Regex Library

JavaScript Lookbehind 📘 Reference · Page

Positive lookbehind JavaScript Lookbehind 📘 Reference · Concept Negative lookbehind JavaScript Lookbehind 📘 Reference · Concept Compatibility JavaScript Lookbehind 📘 Reference · Concept Combining lookahead and lookbehind Lookarounds 📘 Reference · Concept Lookarounds with groups and alternation Lookarounds 📘 Reference · Concept

What is a lookbehind assertion? JavaScript Lookbehind 📘 Reference · Guide Positive vs negative lookbehind JavaScript Lookbehind 📘 Reference · Guide Why use lookbehind in JavaScript? JavaScript Lookbehind 📘 Reference · Guide Common lookbehind use cases JavaScript Lookbehind 📘 Reference · Guide Browser compatibility considerations JavaScript Lookbehind 📘 Reference · Guide Lookbehind vs capturing groups JavaScript Lookbehind 📘 Reference · Guide When not to use lookbehind JavaScript Lookbehind 📘 Reference · Guide

UUID Regex 🧩 Pattern · Page

Standard UUID UUID Regex 🧩 Pattern · Example UUID v4 UUID Regex 🧩 Pattern · Example UUID v1 to v5 UUID Regex 🧩 Pattern · Example UUID without hyphens UUID Regex 🧩 Pattern · Example

What is a UUID? UUID Regex 🧩 Pattern · Guide Common UUID format UUID Regex 🧩 Pattern · Guide UUID versions UUID Regex 🧩 Pattern · Guide Can regex verify UUID uniqueness? UUID Regex 🧩 Pattern · Guide When to use a UUID regex UUID Regex 🧩 Pattern · Guide

Common Regex Mistakes 🎓 Learning · Page

Non-capturing groups Groups 📘 Reference · Concept Shared prefix Alternation 📘 Reference · Concept Shared suffix Alternation 📘 Reference · Concept

Combining regex with other validation Regex Limitations 🎓 Learning · Guide Keep a reusable test suite Testing Regex 🎓 Learning · Guide Portable regexes use simpler syntax Regex Compatibility 🎓 Learning · Guide Be as specific as possible Regex Performance 🎓 Learning · Guide Readability is part of performance Regex Performance 🎓 Learning · Guide Measure before optimizing Regex Performance 🎓 Learning · Guide Normalize input before validation Input Validation Best Practices 🎓 Learning · Guide Separate formatting from validation Input Validation Best Practices 🎓 Learning · Guide Keep validation maintainable Input Validation Best Practices 🎓 Learning · Guide Combine multiple validation layers Input Validation Best Practices 🎓 Learning · Guide Character classes are the foundation of most regexes Character Classes 📘 Reference · Guide Choosing the right character class Character Classes 📘 Reference · Guide Choosing the right quantifier Quantifiers 📘 Reference · Guide Common quantifier mistakes Quantifiers 📘 Reference · Guide Capturing groups versus non-capturing groups Groups 📘 Reference · Guide Common grouping mistakes Groups 📘 Reference · Guide Groups are everywhere in practical regexes Groups 📘 Reference · Guide Common anchor mistakes Anchors 📘 Reference · Guide Anchors improve regex reliability Anchors 📘 Reference · Guide Group alternatives whenever possible Alternation 📘 Reference · Guide Reduce duplication in alternatives Alternation 📘 Reference · Guide Common alternation mistakes Alternation 📘 Reference · Guide Common lookaround mistakes Lookarounds 📘 Reference · Guide

JavaScript Regex vs PCRE 📘 Reference · Page

Common features JavaScript Regex vs PCRE 📘 Reference · Concept PCRE-specific strengths JavaScript Regex vs PCRE 📘 Reference · Concept JavaScript-specific behavior JavaScript Regex vs PCRE 📘 Reference · Concept

Ignoring regex engine differences Common Regex Mistakes 🎓 Learning · Guide What is PCRE? JavaScript Regex vs PCRE 📘 Reference · Guide Why do regex engines behave differently? JavaScript Regex vs PCRE 📘 Reference · Guide JavaScript regex vs PCRE JavaScript Regex vs PCRE 📘 Reference · Guide Lookbehind and modern regex features JavaScript Regex vs PCRE 📘 Reference · Guide Unicode support differences JavaScript Regex vs PCRE 📘 Reference · Guide Why does a regex work in a testing tool but not in my code? JavaScript Regex vs PCRE 📘 Reference · Guide Which regex engine should you learn? JavaScript Regex vs PCRE 📘 Reference · Guide

Date Regex 🧩 Pattern · Page UUID Regex 🧩 Pattern · Page Credit Card Regex 🧩 Pattern · Page

DD/MM/YYYY Date Regex 🧩 Pattern · Example DD-MM-YYYY Date Regex 🧩 Pattern · Example ISO date Date Regex 🧩 Pattern · Example ISO datetime Date Regex 🧩 Pattern · Example Stricter DD/MM/YYYY Date Regex 🧩 Pattern · Example Standard UUID UUID Regex 🧩 Pattern · Example UUID v4 UUID Regex 🧩 Pattern · Example UUID v1 to v5 UUID Regex 🧩 Pattern · Example UUID without hyphens UUID Regex 🧩 Pattern · Example Generic Credit Card Number Credit Card Regex 🧩 Pattern · Example Visa Card Credit Card Regex 🧩 Pattern · Example Mastercard Credit Card Regex 🧩 Pattern · Example American Express Credit Card Regex 🧩 Pattern · Example Credit Card with Spaces or Hyphens Credit Card Regex 🧩 Pattern · Example Discover Card Credit Card Regex 🧩 Pattern · Example JCB Card Credit Card Regex 🧩 Pattern · Example Diners Club Credit Card Regex 🧩 Pattern · Example Last 4 Digits Credit Card Regex 🧩 Pattern · Example Extract Credit Card Numbers Credit Card Regex 🧩 Pattern · Example Accept Multiple Credit Card Formats Credit Card Regex 🧩 Pattern · Example Mask Credit Card Numbers Credit Card Regex 🧩 Pattern · Example Payment Form Validation Credit Card Regex 🧩 Pattern · Example Strict 16-Digit Card Number Credit Card Regex 🧩 Pattern · Example Find Card Numbers in Logs Credit Card Regex 🧩 Pattern · Example

Why validate dates with regex? Date Regex 🧩 Pattern · Guide Common date formats Date Regex 🧩 Pattern · Guide ISO 8601 date validation Date Regex 🧩 Pattern · Guide Can regex validate real calendar dates? Date Regex 🧩 Pattern · Guide Date format validation vs date parsing Date Regex 🧩 Pattern · Guide Common date validation mistakes Date Regex 🧩 Pattern · Guide When to use a date regex Date Regex 🧩 Pattern · Guide What is a UUID? UUID Regex 🧩 Pattern · Guide Common UUID format UUID Regex 🧩 Pattern · Guide UUID versions UUID Regex 🧩 Pattern · Guide Can regex verify UUID uniqueness? UUID Regex 🧩 Pattern · Guide When to use a UUID regex UUID Regex 🧩 Pattern · Guide Checksums and algorithms need code Regex Limitations 🎓 Learning · Guide Dates and numbers often need parsing Regex Limitations 🎓 Learning · Guide Unicode support is especially tricky Regex Compatibility 🎓 Learning · Guide

