Validation

^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$

Phone Number (US)

^\(?\d{3}\)?[-. ]?\d{3}[-. ]?\d{4}$

URL

https?:\/\/(www\.)?[-a-zA-Z0-9@:%._\+~#=]{1,256}\.[a-zA-Z0-9()]{1,6}\b([-a-zA-Z0-9()@:%_\+.~#?&//=]*)

Character Classes

Square brackets define a set of characters to match:

[aeiou]     → any vowel
[0-9]       → any digit (same as \d)
[A-Za-z]    → any letter
[^0-9]      → NOT a digit

Groups and Alternation

(cat|dog)   → matches "cat" or "dog"
(\d{3})     → captures 3 digits as a group

⚠️ Performance Warning: Complex regex patterns can be slow. Avoid nested quantifiers like (a+)+ which can cause catastrophic backtracking.