How does this tool handle possessive quantifiers and atomic groups?

They are translated using lookahead-capture emulation to ensure they strictly prevent backtracking, mirroring Java engine behavior.

Are \d, \w, and \s matching ASCII or Unicode by default?

By default, they match ASCII characters only, matching Java defaults unless the UNICODE_CHARACTER_CLASS option is enabled.

What replacement syntax is supported in the preview?

The preview supports Java replacement syntax, including numeric index backreferences like $1 and named backreferences like ${name}.

Does the tester generate usable Java source code?

Yes, it generates ready-to-paste Java code with the appropriate Pattern flags and Matcher named-group extraction calls.

Are lookbehind limitations identical to Java?

The browser execution environment may permit variable-length lookbehind, but differences where Java's engine is stricter are noted in the results.

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Java Regex Tester

Test java.util.regex patterns online: (?<name>…) named groups, possessive quantifiers and atomic groups, Pattern flags, ASCII vs UNICODE_CHARACTER_CLASS \d semantics, and ready-to-paste Java code.

Details

What this tool helps you do

A regex tester for the java.util.regex flavor (java.util.regex.Pattern / Matcher). Write the pattern exactly as in Java — named groups (?…), backreference \k, possessive quantifiers like a*+, atomic groups (?>…) — and it is translated to the platform's ECMAScript engine and executed against your test string, reporting every match, capture groups with names, and a replace preview using $1 / ${name} Java substitution syntax. Ready-to-paste Java code with the right Pattern.CASEINSENSITIVE / MULTILINE / DOTALL / COMMENTS / UNICODECHARACTERCLASS flags and matcher.group("name") access is generated for you. Java-specific semantics are honored: \d \w \s match ASCII only unless you enable Unicode character classes (emulating UNICODECHARACTER_CLASS), possessive and atomic constructs are emulated with lookahead-capture so they really do prevent backtracking, and constructs Java rejects (variable-length lookbehind) are noted where this emulator is more permissive than the real engine.

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Samples

Java Regex Tester

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Key facts

Overview

When to use

How it works

Use cases

Examples

1. Named capture groups for log parsing

2. Validating possessive quantifiers against backtracking

FAQ

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