Hex Color Regex 🧩 Pattern · Page

Short hex color Hex Color Regex 🧩 Pattern · Example Long hex color Hex Color Regex 🧩 Pattern · Example Short or long hex color Hex Color Regex 🧩 Pattern · Example Hex color with alpha Hex Color Regex 🧩 Pattern · Example Any CSS hex color Hex Color Regex 🧩 Pattern · Example Hex color without # Hex Color Regex 🧩 Pattern · Example

What is a hex color? Hex Color Regex 🧩 Pattern · Guide Short vs long hex colors Hex Color Regex 🧩 Pattern · Guide Hex colors with alpha transparency Hex Color Regex 🧩 Pattern · Guide Common hex color validation mistakes Hex Color Regex 🧩 Pattern · Guide When to use a hex color regex Hex Color Regex 🧩 Pattern · Guide

Alternation appears in many real-world regexes Alternation 📘 Reference · Guide Lookarounds in real-world regexes Lookarounds 📘 Reference · Guide

Extract email addresses Email Regex 🧩 Pattern · Example Email domain only Email Regex 🧩 Pattern · Example Username part of email Email Regex 🧩 Pattern · Example Extract multiple email addresses Email Regex 🧩 Pattern · Example

Multiple capturing groups Groups 📘 Reference · Concept Named capturing groups Groups 📘 Reference · Concept Anchors in extraction Anchors 📘 Reference · Concept Capturing alternatives Alternation 📘 Reference · Concept Positive lookbehind Lookarounds 📘 Reference · Concept Negative lookbehind Lookarounds 📘 Reference · Concept

Anchors and multiline mode Anchors 📘 Reference · Guide

Username Regex 🧩 Pattern · Page

Letters and numbers Username Regex 🧩 Pattern · Example Letters, numbers and underscore Username Regex 🧩 Pattern · Example Username with length limit Username Regex 🧩 Pattern · Example Lowercase username Username Regex 🧩 Pattern · Example Username with hyphen Username Regex 🧩 Pattern · Example International username Username Regex 🧩 Pattern · Example

What is a username regex? Username Regex 🧩 Pattern · Guide Common username validation rules Username Regex 🧩 Pattern · Guide ASCII usernames vs Unicode usernames Username Regex 🧩 Pattern · Guide What regex cannot validate Username Regex 🧩 Pattern · Guide

JavaScript Lookbehind 📘 Reference · Page

Positive lookbehind JavaScript Lookbehind 📘 Reference · Concept Negative lookbehind JavaScript Lookbehind 📘 Reference · Concept Compatibility JavaScript Lookbehind 📘 Reference · Concept

What is a lookbehind assertion? JavaScript Lookbehind 📘 Reference · Guide Positive vs negative lookbehind JavaScript Lookbehind 📘 Reference · Guide Why use lookbehind in JavaScript? JavaScript Lookbehind 📘 Reference · Guide Common lookbehind use cases JavaScript Lookbehind 📘 Reference · Guide Browser compatibility considerations JavaScript Lookbehind 📘 Reference · Guide Lookbehind vs capturing groups JavaScript Lookbehind 📘 Reference · Guide When not to use lookbehind JavaScript Lookbehind 📘 Reference · Guide

Regex Cheatsheet 📘 Reference · Page Regex Tutorial 🎓 Learning · Page Regex Limitations 🎓 Learning · Page Testing Regex 🎓 Learning · Page Regex Compatibility 🎓 Learning · Page Regex Performance 🎓 Learning · Page Input Validation Best Practices 🎓 Learning · Page Common Regex Mistakes 🎓 Learning · Page JavaScript Lookbehind 📘 Reference · Page JavaScript Regex vs PCRE 📘 Reference · Page Character Classes 📘 Reference · Page Quantifiers 📘 Reference · Page Groups 📘 Reference · Page Anchors 📘 Reference · Page Alternation 📘 Reference · Page Lookarounds 📘 Reference · Page

Anchors and boundaries Regex Cheatsheet 📘 Reference · Concept Character classes Regex Cheatsheet 📘 Reference · Concept Quantifiers Regex Cheatsheet 📘 Reference · Concept Groups and alternatives Regex Cheatsheet 📘 Reference · Concept Lookarounds Regex Cheatsheet 📘 Reference · Concept Start with exact text Regex Tutorial 🎓 Learning · Concept Use character classes Regex Tutorial 🎓 Learning · Concept Add repetition Regex Tutorial 🎓 Learning · Concept Test often Regex Tutorial 🎓 Learning · Concept Positive lookbehind JavaScript Lookbehind 📘 Reference · Concept Negative lookbehind JavaScript Lookbehind 📘 Reference · Concept Compatibility JavaScript Lookbehind 📘 Reference · Concept Common features JavaScript Regex vs PCRE 📘 Reference · Concept PCRE-specific strengths JavaScript Regex vs PCRE 📘 Reference · Concept JavaScript-specific behavior JavaScript Regex vs PCRE 📘 Reference · Concept Dot wildcard Character Classes 📘 Reference · Concept Custom character classes Character Classes 📘 Reference · Concept Character ranges Character Classes 📘 Reference · Concept Negated character classes Character Classes 📘 Reference · Concept Zero or more with * Quantifiers 📘 Reference · Concept One or more with + Quantifiers 📘 Reference · Concept Optional with ? Quantifiers 📘 Reference · Concept Greedy quantifiers Quantifiers 📘 Reference · Concept Lazy quantifiers Quantifiers 📘 Reference · Concept Capturing groups Groups 📘 Reference · Concept Nested groups Groups 📘 Reference · Concept Groups with alternation Groups 📘 Reference · Concept Groups with quantifiers Groups 📘 Reference · Concept Anchors without multiline mode Anchors 📘 Reference · Concept Basic alternation with | Alternation 📘 Reference · Concept Alternation inside a group Alternation 📘 Reference · Concept Alternation precedence Alternation 📘 Reference · Concept Multiple alternatives Alternation 📘 Reference · Concept Zero-width assertions Lookarounds 📘 Reference · Concept

