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    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