Regex Cheatsheet

Regex Pattern Library

Copy-paste ready regular expressions for common use cases. Test before production use.

---

📧 Email & Contact Patterns

Email Address

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

# More permissive (real-world)
^[^\s@]+@[^\s@]+\.[^\s@]{2,}$

Phone Numbers

# US phone (123) 456-7890 or 123-456-7890
^\(?\d{3}\)?[-.\s]?\d{3}[-.\s]?\d{4}$

# International E.164 format (+1234567890)
^\+?[1-9]\d{1,14}$

URLs

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

# URL (optional protocol)
(https?://)?[a-zA-Z0-9-]+(\.[a-zA-Z0-9-]+)+[^\s]*

# Domain name
^[a-zA-Z0-9]([a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(\.[a-zA-Z]{2,})+$

IP Addresses

# 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]?)$

# IPv6 (simplified)
^([0-9a-fA-F]{1,4}:){7}[0-9a-fA-F]{1,4}$

---

📅 Date & Time Patterns

Dates

# YYYY-MM-DD (ISO 8601)
^\d{4}-(0[1-9]|1[0-2])-(0[1-9]|[12]\d|3[01])$

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

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

Times

# 24-hour format (HH:MM)
^([01]\d|2[0-3]):[0-5]\d$

# 24-hour with seconds
^([01]\d|2[0-3]):[0-5]\d:[0-5]\d$

# 12-hour format (HH:MM AM/PM)
^(0?[1-9]|1[0-2]):[0-5]\d\s?(AM|PM|am|pm)$

Timestamps

# ISO 8601 with timezone
^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}(Z|[+-]\d{2}:\d{2})$

---

🔐 Password & Security Patterns

Password Strength

# Minimum 8 chars, at least one letter and one number
^(?=.*[A-Za-z])(?=.*\d).{8,}$

# Minimum 8 chars, uppercase, lowercase, number
^(?=.*[a-z])(?=.*[A-Z])(?=.*\d).{8,}$

# Strong: 8+ chars, upper, lower, number, special
^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&]).{8,}$

Common Identifiers

# UUID v4
^[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$

# US Social Security Number (SSN)
^\d{3}-\d{2}-\d{4}$

# US ZIP Code
^\d{5}(-\d{4})?$

# UK Postcode
^[A-Z]{1,2}\d[A-Z\d]?\s?\d[A-Z]{2}$

---

📝 Text Processing Patterns

Whitespace

# Leading whitespace
^\s+

# Trailing whitespace
\s+$

# Multiple spaces (replace with single space)
\s{2,}

# Empty lines
^\s*$

Numbers

# Integer (positive/negative)
^-?\d+$

# Decimal number
^-?\d+(\.\d+)?$

# Currency ($123.45)
^\$?\d{1,3}(,\d{3})*(\.\d{2})?$

# Percentage (12.5%)
^\d+(\.\d+)?%$

Hex Colors

# Hex color (#fff or #ffffff)
^#([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3})$

# Hex with alpha
^#([A-Fa-f0-9]{8}|[A-Fa-f0-9]{4})$

---

🌐 HTML & Web Patterns

HTML Tags

# Opening tag
<([a-zA-Z][a-zA-Z0-9]*)\b[^>]*>

# Closing tag
</([a-zA-Z][a-zA-Z0-9]*)>

# Self-closing tag
<([a-zA-Z][a-zA-Z0-9]*)\b[^>]*/>

HTML Comments

<!--[\s\S]*?-->

CSS Classes/IDs

# Class selector
class="([^"]*)"

# ID selector
id="([^"]*)"

Markdown

# Heading level 1-6
^#{1,6}\s.+$

# Bold text
\*\*([^*]+)\*\*

# Italic text
\*([^*]+)\*

# Link
\[([^\]]*)\]\(([^)]*)\)

---

📂 File & Path Patterns

File Extensions

# Image files
\.(jpg|jpeg|png|gif|bmp|webp|svg)$

# Document files
\.(pdf|doc|docx|txt|xls|xlsx|ppt|pptx)$

# Code files
\.(js|ts|py|java|cpp|c|h|go|rs|rb|php)$

File Paths

# Unix path
^(/[^/]+)+/?$

# Windows path
^[A-Za-z]:\\([^\\]+\\)*[^\\]*$

# Filename (no path)
[^/\\]+$

---

🔍 Useful Regex Constructs

Lookarounds

# Positive lookahead
foo(?=bar)       # "foo" followed by "bar"

# Negative lookahead
foo(?!bar)       # "foo" NOT followed by "bar"

# Positive lookbehind
(?<=foo)bar      # "bar" preceded by "foo"

# Negative lookbehind
(?<!foo)bar      # "bar" NOT preceded by "foo"

Groups & Capture

# Capture group
(foo|bar)

# Non-capturing group
(?:foo|bar)

# Named group (Python/JS)
(?Ppattern)

# Backreference
(["'])\w+\1      # Matches quoted text with same quotes

---

💡 Quick Reference Table

---

⚠️ Important Notes

1. Test thoroughly: Always test regex patterns with your specific data
2. Escape properly: Different languages/engines may require different escaping
3. Performance: Complex regex can be slow on large inputs
4. HTML parsing: Regex cannot fully parse HTML - use proper parsers when possible
5. Email validation: No perfect email regex exists - send verification emails instead
6. Security: Regex denial-of-service (ReDoS) is possible with certain patterns

Testing Tools

• https://regex101.com - Interactive testing with explanation
• https://regexr.com - Visual regex builder
• https://www.debuggex.com - Visualize regex state machines