Why validate passwords with regex? Password Regex 🧩 Pattern · Guide Common password requirements Password Regex 🧩 Pattern · Guide Strong passwords vs complex passwords Password Regex 🧩 Pattern · Guide Can regex measure password strength? Password Regex 🧩 Pattern · Guide Client-side and server-side validation Password Regex 🧩 Pattern · Guide Common password validation mistakes Password Regex 🧩 Pattern · Guide Modern password recommendations Password Regex 🧩 Pattern · Guide Why email regex validation is difficult Email Regex 🧩 Pattern · Guide Practical email regex vs RFC-compliant regex Email Regex 🧩 Pattern · Guide Why use a URL regex? URL Regex 🧩 Pattern · Guide What is a valid URL? URL Regex 🧩 Pattern · Guide HTTP vs HTTPS URL validation URL Regex 🧩 Pattern · Guide Should a regex validate URLs completely? URL Regex 🧩 Pattern · Guide Common URL validation mistakes URL Regex 🧩 Pattern · Guide When to use URL regex validation URL Regex 🧩 Pattern · Guide Why validate phone numbers with regex? Phone Number Regex 🧩 Pattern · Guide What is the E.164 phone number format? Phone Number Regex 🧩 Pattern · Guide International phone numbers are difficult to validate Phone Number Regex 🧩 Pattern · Guide Mobile numbers vs landline numbers Phone Number Regex 🧩 Pattern · Guide Formatting and user-friendly input with spaces, parentheses, dashes and other Phone Number Regex 🧩 Pattern · Guide Can regex verify a real phone number? Phone Number Regex 🧩 Pattern · Guide Best practices for phone number validation Phone Number Regex 🧩 Pattern · Guide What is a username regex? Username Regex 🧩 Pattern · Guide Common username validation rules Username Regex 🧩 Pattern · Guide ASCII usernames vs Unicode usernames Username Regex 🧩 Pattern · Guide What regex cannot validate Username Regex 🧩 Pattern · Guide Why validate dates with regex? Date Regex 🧩 Pattern · Guide Common date formats Date Regex 🧩 Pattern · Guide ISO 8601 date validation Date Regex 🧩 Pattern · Guide Can regex validate real calendar dates? Date Regex 🧩 Pattern · Guide Date format validation vs date parsing Date Regex 🧩 Pattern · Guide Common date validation mistakes Date Regex 🧩 Pattern · Guide When to use a date regex Date Regex 🧩 Pattern · Guide Why validate IP addresses with regex? IP Address Regex 🧩 Pattern · Guide IPv4 vs IPv6 IP Address Regex 🧩 Pattern · Guide What makes IPv6 validation difficult? IP Address Regex 🧩 Pattern · Guide CIDR notation explained IP Address Regex 🧩 Pattern · Guide Private vs public IP addresses IP Address Regex 🧩 Pattern · Guide Can regex verify that an IP address exists? IP Address Regex 🧩 Pattern · Guide When to use IP address regex validation IP Address Regex 🧩 Pattern · Guide What is a UUID? UUID Regex 🧩 Pattern · Guide Common UUID format UUID Regex 🧩 Pattern · Guide UUID versions UUID Regex 🧩 Pattern · Guide Can regex verify UUID uniqueness? UUID Regex 🧩 Pattern · Guide When to use a UUID regex UUID Regex 🧩 Pattern · Guide What is a hex color? Hex Color Regex 🧩 Pattern · Guide Short vs long hex colors Hex Color Regex 🧩 Pattern · Guide Hex colors with alpha transparency Hex Color Regex 🧩 Pattern · Guide Common hex color validation mistakes Hex Color Regex 🧩 Pattern · Guide When to use a hex color regex Hex Color Regex 🧩 Pattern · Guide What is a domain name? Domain Name Regex 🧩 Pattern · Guide Domain name vs URL Domain Name Regex 🧩 Pattern · Guide Subdomains and labels Domain Name Regex 🧩 Pattern · Guide Common domain validation mistakes Domain Name Regex 🧩 Pattern · Guide Can regex verify that a domain exists? Domain Name Regex 🧩 Pattern · Guide Internationalized domain names Domain Name Regex 🧩 Pattern · Guide What is a slug? Slug Regex 🧩 Pattern · Guide Why are slugs important for SEO? Slug Regex 🧩 Pattern · Guide Common slug rules Slug Regex 🧩 Pattern · Guide Slug validation vs slug generation Slug Regex 🧩 Pattern · Guide Unicode slugs and international websites Slug Regex 🧩 Pattern · Guide When to use a slug regex Slug Regex 🧩 Pattern · Guide Can Regex Validate a Credit Card? Credit Card Regex 🧩 Pattern · Guide Understanding Credit Card Number Structure Credit Card Regex 🧩 Pattern · Guide Visa vs Mastercard Regex Credit Card Regex 🧩 Pattern · Guide Common Regex Mistakes Credit Card Regex 🧩 Pattern · Guide Cross-Engine Compatibility Credit Card Regex 🧩 Pattern · Guide Performance Considerations Credit Card Regex 🧩 Pattern · Guide What is a regex cheatsheet? Regex Cheatsheet 📘 Reference · Guide The most important regex tokens to know Regex Cheatsheet 📘 Reference · Guide Character classes explained Regex Cheatsheet 📘 Reference · Guide Understanding regex quantifiers Regex Cheatsheet 📘 Reference · Guide Groups, alternation and anchors Regex Cheatsheet 📘 Reference · Guide Regex syntax differences between languages Regex Cheatsheet 📘 Reference · Guide How to learn regex efficiently Regex Cheatsheet 📘 Reference · Guide How to learn regex step by step Regex Tutorial 🎓 Learning · Guide Start with literal text Regex Tutorial 🎓 Learning · Guide Use character classes for flexibility Regex Tutorial 🎓 Learning · Guide Add quantifiers to repeat patterns Regex Tutorial 🎓 Learning · Guide Use groups to organize your regex Regex Tutorial 🎓 Learning · Guide Test regex patterns often Regex Tutorial 🎓 Learning · Guide Common beginner mistakes Regex Tutorial 🎓 Learning · Guide Regex validates format, not meaning Regex Limitations 🎓 Learning · Guide Regex cannot replace business logic Regex Limitations 🎓 Learning · Guide Regex can be too strict or too permissive Regex Limitations 🎓 Learning · Guide Regex cannot query external systems Regex Limitations 🎓 Learning · Guide Regex cannot understand context Regex Limitations 🎓 Learning · Guide Regex is not a parser Regex Limitations 🎓 Learning · Guide Combining regex with other validation Regex Limitations 🎓 Learning · Guide Test both matches and non-matches Testing Regex 🎓 Learning · Guide Use realistic input Testing Regex 🎓 Learning · Guide Include edge cases Testing Regex 🎓 Learning · Guide Test anchors carefully Testing Regex 🎓 Learning · Guide Test flags and modes Testing Regex 🎓 Learning · Guide Test extraction separately from validation Testing Regex 🎓 Learning · Guide Test replacement patterns Testing Regex 🎓 Learning · Guide Test in the target regex engine Testing Regex 🎓 Learning · Guide Keep a reusable test suite Testing Regex 🎓 Learning · Guide Regex engines are not all the same Regex Compatibility 🎓 Learning · Guide Lookbehind support varies Regex Compatibility 🎓 Learning · Guide Named groups use different syntaxes Regex Compatibility 🎓 Learning · Guide Replacement syntax is language-specific Regex Compatibility 🎓 Learning · Guide Flags can change behavior Regex Compatibility 🎓 Learning · Guide Unicode support is especially tricky Regex Compatibility 🎓 Learning · Guide Anchors and line modes can differ Regex Compatibility 🎓 Learning · Guide Portable regexes use simpler syntax Regex Compatibility 🎓 Learning · Guide Always test in the target environment Regex Compatibility 🎓 Learning · Guide Most regexes are already fast Regex Performance 🎓 Learning · Guide Understand backtracking Regex Performance 🎓 Learning · Guide Avoid catastrophic backtracking Regex Performance 🎓 Learning · Guide Anchor validation regexes Regex Performance 🎓 Learning · Guide Be as specific as possible Regex Performance 🎓 Learning · Guide Large inputs deserve extra testing Regex Performance 🎓 Learning · Guide Readability is part of performance Regex Performance 🎓 Learning · Guide Measure before optimizing Regex Performance 🎓 Learning · Guide Validate structure, not meaning Input Validation Best Practices 🎓 Learning · Guide Normalize input before validation Input Validation Best Practices 🎓 Learning · Guide Validate on both client and server Input Validation Best Practices 🎓 Learning · Guide Provide clear validation messages Input Validation Best Practices 🎓 Learning · Guide Never trust client-side input Input Validation Best Practices 🎓 Learning · Guide Separate formatting from validation Input Validation Best Practices 🎓 Learning · Guide Accept reasonable user input Input Validation Best Practices 🎓 Learning · Guide Keep validation maintainable Input Validation Best Practices 🎓 Learning · Guide Combine multiple validation layers Input Validation Best Practices 🎓 Learning · Guide Forgetting anchors Common Regex Mistakes 🎓 Learning · Guide Using .* everywhere Common Regex Mistakes 🎓 Learning · Guide Forgetting to escape special characters Common Regex Mistakes 🎓 Learning · Guide Trying to validate everything with regex Common Regex Mistakes 🎓 Learning · Guide Writing unreadable regexes Common Regex Mistakes 🎓 Learning · Guide Ignoring edge cases Common Regex Mistakes 🎓 Learning · Guide Parsing HTML or JSON with regex Common Regex Mistakes 🎓 Learning · Guide Ignoring regex engine differences Common Regex Mistakes 🎓 Learning · Guide Never measuring performance Common Regex Mistakes 🎓 Learning · Guide Not documenting important regexes Common Regex Mistakes 🎓 Learning · Guide What is a lookbehind assertion? JavaScript Lookbehind 📘 Reference · Guide Positive vs negative lookbehind JavaScript Lookbehind 📘 Reference · Guide Why use lookbehind in JavaScript? JavaScript Lookbehind 📘 Reference · Guide Common lookbehind use cases JavaScript Lookbehind 📘 Reference · Guide Browser compatibility considerations JavaScript Lookbehind 📘 Reference · Guide Lookbehind vs capturing groups JavaScript Lookbehind 📘 Reference · Guide When not to use lookbehind JavaScript Lookbehind 📘 Reference · Guide What is PCRE? JavaScript Regex vs PCRE 📘 Reference · Guide Why do regex engines behave differently? JavaScript Regex vs PCRE 📘 Reference · Guide JavaScript regex vs PCRE JavaScript Regex vs PCRE 📘 Reference · Guide Lookbehind and modern regex features JavaScript Regex vs PCRE 📘 Reference · Guide Unicode support differences JavaScript Regex vs PCRE 📘 Reference · Guide Why does a regex work in a testing tool but not in my code? JavaScript Regex vs PCRE 📘 Reference · Guide Which regex engine should you learn? JavaScript Regex vs PCRE 📘 Reference · Guide Why character classes matter Character Classes 📘 Reference · Guide Shortcut classes versus custom classes Character Classes 📘 Reference · Guide ASCII versus Unicode Character Classes 📘 Reference · Guide ASCII versus Unicode Character Classes 📘 Reference · Guide Character classes are the foundation of most regexes Character Classes 📘 Reference · Guide Choosing the right character class Character Classes 📘 Reference · Guide Why quantifiers are essential Quantifiers 📘 Reference · Guide Choosing the right quantifier Quantifiers 📘 Reference · Guide Greedy versus lazy matching Quantifiers 📘 Reference · Guide Quantifiers and performance Quantifiers 📘 Reference · Guide Fixed-length versus variable-length patterns Quantifiers 📘 Reference · Guide Common quantifier mistakes Quantifiers 📘 Reference · Guide Why groups matter Groups 📘 Reference · Guide Capturing groups versus non-capturing groups Groups 📘 Reference · Guide Named groups improve readability Groups 📘 Reference · Guide Backreferences are powerful but should stay readable Groups 📘 Reference · Guide Common grouping mistakes Groups 📘 Reference · Guide Groups are everywhere in practical regexes Groups 📘 Reference · Guide Why anchors matter Anchors 📘 Reference · Guide Validation versus extraction Anchors 📘 Reference · Guide Understanding word boundaries Anchors 📘 Reference · Guide Anchors and multiline mode Anchors 📘 Reference · Guide Common anchor mistakes Anchors 📘 Reference · Guide Anchors improve regex reliability Anchors 📘 Reference · Guide Why alternation matters Alternation 📘 Reference · Guide Group alternatives whenever possible Alternation 📘 Reference · Guide Reduce duplication in alternatives Alternation 📘 Reference · Guide Order alternatives carefully Alternation 📘 Reference · Guide Common alternation mistakes Alternation 📘 Reference · Guide Alternation appears in many real-world regexes Alternation 📘 Reference · Guide Why lookarounds matter Lookarounds 📘 Reference · Guide Lookarounds do not consume characters Lookarounds 📘 Reference · Guide Lookaheads versus lookbehinds Lookarounds 📘 Reference · Guide Lookarounds simplify validation Lookarounds 📘 Reference · Guide Common lookaround mistakes Lookarounds 📘 Reference · Guide Lookarounds in real-world regexes Lookarounds 📘 Reference · Guide

