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Regex Checker (Java)

Check a Java regex pattern against your text and inspect matches, capture groups, and replacements. You can also paste a Java "string literal" (with double backslashes) and have it unescaped before testing.

Note: This runs in your browser using a JavaScript regex engine, so a few Java-specific constructs may differ. The tool highlights common incompatibilities.

Category: Developer · URL: /tools/regex-checker-java.html
Runs locally in your browser. Engine is JavaScript RegExp with Java-oriented helpers (Java string literal unescape + Java code snippet). Some Java-only features may not behave exactly the same.
Tip: If your Java code uses "\\d+", check “Pattern is Java string literal” so it tests as \d+. 

      

      

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            Privacy: runs locally in your browser. No uploads, no tracking scripts.
          

        

      


      
    


    

      
How to use

      
Use it like a quick Pattern + Matcher sandbox.
  1. Enter your regex in Pattern. If you copied it from Java source code (double slashes), enable Pattern is Java string literal.
  2. Paste your test text in Text (optionally unescape as Java string literal).
  3. Select flags (Case-insensitive / Multiline / Dotall / Unicode) and choose Find all if you want every match.
  4. Click Test to see matches and groups. Use Replace All / Replace First to preview replacements.

      

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        Secondary intents covered: Validate whether a Java regex pattern is syntactically correct, Find all matches and inspect capture groups and indices, Convert a Java string literal (e.g., "\\d+") into the real regex for testing, Generate a Java Pattern/Matcher snippet with the chosen flags, Test replaceAll / replaceFirst output quickly, Troubleshoot why a pattern behaves differently than expected (flags, anchors, boundaries), Spot Java-vs-JS incompatibilities before copying into code, Check multiline behavior for ^ and $ across lines
      

    


    

      
FAQ

    
Is this running real Java Pattern/Matcher?
No. It runs in your browser using JavaScript RegExp, plus helpers for Java string-literal unescaping and a Java code snippet.
Why does my Java regex behave differently here?
Some Java-only features (like inline flags, possessive quantifiers, or tokens like \A/\Z) may not be supported or may differ in JavaScript.
What does “Pattern is Java string literal” mean?
If you copied from Java code (e.g., "\\d+"), enable it so the tool converts \\ into a single backslash before testing.
Which flags correspond to Java Pattern flags?
i ≈ CASE_INSENSITIVE, m ≈ MULTILINE, s ≈ DOTALL. The u flag enables Unicode mode in JavaScript and may affect character classes.
Does “Find all” match Java’s find() loop?
Yes, it approximates repeated m.find() by using global matching and listing each match with captured groups.
How do I test replaceAll vs replaceFirst?
Use Replace All to replace every match (like Java replaceAll) and Replace First for only the first occurrence.
How many matches will it list?
It caps output to keep the page fast (up to 200 matches) and applies a short time guard for very expensive patterns.