Converting regex between flavors

"Regex" is a family of related languages, not a single language. A pattern that works perfectly in Python may fail silently in JavaScript, or work but mean something different. This guide is the survival map.

The major flavors

• PCRE — Perl-Compatible Regular Expressions. The reference for "advanced" regex. Used in PHP, nginx, BBEdit, many tools.
• Python — the re module. Close to PCRE but with some unique syntax.
• JavaScript — the RegExp builtin. Common features only, no \K or recursion.
• Java — java.util.regex. Backtracking engine with most features.
• .NET — System.Text.RegularExpressions. Most powerful backtracking engine in widespread use.
• Go — uses RE2 (Russ Cox's library). Linear time, no backtracking, fewer features.
• Ruby — Onigmo engine. Close to Perl, full feature set.

What's identical across all of them

The basic building blocks work the same everywhere:

• Character classes: [abc], [a-z], [^abc]
• Shorthand: \d, \w, \s
• Quantifiers: *, +, ?, {n,m}
• Anchors: ^, $, \b
• Capturing groups: (...)
• Non-capturing groups: (?:...)
• Alternation: a|b
• Lookahead: (?=...), (?!...)

If your pattern uses only these, it almost certainly works everywhere.

Named group syntax — the most common gotcha

The P in Python's (?P<name>...) is a Python-specific quirk. Converting between Python and others almost always needs you to add or remove the P.

Lookbehind

• PCRE, Java, .NET, Ruby: variable-length lookbehind supported (some restrictions).
• JavaScript: variable-length lookbehind supported (ES2018+).
• Python: variable-length lookbehind supported in the regex third-party module; re requires fixed length.
• Go: not supported at all (RE2 is linear-time and rejects lookbehind).

Features Go doesn't have

Go's regexp is RE2, which trades features for linear-time guarantees. If you're porting to Go, watch for:

• No backreferences (\1)
• No lookahead or lookbehind
• No atomic groups or possessive quantifiers
• No recursion
• Some Unicode properties differ slightly

If your pattern uses any of these, you need to rewrite it for Go.

Flag syntax

Go uses inline flags (?i), (?m), etc. — no external flag arg.

String literal escaping

This trips up everyone. The regex \d+ as a literal in source code:

Languages with raw/verbatim string syntax (Python, Go, C#) let you write the regex once. Others require doubling every backslash.

Use our converter

Our flavor converter handles the syntax differences automatically. Paste a pattern, pick source and target flavors, and it shows the converted pattern plus warnings about features that don't carry over.

See also

• Regex in Python — complete guide
• Regex in JavaScript — complete guide
• Python vs JavaScript regex — quick comparison
• Convert flavors tool