Writing unreadable regexes Common Regex Mistakes 🎓 Learning · Guide Not documenting important regexes Common Regex Mistakes 🎓 Learning · Guide

IP Address Regex 🧩 Pattern · Page Domain Name Regex 🧩 Pattern · Page

Simple IPv4 IP Address Regex 🧩 Pattern · Example Strict IPv4 IP Address Regex 🧩 Pattern · Example Private IPv4 ranges IP Address Regex 🧩 Pattern · Example IPv4 with CIDR IP Address Regex 🧩 Pattern · Example Basic IPv6 IP Address Regex 🧩 Pattern · Example Simple domain name Domain Name Regex 🧩 Pattern · Example Domain with subdomains Domain Name Regex 🧩 Pattern · Example Strict domain labels Domain Name Regex 🧩 Pattern · Example Lowercase domain name Domain Name Regex 🧩 Pattern · Example Domain with optional trailing dot Domain Name Regex 🧩 Pattern · Example Punycode domain Domain Name Regex 🧩 Pattern · Example

Why validate IP addresses with regex? IP Address Regex 🧩 Pattern · Guide IPv4 vs IPv6 IP Address Regex 🧩 Pattern · Guide What makes IPv6 validation difficult? IP Address Regex 🧩 Pattern · Guide CIDR notation explained IP Address Regex 🧩 Pattern · Guide Private vs public IP addresses IP Address Regex 🧩 Pattern · Guide Can regex verify that an IP address exists? IP Address Regex 🧩 Pattern · Guide When to use IP address regex validation IP Address Regex 🧩 Pattern · Guide What is a domain name? Domain Name Regex 🧩 Pattern · Guide Domain name vs URL Domain Name Regex 🧩 Pattern · Guide Subdomains and labels Domain Name Regex 🧩 Pattern · Guide Common domain validation mistakes Domain Name Regex 🧩 Pattern · Guide Can regex verify that a domain exists? Domain Name Regex 🧩 Pattern · Guide Internationalized domain names Domain Name Regex 🧩 Pattern · Guide

Regex is not a parser Regex Limitations 🎓 Learning · Guide Parsing HTML or JSON with regex Common Regex Mistakes 🎓 Learning · Guide

Regex Performance 🎓 Learning · Page

Most regexes are already fast Regex Performance 🎓 Learning · Guide Understand backtracking Regex Performance 🎓 Learning · Guide Avoid catastrophic backtracking Regex Performance 🎓 Learning · Guide Using .* everywhere Common Regex Mistakes 🎓 Learning · Guide Never measuring performance Common Regex Mistakes 🎓 Learning · Guide Quantifiers and performance Quantifiers 📘 Reference · Guide Order alternatives carefully Alternation 📘 Reference · Guide

