Regex Flavors Demystified: JavaScript vs Python vs PCRE vs Java
Same pattern. Different language. Suddenly it breaks.
Welcome to the world of regex flavors.
Major Differences
- JavaScript: No lookbehind, no named groups (until recently), limited Unicode
- Python: Full lookbehind, named groups, verbose mode
- PCRE: Conditional patterns, recursion, used in PHP/NGINX
- Java: Possessive quantifiers, Unicode scripts
Real Example
Pattern: (?<=@)\w+ (lookbehind)
- JavaScript → Invalid
- Python/PCRE/Java → Matches domain in email
This Tool Lets You Switch Instantly
Use the dropdown → see how your pattern behaves across engines. No setup. No confusion.
Best Practices
- Write for your target engine
- Avoid advanced features unless necessary
- Test in the actual environment
FAQ
Which flavor should I learn first?
JavaScript — it’s the most restrictive and widely used.
Are regexes standardized?
No. Each engine has its own quirks.
Write once. Test everywhere.