Regex flavor guides

"Regex" isn't one thing — grep, Python, Java, .NET and JavaScript all disagree about what a pattern actually does. Each guide below covers one real engine's own rules, and links straight into the regex tester with that flavor already selected.

ECMAScript (RegExp)

JavaScript, TypeScript, Node.js, browsers

ECMAScript RegExp, exactly as your browser or Node runs it

PCRE2

PHP (preg_*), R, Apache/nginx rewrite rules, many C/C++ tools

the engine behind PHP's preg_* and most Apache/nginx rewrite rules

Python 3.11+ (re)

Python 3.11+

re module rules, Unicode-by-default, 3.11+ possessive quantifiers

Java (java.util.regex)

Java, Kotlin, Scala, Android

java.util.regex rules for Java, Kotlin, Scala and Android

.NET / PowerShell (Regex)

C#, F#, VB.NET, PowerShell

System.Text.RegularExpressions rules, including the group-numbering quirk

Go RE2 (regexp, Go 1.22+)

Go, ripgrep (default mode), Rust regex (closely related engine)

RE2 rules, including why lookaround and backreferences don't compile

POSIX ERE (grep -E, sed -E, awk)

grep -E, egrep, sed -E / -r, awk

POSIX ERE and leftmost-longest matching, not leftmost-first

POSIX BRE (grep, sed default)

grep (no -E), sed (no -E/-r)

POSIX BRE, where ( ) and { } are literal and \( \) \{ \} do the work