Regex Cheatsheet 📘 Reference · Page

Anchors and boundaries Regex Cheatsheet 📘 Reference · Concept Character classes Regex Cheatsheet 📘 Reference · Concept Quantifiers Regex Cheatsheet 📘 Reference · Concept Groups and alternatives Regex Cheatsheet 📘 Reference · Concept Lookarounds Regex Cheatsheet 📘 Reference · Concept

What is a regex cheatsheet? Regex Cheatsheet 📘 Reference · Guide The most important regex tokens to know Regex Cheatsheet 📘 Reference · Guide Character classes explained Regex Cheatsheet 📘 Reference · Guide Understanding regex quantifiers Regex Cheatsheet 📘 Reference · Guide Groups, alternation and anchors Regex Cheatsheet 📘 Reference · Guide Regex syntax differences between languages Regex Cheatsheet 📘 Reference · Guide How to learn regex efficiently Regex Cheatsheet 📘 Reference · Guide

JavaScript Regex vs PCRE 📘 Reference · Page

Common features JavaScript Regex vs PCRE 📘 Reference · Concept PCRE-specific strengths JavaScript Regex vs PCRE 📘 Reference · Concept JavaScript-specific behavior JavaScript Regex vs PCRE 📘 Reference · Concept

Test flags and modes Testing Regex 🎓 Learning · Guide Regex engines are not all the same Regex Compatibility 🎓 Learning · Guide Flags can change behavior Regex Compatibility 🎓 Learning · Guide What is PCRE? JavaScript Regex vs PCRE 📘 Reference · Guide Why do regex engines behave differently? JavaScript Regex vs PCRE 📘 Reference · Guide JavaScript regex vs PCRE JavaScript Regex vs PCRE 📘 Reference · Guide Lookbehind and modern regex features JavaScript Regex vs PCRE 📘 Reference · Guide Unicode support differences JavaScript Regex vs PCRE 📘 Reference · Guide Why does a regex work in a testing tool but not in my code? JavaScript Regex vs PCRE 📘 Reference · Guide Which regex engine should you learn? JavaScript Regex vs PCRE 📘 Reference · Guide

Mask email addresses Email Regex 🧩 Pattern · Example

Password Regex 🧩 Pattern · Page Credit Card Regex 🧩 Pattern · Page

Minimum 8 characters Password Regex 🧩 Pattern · Example Letters and numbers Password Regex 🧩 Pattern · Example Strong password Password Regex 🧩 Pattern · Example Strong password (12+ chars) Password Regex 🧩 Pattern · Example Generic Credit Card Number Credit Card Regex 🧩 Pattern · Example Visa Card Credit Card Regex 🧩 Pattern · Example Mastercard Credit Card Regex 🧩 Pattern · Example American Express Credit Card Regex 🧩 Pattern · Example Credit Card with Spaces or Hyphens Credit Card Regex 🧩 Pattern · Example Discover Card Credit Card Regex 🧩 Pattern · Example JCB Card Credit Card Regex 🧩 Pattern · Example Diners Club Credit Card Regex 🧩 Pattern · Example Last 4 Digits Credit Card Regex 🧩 Pattern · Example Extract Credit Card Numbers Credit Card Regex 🧩 Pattern · Example Accept Multiple Credit Card Formats Credit Card Regex 🧩 Pattern · Example Mask Credit Card Numbers Credit Card Regex 🧩 Pattern · Example Payment Form Validation Credit Card Regex 🧩 Pattern · Example Strict 16-Digit Card Number Credit Card Regex 🧩 Pattern · Example Find Card Numbers in Logs Credit Card Regex 🧩 Pattern · Example

Why validate passwords with regex? Password Regex 🧩 Pattern · Guide Common password requirements Password Regex 🧩 Pattern · Guide Strong passwords vs complex passwords Password Regex 🧩 Pattern · Guide Can regex measure password strength? Password Regex 🧩 Pattern · Guide Client-side and server-side validation Password Regex 🧩 Pattern · Guide Common password validation mistakes Password Regex 🧩 Pattern · Guide Modern password recommendations Password Regex 🧩 Pattern · Guide Confirming an email address Email Regex 🧩 Pattern · Guide Can Regex Validate a Credit Card? Credit Card Regex 🧩 Pattern · Guide Extracting Credit Card Numbers from Text Credit Card Regex 🧩 Pattern · Guide Masking Sensitive Card Numbers Credit Card Regex 🧩 Pattern · Guide Performance Considerations Credit Card Regex 🧩 Pattern · Guide Security-sensitive validation needs layers Regex Limitations 🎓 Learning · Guide Validate on both client and server Input Validation Best Practices 🎓 Learning · Guide Never trust client-side input Input Validation Best Practices 🎓 Learning · Guide

Slug Regex 🧩 Pattern · Page

Basic slug Slug Regex 🧩 Pattern · Example SEO-friendly slug Slug Regex 🧩 Pattern · Example Slug with underscores Slug Regex 🧩 Pattern · Example Fixed-length slug Slug Regex 🧩 Pattern · Example Unicode slug Slug Regex 🧩 Pattern · Example

What is a slug? Slug Regex 🧩 Pattern · Guide Why are slugs important for SEO? Slug Regex 🧩 Pattern · Guide Common slug rules Slug Regex 🧩 Pattern · Guide Slug validation vs slug generation Slug Regex 🧩 Pattern · Guide Unicode slugs and international websites Slug Regex 🧩 Pattern · Guide When to use a slug regex Slug Regex 🧩 Pattern · Guide

Character Classes 📘 Reference · Page Quantifiers 📘 Reference · Page Groups 📘 Reference · Page Anchors 📘 Reference · Page Alternation 📘 Reference · Page Lookarounds 📘 Reference · Page

Dot wildcard Character Classes 📘 Reference · Concept Digits with \d Character Classes 📘 Reference · Concept Non-digits with \D Character Classes 📘 Reference · Concept Word characters with \w Character Classes 📘 Reference · Concept Non-word characters with \W Character Classes 📘 Reference · Concept Whitespace with \s Character Classes 📘 Reference · Concept Non-whitespace with \S Character Classes 📘 Reference · Concept Custom character classes Character Classes 📘 Reference · Concept Character ranges Character Classes 📘 Reference · Concept Negated character classes Character Classes 📘 Reference · Concept Zero or more with * Quantifiers 📘 Reference · Concept One or more with + Quantifiers 📘 Reference · Concept Optional with ? Quantifiers 📘 Reference · Concept Exact repetition with {n} Quantifiers 📘 Reference · Concept At least n repetitions with {n,} Quantifiers 📘 Reference · Concept Between n and m repetitions with {n,m} Quantifiers 📘 Reference · Concept Greedy quantifiers Quantifiers 📘 Reference · Concept Lazy quantifiers Quantifiers 📘 Reference · Concept Capturing groups Groups 📘 Reference · Concept Multiple capturing groups Groups 📘 Reference · Concept Nested groups Groups 📘 Reference · Concept Optional groups Groups 📘 Reference · Concept Non-capturing groups Groups 📘 Reference · Concept Named capturing groups Groups 📘 Reference · Concept Backreferences Groups 📘 Reference · Concept Groups with alternation Groups 📘 Reference · Concept Groups with quantifiers Groups 📘 Reference · Concept Start of string with ^ Anchors 📘 Reference · Concept End of string with $ Anchors 📘 Reference · Concept Validate the whole input Anchors 📘 Reference · Concept Word boundary with \b Anchors 📘 Reference · Concept Non-word boundary with \B Anchors 📘 Reference · Concept Multiline anchors Anchors 📘 Reference · Concept Anchors without multiline mode Anchors 📘 Reference · Concept Anchors in extraction Anchors 📘 Reference · Concept Basic alternation with | Alternation 📘 Reference · Concept Alternation inside a group Alternation 📘 Reference · Concept Shared prefix Alternation 📘 Reference · Concept Shared suffix Alternation 📘 Reference · Concept Alternation precedence Alternation 📘 Reference · Concept Multiple alternatives Alternation 📘 Reference · Concept Alternation with optional text Alternation 📘 Reference · Concept Capturing alternatives Alternation 📘 Reference · Concept Positive lookahead Lookarounds 📘 Reference · Concept Negative lookahead Lookarounds 📘 Reference · Concept Positive lookbehind Lookarounds 📘 Reference · Concept Negative lookbehind Lookarounds 📘 Reference · Concept Multiple lookaheads Lookarounds 📘 Reference · Concept Zero-width assertions Lookarounds 📘 Reference · Concept Combining lookahead and lookbehind Lookarounds 📘 Reference · Concept Lookarounds with groups and alternation Lookarounds 📘 Reference · Concept

