Test and debug regular expressions with live matching. Essential for writing and validating regex patterns.

Note: This tool uses JavaScript regex engine. See below for differences between regex flavors.

Test Regex

Common Use Cases

  • Input Validation: Test email, phone, SSN patterns
  • Log Parsing: Extract data from log files
  • Data Extraction: Pull specific patterns from text
  • Search & Replace: Test patterns before using in code
  • URL Matching: Validate or extract URL components
  • Password Requirements: Test password complexity rules

Common Patterns

Email:

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

Phone (US):

Lang: 
^\d{3}-\d{3}-\d{4}$

IP Address:

Lang: 
^(\d{1,3}\.){3}\d{1,3}$

URL:

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

Date (YYYY-MM-DD):

Lang: 
^\d{4}-\d{2}-\d{2}$

Hexadecimal Color:

Lang: 
^#?([a-fA-F0-9]{6}|[a-fA-F0-9]{3})$

Regex Flags

  • g (global): Find all matches, not just the first
  • i (ignore case): Case-insensitive matching
  • m (multiline): ^ and $ match line starts/ends
  • s (dotall): . matches newlines
  • u (unicode): Enable full Unicode support
  • y (sticky): Match from lastIndex position

Special Characters

  • . - Any character except newline
  • \d - Digit (0-9)
  • \w - Word character (a-z, A-Z, 0-9, _)
  • \s - Whitespace
  • ^ - Start of string/line
  • $ - End of string/line
  • * - 0 or more
  • + - 1 or more
  • ? - 0 or 1
  • {n} - Exactly n times
  • {n,m} - Between n and m times

Regex Flavor Differences

Different programming languages use different regex engines with varying features. This tool uses JavaScript (ECMAScript) regex.

JavaScript (ECMAScript)

  • Lookahead: (?=...) and (?!...) ✅ Supported
  • Lookbehind: (?<=...) and (?<!...) ✅ Supported (ES2018+)
  • Named Groups: (?<name>...) ✅ Supported (ES2018+)
  • Atomic Groups: (?>...) ❌ Not supported
  • Possessive Quantifiers: *+, ++ ❌ Not supported
  • Unicode: \p{...} ✅ Supported with u flag

Python (re module)

  • Lookahead/Lookbehind: ✅ Full support
  • Named Groups: (?P<name>...) (different syntax)
  • Atomic Groups: ❌ Not supported
  • Conditional: (?(id)yes|no) ✅ Supported
  • Verbose Mode: (?x) for comments ✅ Supported

PCRE (PHP, grep -P, Perl)

  • Most Features: ✅ Most comprehensive regex engine
  • Atomic Groups: (?>...) ✅ Supported
  • Possessive Quantifiers: ✅ Supported
  • Recursion: (?R) ✅ Supported
  • Conditionals: ✅ Supported
  • Subroutines: (?&name) ✅ Supported

Java

  • Lookahead/Lookbehind: ✅ Supported
  • Named Groups: (?<name>...) ✅ Supported
  • Atomic Groups: (?>...) ✅ Supported
  • Possessive Quantifiers: ✅ Supported
  • Unicode: Full Unicode support

.NET (C#)

  • Balancing Groups: (?<name1-name2>...) ✅ Unique feature
  • Conditional: (?(expression)yes|no) ✅ Supported
  • Right-to-left: RegexOptions.RightToLeft ✅ Supported
  • Most Features: Very comprehensive

Key Differences Summary

FeatureJavaScriptPythonPCREJava.NET
Lookbehind✅✅✅✅✅
Named Groups✅✅✅✅✅
Atomic Groups❌❌✅✅✅
Possessive Quantifiers❌❌✅✅✅
Recursion❌❌✅❌❌
Balancing Groups❌❌❌❌✅

Recommendations

  • For testing JavaScript patterns: Use this tool
  • For Python patterns: Test with python3 -c "import re; ..."
  • For PCRE patterns: Use grep -P or regex101.com
  • For production: Always test in your target language/environment