Regex Tester & Debugger

Test regular expressions against sample text with Match, Test, and Replace modes, including named groups and $1 references — entirely in your browser.

Ready
 
 
  
 
 Reference 
Regular Expressions notes
 
 
 
 
What is Regular Expressions?
 
Regular expressions (regex) are patterns used to match character combinations in strings.
 
 
 
How does the Regular Expressions tool work?
 
Enter a regex pattern and optional flags, then choose a mode: 'Match' lists all matches with their index, positional groups and named groups; 'Test' returns whether the pattern exists; 'Replace' applies the replacement text (supports $1, $&, and named-group references) and shows the replaced result. Use the 'g' flag to find multiple matches and include other flags (i, m, u, y) as needed.
 
 
 
When should you use Regular Expressions?
 
     
  • Validating user input such as emails, phone numbers, and postal codes
  • Finding and replacing text across logs and configuration files
  • Extracting structured data like dates, URLs, and IDs from raw text
  • Debugging capture groups and named groups before using a pattern in code
  • Testing pattern behavior across flags (g, i, m, u, y) before deployment
    •  
 
 
 
What are the limitations of Regular Expressions?
 
     
  • Poorly written patterns can trigger catastrophic backtracking and freeze on long inputs
  • This tool uses the JavaScript regex engine, which differs from PCRE, RE2, and other engines in supported syntax
  • Complex patterns are hard to read and maintain without comments or tests
  • Regular expressions are not suited for parsing nested formats such as HTML or JSON
    •  
 
 
  
 
Regular Expressions examples
 
 
 email ([a-z0-9._%+-]+)@([a-z0-9.-]+)\.([a-z]{2,}) -> Matches email addresses and captures user, domain and TLD groups 
 phone \+?\d{1,3}[ -]?\(?\d{1,4}\)?[ -]?\d{3,4}[ -]?\d{3,4} -> Matches many international phone number formats 
 groups (\d{4})-(\d{2})-(\d{2}) -> Captures year, month and day from YYYY-MM-DD 
 url https?:\/\/[^\s]+ -> Matches http/https URLs 
 ipv4 \b(?:(?:25[0-5]|2[0-4]\\d|1?\\d?\\d)\.){3}(?:25[0-5]|2[0-4]\\d|1?\\d?\\d)\b -> Matches IPv4 addresses with valid 0-255 ranges 
 uuid [0-9a-f]{8}-[0-9a-f]{4}-[1-5][0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12} -> Matches RFC4122 UUID strings 
 
 
 
  
 
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