Test anchors carefully Testing Regex 🎓 Learning · Guide Named groups use different syntaxes Regex Compatibility 🎓 Learning · Guide Anchors and line modes can differ Regex Compatibility 🎓 Learning · Guide Forgetting to escape special characters Common Regex Mistakes 🎓 Learning · Guide Why character classes matter Character Classes 📘 Reference · Guide Shortcut classes versus custom classes Character Classes 📘 Reference · Guide Why quantifiers are essential Quantifiers 📘 Reference · Guide Greedy versus lazy matching Quantifiers 📘 Reference · Guide Why groups matter Groups 📘 Reference · Guide Named groups improve readability Groups 📘 Reference · Guide Why anchors matter Anchors 📘 Reference · Guide Why alternation matters Alternation 📘 Reference · Guide Why lookarounds matter Lookarounds 📘 Reference · Guide Lookarounds do not consume characters Lookarounds 📘 Reference · Guide Lookaheads versus lookbehinds Lookarounds 📘 Reference · Guide

Phone Number Regex 🧩 Pattern · Page

Simple phone number Phone Number Regex 🧩 Pattern · Example E.164 international format Phone Number Regex 🧩 Pattern · Example French mobile number Phone Number Regex 🧩 Pattern · Example US phone number Phone Number Regex 🧩 Pattern · Example Mobile numbers only Phone Number Regex 🧩 Pattern · Example

Why validate phone numbers with regex? Phone Number Regex 🧩 Pattern · Guide What is the E.164 phone number format? Phone Number Regex 🧩 Pattern · Guide International phone numbers are difficult to validate Phone Number Regex 🧩 Pattern · Guide Mobile numbers vs landline numbers Phone Number Regex 🧩 Pattern · Guide Formatting and user-friendly input with spaces, parentheses, dashes and other Phone Number Regex 🧩 Pattern · Guide Can regex verify a real phone number? Phone Number Regex 🧩 Pattern · Guide Best practices for phone number validation Phone Number Regex 🧩 Pattern · Guide

Always test in the target environment Regex Compatibility 🎓 Learning · Guide Large inputs deserve extra testing Regex Performance 🎓 Learning · Guide Ignoring edge cases Common Regex Mistakes 🎓 Learning · Guide

Non-digits with \D Character Classes 📘 Reference · Concept Non-word characters with \W Character Classes 📘 Reference · Concept Whitespace with \s Character Classes 📘 Reference · Concept Non-whitespace with \S Character Classes 📘 Reference · Concept Backreferences Groups 📘 Reference · Concept Word boundary with \b Anchors 📘 Reference · Concept Non-word boundary with \B Anchors 📘 Reference · Concept Multiline anchors Anchors 📘 Reference · Concept

Extracting Credit Card Numbers from Text Credit Card Regex 🧩 Pattern · Guide Masking Sensitive Card Numbers Credit Card Regex 🧩 Pattern · Guide Regex cannot understand context Regex Limitations 🎓 Learning · Guide Test extraction separately from validation Testing Regex 🎓 Learning · Guide Test replacement patterns Testing Regex 🎓 Learning · Guide Replacement syntax is language-specific Regex Compatibility 🎓 Learning · Guide Backreferences are powerful but should stay readable Groups 📘 Reference · Guide Understanding word boundaries Anchors 📘 Reference · Guide

Regex Tutorial 🎓 Learning · Page

Start with exact text Regex Tutorial 🎓 Learning · Concept Use character classes Regex Tutorial 🎓 Learning · Concept Add repetition Regex Tutorial 🎓 Learning · Concept Test often Regex Tutorial 🎓 Learning · Concept

How to learn regex step by step Regex Tutorial 🎓 Learning · Guide Start with literal text Regex Tutorial 🎓 Learning · Guide Use character classes for flexibility Regex Tutorial 🎓 Learning · Guide Add quantifiers to repeat patterns Regex Tutorial 🎓 Learning · Guide Use groups to organize your regex Regex Tutorial 🎓 Learning · Guide Test regex patterns often Regex Tutorial 🎓 Learning · Guide Common beginner mistakes Regex Tutorial 🎓 Learning · Guide

International email addresses Email Regex 🧩 Pattern · Guide ASCII versus Unicode Character Classes 📘 Reference · Guide ASCII versus Unicode Character Classes 📘 Reference · Guide

Provide clear validation messages Input Validation Best Practices 🎓 Learning · Guide Accept reasonable user input Input Validation Best Practices 🎓 Learning · Guide

Password Regex 🧩 Pattern · Page Email Regex 🧩 Pattern · Page URL Regex 🧩 Pattern · Page Phone Number Regex 🧩 Pattern · Page Username Regex 🧩 Pattern · Page Date Regex 🧩 Pattern · Page IP Address Regex 🧩 Pattern · Page UUID Regex 🧩 Pattern · Page Hex Color Regex 🧩 Pattern · Page Domain Name Regex 🧩 Pattern · Page Slug Regex 🧩 Pattern · Page Credit Card Regex 🧩 Pattern · Page Regex Limitations 🎓 Learning · Page Testing Regex 🎓 Learning · Page Input Validation Best Practices 🎓 Learning · Page

