Regex Cheat Sheet 2026: The Only Reference You Need

Bookmark this page. Every developer needs a regex reference they can scan in 30 seconds. This one covers the syntax, the common patterns, and the language-specific differences that trip people up.

Character Classes

Quantifiers

Greedy vs Lazy: By default, * and + are greedy (match as much as possible). Adding ? makes them lazy (match as little as possible). This matters most when matching quoted strings or HTML tags.

Anchors and Boundaries

\bcat\b    matches "cat" in "the cat sat"
           does NOT match "cat" in "concatenate"

Groups and Backreferences

Lookahead and Lookbehind

Lookarounds are zero-width assertions -- they check for a pattern without consuming characters.

Common Patterns

Email Address

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

This covers 99% of valid email addresses. For true RFC 5322 compliance, the regex is absurdly long and you should use a library instead.

URL

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

IP Address (IPv4)

\b(?:(?:25[0-5]|2[0-4]\d|[01]?\d\d?)\.){3}(?:25[0-5]|2[0-4]\d|[01]?\d\d?)\b

The naive \d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3} matches invalid IPs like 999.999.999.999. The pattern above validates each octet is 0-255.

Phone Number (US)

^(\+1)?[-.\s]?\(?[0-9]{3}\)?[-.\s]?[0-9]{3}[-.\s]?[0-9]{4}$

Matches: 555-123-4567, (555) 123-4567, +1 555.123.4567

Date (YYYY-MM-DD)

^\d{4}-(0[1-9]|1[0-2])-(0[1-9]|[12]\d|3[01])$

Password Strength (min 8 chars, upper + lower + digit + special)

^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&])[A-Za-z\d@$!%*?&]{8,}$

This uses four lookaheads to require one of each character type.

UUID v4

^[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$

Hex Color

^#([0-9a-fA-F]{3}|[0-9a-fA-F]{6})$

Slug (URL-friendly string)

^[a-z0-9]+(?:-[a-z0-9]+)*$

Language-Specific Differences

JavaScript

// Literal syntax
const re = /\d+/g;

// Constructor (for dynamic patterns)
const re2 = new RegExp('\\d+', 'g');

// Methods
'hello 123'.match(/\d+/);          // ['123']
'hello 123'.replace(/\d+/, 'NUM'); // 'hello NUM'
/\d+/.test('hello 123');           // true

// Named groups
const m = '2025-03-15'.match(/(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/);
m.groups.year;  // '2025'

// matchAll (returns iterator)
const matches = [...'a1 b2 c3'.matchAll(/([a-z])(\d)/g)];

JS-specific flags: d (indices), s (dotAll -- . matches newline), u (unicode), v (unicodeSets, new in ES2024).

Python

import re

# Search (first match)
m = re.search(r'\d+', 'hello 123 world 456')
m.group()  # '123'

# Find all
re.findall(r'\d+', 'hello 123 world 456')  # ['123', '456']

# Replace
re.sub(r'\d+', 'NUM', 'hello 123')  # 'hello NUM'

# Compile for reuse
pattern = re.compile(r'(?P<year>\d{4})-(?P<month>\d{2})')
m = pattern.match('2025-03')
m.group('year')  # '2025'

# Verbose mode (for readable complex patterns)
pattern = re.compile(r'''
    ^(?P<protocol>https?)://  # protocol
    (?P<domain>[^/]+)         # domain
    (?P<path>/.*)?$           # path (optional)
''', re.VERBOSE)

Python uses (?P...) for named groups (not (?...) like JS).

Go

import "regexp"

re := regexp.MustCompile(`\d+`)
re.FindString("hello 123")           // "123"
re.FindAllString("a1 b2 c3", -1)     // ["1", "2", "3"]
re.ReplaceAllString("hello 123", "N") // "hello N"

Go does not support lookaheads or lookbehinds. The regexp package uses RE2 syntax, which guarantees linear-time matching but sacrifices some features. If you need lookarounds in Go, restructure your approach or use multiple passes.

Testing Your Regex

Before deploying a regex, test it against edge cases. regex101.com is the standard -- it shows matches in real time, explains each part of the pattern, and lets you switch between PCRE, JavaScript, Python, and Go flavors.

devdash.io also has a regex tester with a simpler interface if you just need quick validation without the full breakdown.

For unit testing, always test these edge cases:

• Empty string
• String with only whitespace
• Unicode characters (accented letters, emoji)
• Very long strings (check for catastrophic backtracking)
• Strings that almost match but should not

Performance: Avoiding Catastrophic Backtracking

This regex looks harmless but can freeze your program:

(a+)+b

On input aaaaaaaaaaaaaaaaac, the engine backtracks exponentially. This is called "catastrophic backtracking" or "ReDoS."

Rules to avoid it:

1. Never nest quantifiers ((a+)+, (a))
2. Use atomic groups or possessive quantifiers when available
3. Be specific -- [a-z]+ is safer than .*
4. Set a timeout on regex execution in production code
5. Test with long non-matching strings, not just matching ones

Regex is powerful but it is a sharp tool. Write the simplest pattern that matches your requirements, test it thoroughly, and add a comment explaining what it does. Your future self will thank you.