About Regular Expressions

Regular expressions (regex) are patterns used to match character combinations in strings. They're essential for text searching, validation, parsing, and transformation across all programming languages.

Basic Syntax

.: Any character except newline
*: Zero or more of the preceding
+: One or more of the preceding
?: Zero or one of the preceding
^: Start of string/line
$: End of string/line
[abc]: Character class: a, b, or c
[^abc]: Negated class: not a, b, or c
(group): Capturing group
a|b: Alternation: a or b

Character Classes

\d: Digit [0-9]
\D: Non-digit [^0-9]
\w: Word character [A-Za-z0-9_]
\W: Non-word character
\s: Whitespace (space, tab, newline)
\S: Non-whitespace
\b: Word boundary

Common Regex Patterns

Email Validation

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

Phone Number (US)

^$?[0-9]{3}$?[-.\s]?[0-9]{3}[-.\s]?[0-9]{4}$

URL

https?:\/\/[^\s/$.?#].[^\s]*

IP Address (IPv4)

^(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)$

Regex Flags

g: Global — find all matches
i: Case-insensitive
m: Multiline — ^ and $ match line breaks
s: Dotall — . matches newlines
u: Unicode mode

Regex in Code

JavaScript

// Test if matches
/\d+/.test("123")           // true

// Find all matches
"a1b2c3".match(/\d/g)       // ["1", "2", "3"]

// Replace
"hello".replace(/l/g, "L")  // "heLLo"

Python

import re

re.search(r'\d+', 'abc123')     # Match object
re.findall(r'\d', 'a1b2c3')     # ['1', '2', '3']
re.sub(r'l', 'L', 'hello')      # 'heLLo'

💡 Pro Tips

Start simple and add complexity gradually
Use non-greedy quantifiers (*?, +?) when needed
Escape special characters with backslash: \., \*
Use named groups for readability: (?<name>pattern)
Test thoroughly — regex edge cases are common bugs

Regex Tester

How to Use