Minimum 8 characters Password Regex 🧩 Pattern · Example Letters and numbers Password Regex 🧩 Pattern · Example Strong password Password Regex 🧩 Pattern · Example Strong password (12+ chars) Password Regex 🧩 Pattern · Example Basic email validation Email Regex 🧩 Pattern · Example Practical email validation Email Regex 🧩 Pattern · Example Email with subdomains Email Regex 🧩 Pattern · Example Gmail addresses only Email Regex 🧩 Pattern · Example Corporate email address Email Regex 🧩 Pattern · Example Comma-separated email list Email Regex 🧩 Pattern · Example Email with plus alias Email Regex 🧩 Pattern · Example Email ending with specific TLD Email Regex 🧩 Pattern · Example Business email address Email Regex 🧩 Pattern · Example Email without plus aliases Email Regex 🧩 Pattern · Example Allowed top-level domains Email Regex 🧩 Pattern · Example Simple URL regex URL Regex 🧩 Pattern · Example HTTPS-only URL regex URL Regex 🧩 Pattern · Example Simple phone number Phone Number Regex 🧩 Pattern · Example E.164 international format Phone Number Regex 🧩 Pattern · Example French mobile number Phone Number Regex 🧩 Pattern · Example US phone number Phone Number Regex 🧩 Pattern · Example Mobile numbers only Phone Number Regex 🧩 Pattern · Example Letters and numbers Username Regex 🧩 Pattern · Example Letters, numbers and underscore Username Regex 🧩 Pattern · Example Username with length limit Username Regex 🧩 Pattern · Example Lowercase username Username Regex 🧩 Pattern · Example Username with hyphen Username Regex 🧩 Pattern · Example International username Username Regex 🧩 Pattern · Example DD/MM/YYYY Date Regex 🧩 Pattern · Example DD-MM-YYYY Date Regex 🧩 Pattern · Example ISO date Date Regex 🧩 Pattern · Example ISO datetime Date Regex 🧩 Pattern · Example Stricter DD/MM/YYYY Date Regex 🧩 Pattern · Example Simple IPv4 IP Address Regex 🧩 Pattern · Example Strict IPv4 IP Address Regex 🧩 Pattern · Example Private IPv4 ranges IP Address Regex 🧩 Pattern · Example IPv4 with CIDR IP Address Regex 🧩 Pattern · Example Basic IPv6 IP Address Regex 🧩 Pattern · Example Standard UUID UUID Regex 🧩 Pattern · Example UUID v4 UUID Regex 🧩 Pattern · Example UUID v1 to v5 UUID Regex 🧩 Pattern · Example UUID without hyphens UUID Regex 🧩 Pattern · Example Short hex color Hex Color Regex 🧩 Pattern · Example Long hex color Hex Color Regex 🧩 Pattern · Example Short or long hex color Hex Color Regex 🧩 Pattern · Example Hex color with alpha Hex Color Regex 🧩 Pattern · Example Any CSS hex color Hex Color Regex 🧩 Pattern · Example Hex color without # Hex Color Regex 🧩 Pattern · Example Simple domain name Domain Name Regex 🧩 Pattern · Example Domain with subdomains Domain Name Regex 🧩 Pattern · Example Strict domain labels Domain Name Regex 🧩 Pattern · Example Lowercase domain name Domain Name Regex 🧩 Pattern · Example Domain with optional trailing dot Domain Name Regex 🧩 Pattern · Example Punycode domain Domain Name Regex 🧩 Pattern · Example Basic slug Slug Regex 🧩 Pattern · Example SEO-friendly slug Slug Regex 🧩 Pattern · Example Slug with underscores Slug Regex 🧩 Pattern · Example Fixed-length slug Slug Regex 🧩 Pattern · Example Unicode slug Slug Regex 🧩 Pattern · Example Generic Credit Card Number Credit Card Regex 🧩 Pattern · Example Visa Card Credit Card Regex 🧩 Pattern · Example Mastercard Credit Card Regex 🧩 Pattern · Example American Express Credit Card Regex 🧩 Pattern · Example Credit Card with Spaces or Hyphens Credit Card Regex 🧩 Pattern · Example Discover Card Credit Card Regex 🧩 Pattern · Example JCB Card Credit Card Regex 🧩 Pattern · Example Diners Club Credit Card Regex 🧩 Pattern · Example Last 4 Digits Credit Card Regex 🧩 Pattern · Example Extract Credit Card Numbers Credit Card Regex 🧩 Pattern · Example Accept Multiple Credit Card Formats Credit Card Regex 🧩 Pattern · Example Mask Credit Card Numbers Credit Card Regex 🧩 Pattern · Example Payment Form Validation Credit Card Regex 🧩 Pattern · Example Strict 16-Digit Card Number Credit Card Regex 🧩 Pattern · Example Find Card Numbers in Logs Credit Card Regex 🧩 Pattern · Example

Digits with \d Character Classes 📘 Reference · Concept Word characters with \w Character Classes 📘 Reference · Concept Exact repetition with {n} Quantifiers 📘 Reference · Concept At least n repetitions with {n,} Quantifiers 📘 Reference · Concept Between n and m repetitions with {n,m} Quantifiers 📘 Reference · Concept Optional groups Groups 📘 Reference · Concept Start of string with ^ Anchors 📘 Reference · Concept End of string with $ Anchors 📘 Reference · Concept Validate the whole input Anchors 📘 Reference · Concept Alternation with optional text Alternation 📘 Reference · Concept Positive lookahead Lookarounds 📘 Reference · Concept Negative lookahead Lookarounds 📘 Reference · Concept Multiple lookaheads Lookarounds 📘 Reference · Concept

