Regex tester

Runs in your browser, nothing uploaded

Input

Pattern Without slashes or flags. Enter runs now.

Written for Java and Go patterns are checked, not run. Try in JS runs a translated copy.

Flags g all matches i ignore case m ^ and $ per line s dot matches newline u Unicode mode y sticky, contiguous only d group indices

Inline flags such as (?i) are not JavaScript. Try in JS moves them to flags.

Test input

Replacement (optional)

Examples

Also runs as you type.

Output JavaScript runs here. Java and Go are checked.

Type a pattern, or load an example, to see portability warnings and matches.

Portability

Matches

Test input with matches highlighted

Matches found in the test input
#	Index	Text	Groups	Named groups

Use it in code

The pattern as typed, written as a source literal for each language.

Behaviour differences between flavors

Behaviour differences between JavaScript, Java and Go
Topic	JavaScript	Java	Go (RE2)

Help

What it does

Runs the pattern against your test input in JavaScript. Matching runs in a Web Worker, so a pattern that backtracks for a long time cannot freeze the page. A run that passes 1.5 s is stopped.
Checks the pattern for syntax that Java and Go read differently, or reject, and lists each issue for all three flavors.
Runs the replacement in JavaScript, warns about replacement syntax per flavor, and gives Java and Go versions of the replacement.
Writes the pattern as a Java string, a Go literal and a JavaScript literal, ready to paste.

What it does not do

Cannot do: run Java or Go in the browser. Only JavaScript runs. Java and Go results are static checks of known syntax differences, not executions.

Try in JS runs a rewritten copy in JavaScript. Constructs it cannot rewrite are listed, and they may change the result.
Compatible means no construct on the known list was rejected or changed. It does not prove the pattern behaves the same in every engine.
Timing is measured in your browser only. Nothing is uploaded.

The four statuses

Compatible: no construct on the known list is rejected or means something different in this flavor.
Differs: the pattern compiles, but some construct means something else or is ignored in this flavor.
Incompatible: at least one construct is rejected in this flavor, so the pattern will not compile there.
Risky: no other issue found, but the pattern has a shape that can backtrack catastrophically in JavaScript and Java. Go (RE2) runs in linear time and is not affected.

Common portability traps

Lookbehind and lookahead: Go (RE2) rejects them. Java needs lookbehind of bounded length.
Backreferences such as \1 or \k<name>: Go does not support them.
\A and \z: JavaScript reads them as the letters A and z. Use ^ and $ there.
Named groups: (?<name>...) works in Java and JavaScript. (?P<name>...) is Go (before 1.22) and Python. Java group names reject underscores.
Possessive and atomic: a++ and (?>...) are Java only.
Literal {: Java rejects an unescaped { that does not start a quantifier. Escape it as \{.
Catastrophic backtracking: nested quantifiers such as (a+)+ can take exponential time in JavaScript and Java. Go is not affected.
Replacements: JavaScript and Java use $1, Go uses ${1}. A literal dollar sign in Java is \$.
Inline flags: (?i) works in Java and Go. JavaScript has no inline flags, so Try in JS moves a leading one to the flags.

Keyboard

Ctrl+Enter (or ⌘+Enter) runs now.
? opens this help when you are not typing in a field.
Esc closes Help.