Why validate passwords with regex? Password Regex 🧩 Pattern · Guide Common password requirements Password Regex 🧩 Pattern · Guide Strong passwords vs complex passwords Password Regex 🧩 Pattern · Guide Can regex measure password strength? Password Regex 🧩 Pattern · Guide Client-side and server-side validation Password Regex 🧩 Pattern · Guide Common password validation mistakes Password Regex 🧩 Pattern · Guide Modern password recommendations Password Regex 🧩 Pattern · Guide Why email regex validation is difficult Email Regex 🧩 Pattern · Guide Practical email regex vs RFC-compliant regex Email Regex 🧩 Pattern · Guide What an email regex can validate Email Regex 🧩 Pattern · Guide Email aliases and plus addressing Email Regex 🧩 Pattern · Guide International email addresses Email Regex 🧩 Pattern · Guide Confirming an email address Email Regex 🧩 Pattern · Guide Why use a URL regex? URL Regex 🧩 Pattern · Guide What is a valid URL? URL Regex 🧩 Pattern · Guide HTTP vs HTTPS URL validation URL Regex 🧩 Pattern · Guide Should a regex validate URLs completely? URL Regex 🧩 Pattern · Guide Common URL validation mistakes URL Regex 🧩 Pattern · Guide When to use URL regex validation URL Regex 🧩 Pattern · Guide Why validate phone numbers with regex? Phone Number Regex 🧩 Pattern · Guide What is the E.164 phone number format? Phone Number Regex 🧩 Pattern · Guide International phone numbers are difficult to validate Phone Number Regex 🧩 Pattern · Guide Mobile numbers vs landline numbers Phone Number Regex 🧩 Pattern · Guide Formatting and user-friendly input with spaces, parentheses, dashes and other Phone Number Regex 🧩 Pattern · Guide Can regex verify a real phone number? Phone Number Regex 🧩 Pattern · Guide Best practices for phone number validation Phone Number Regex 🧩 Pattern · Guide What is a username regex? Username Regex 🧩 Pattern · Guide Common username validation rules Username Regex 🧩 Pattern · Guide ASCII usernames vs Unicode usernames Username Regex 🧩 Pattern · Guide What regex cannot validate Username Regex 🧩 Pattern · Guide Why validate dates with regex? Date Regex 🧩 Pattern · Guide Common date formats Date Regex 🧩 Pattern · Guide ISO 8601 date validation Date Regex 🧩 Pattern · Guide Can regex validate real calendar dates? Date Regex 🧩 Pattern · Guide Date format validation vs date parsing Date Regex 🧩 Pattern · Guide Common date validation mistakes Date Regex 🧩 Pattern · Guide When to use a date regex Date Regex 🧩 Pattern · Guide Why validate IP addresses with regex? IP Address Regex 🧩 Pattern · Guide IPv4 vs IPv6 IP Address Regex 🧩 Pattern · Guide What makes IPv6 validation difficult? IP Address Regex 🧩 Pattern · Guide CIDR notation explained IP Address Regex 🧩 Pattern · Guide Private vs public IP addresses IP Address Regex 🧩 Pattern · Guide Can regex verify that an IP address exists? IP Address Regex 🧩 Pattern · Guide When to use IP address regex validation IP Address Regex 🧩 Pattern · Guide What is a UUID? UUID Regex 🧩 Pattern · Guide Common UUID format UUID Regex 🧩 Pattern · Guide UUID versions UUID Regex 🧩 Pattern · Guide Can regex verify UUID uniqueness? UUID Regex 🧩 Pattern · Guide When to use a UUID regex UUID Regex 🧩 Pattern · Guide What is a hex color? Hex Color Regex 🧩 Pattern · Guide Short vs long hex colors Hex Color Regex 🧩 Pattern · Guide Hex colors with alpha transparency Hex Color Regex 🧩 Pattern · Guide Common hex color validation mistakes Hex Color Regex 🧩 Pattern · Guide When to use a hex color regex Hex Color Regex 🧩 Pattern · Guide What is a domain name? Domain Name Regex 🧩 Pattern · Guide Domain name vs URL Domain Name Regex 🧩 Pattern · Guide Subdomains and labels Domain Name Regex 🧩 Pattern · Guide Common domain validation mistakes Domain Name Regex 🧩 Pattern · Guide Can regex verify that a domain exists? Domain Name Regex 🧩 Pattern · Guide Internationalized domain names Domain Name Regex 🧩 Pattern · Guide What is a slug? Slug Regex 🧩 Pattern · Guide Why are slugs important for SEO? Slug Regex 🧩 Pattern · Guide Common slug rules Slug Regex 🧩 Pattern · Guide Slug validation vs slug generation Slug Regex 🧩 Pattern · Guide Unicode slugs and international websites Slug Regex 🧩 Pattern · Guide When to use a slug regex Slug Regex 🧩 Pattern · Guide Can Regex Validate a Credit Card? Credit Card Regex 🧩 Pattern · Guide Understanding Credit Card Number Structure Credit Card Regex 🧩 Pattern · Guide Visa vs Mastercard Regex Credit Card Regex 🧩 Pattern · Guide Supporting Spaces and Hyphens Credit Card Regex 🧩 Pattern · Guide Common Regex Mistakes Credit Card Regex 🧩 Pattern · Guide Regex validates format, not meaning Regex Limitations 🎓 Learning · Guide Regex cannot replace business logic Regex Limitations 🎓 Learning · Guide Checksums and algorithms need code Regex Limitations 🎓 Learning · Guide Dates and numbers often need parsing Regex Limitations 🎓 Learning · Guide Regex can be too strict or too permissive Regex Limitations 🎓 Learning · Guide Security-sensitive validation needs layers Regex Limitations 🎓 Learning · Guide Regex cannot query external systems Regex Limitations 🎓 Learning · Guide Test both matches and non-matches Testing Regex 🎓 Learning · Guide Use realistic input Testing Regex 🎓 Learning · Guide Include edge cases Testing Regex 🎓 Learning · Guide Anchor validation regexes Regex Performance 🎓 Learning · Guide Validate structure, not meaning Input Validation Best Practices 🎓 Learning · Guide Forgetting anchors Common Regex Mistakes 🎓 Learning · Guide Trying to validate everything with regex Common Regex Mistakes 🎓 Learning · Guide Fixed-length versus variable-length patterns Quantifiers 📘 Reference · Guide Validation versus extraction Anchors 📘 Reference · Guide Lookarounds simplify validation Lookarounds 📘 Reference · Guide

URL Regex 🧩 Pattern · Page Hex Color Regex 🧩 Pattern · Page Domain Name Regex 🧩 Pattern · Page Slug Regex 🧩 Pattern · Page Regex Compatibility 🎓 Learning · Page

Simple URL regex URL Regex 🧩 Pattern · Example HTTPS-only URL regex URL Regex 🧩 Pattern · Example Short hex color Hex Color Regex 🧩 Pattern · Example Long hex color Hex Color Regex 🧩 Pattern · Example Short or long hex color Hex Color Regex 🧩 Pattern · Example Hex color with alpha Hex Color Regex 🧩 Pattern · Example Any CSS hex color Hex Color Regex 🧩 Pattern · Example Hex color without # Hex Color Regex 🧩 Pattern · Example Simple domain name Domain Name Regex 🧩 Pattern · Example Domain with subdomains Domain Name Regex 🧩 Pattern · Example Strict domain labels Domain Name Regex 🧩 Pattern · Example Lowercase domain name Domain Name Regex 🧩 Pattern · Example Domain with optional trailing dot Domain Name Regex 🧩 Pattern · Example Punycode domain Domain Name Regex 🧩 Pattern · Example Basic slug Slug Regex 🧩 Pattern · Example SEO-friendly slug Slug Regex 🧩 Pattern · Example Slug with underscores Slug Regex 🧩 Pattern · Example Fixed-length slug Slug Regex 🧩 Pattern · Example Unicode slug Slug Regex 🧩 Pattern · Example

Email aliases and plus addressing Email Regex 🧩 Pattern · Guide Why use a URL regex? URL Regex 🧩 Pattern · Guide What is a valid URL? URL Regex 🧩 Pattern · Guide HTTP vs HTTPS URL validation URL Regex 🧩 Pattern · Guide Should a regex validate URLs completely? URL Regex 🧩 Pattern · Guide Common URL validation mistakes URL Regex 🧩 Pattern · Guide When to use URL regex validation URL Regex 🧩 Pattern · Guide What is a hex color? Hex Color Regex 🧩 Pattern · Guide Short vs long hex colors Hex Color Regex 🧩 Pattern · Guide Hex colors with alpha transparency Hex Color Regex 🧩 Pattern · Guide Common hex color validation mistakes Hex Color Regex 🧩 Pattern · Guide When to use a hex color regex Hex Color Regex 🧩 Pattern · Guide What is a domain name? Domain Name Regex 🧩 Pattern · Guide Domain name vs URL Domain Name Regex 🧩 Pattern · Guide Subdomains and labels Domain Name Regex 🧩 Pattern · Guide Common domain validation mistakes Domain Name Regex 🧩 Pattern · Guide Can regex verify that a domain exists? Domain Name Regex 🧩 Pattern · Guide Internationalized domain names Domain Name Regex 🧩 Pattern · Guide What is a slug? Slug Regex 🧩 Pattern · Guide Why are slugs important for SEO? Slug Regex 🧩 Pattern · Guide Common slug rules Slug Regex 🧩 Pattern · Guide Slug validation vs slug generation Slug Regex 🧩 Pattern · Guide Unicode slugs and international websites Slug Regex 🧩 Pattern · Guide When to use a slug regex Slug Regex 🧩 Pattern · Guide Cross-Engine Compatibility Credit Card Regex 🧩 Pattern · Guide Test in the target regex engine Testing Regex 🎓 Learning · Guide Lookbehind support varies Regex Compatibility 🎓 Learning · Guide