Java-Style Regex Browser Preview

What this Java-style regex preview does

Use this page to try a regular-expression pattern against sample text in a browser and inspect what it finds. In Match mode, it highlights each match and lists its position, length, numbered capture groups, and named groups. A capture group is a parenthesized part of a pattern whose matching text is reported separately. In Replace mode, enter replacement text to preview the changed sample; choose whether to replace every match or only the first.

The name describes the patterns and flag concepts you may be preparing for Java. The preview itself runs with the browser's JavaScript regular-expression engine. It is not a Java runtime and does not execute java.util.regex.Pattern. Some familiar expressions work similarly, but syntax and matching details can differ.

Flags are approximations, not a Java compatibility layer

Settings include case-insensitive, multiline, dotall, Unicode case, Unicode character class, comments, and literal modes. Multiline affects how ^ and $ recognize line boundaries; dotall changes whether . can match a newline. Comments mode removes whitespace and hash comments before testing, while literal mode escapes pattern metacharacters. The interface maps these options only partially onto browser flags and preprocessing; the Unicode options, for example, share a JavaScript Unicode flag. Do not infer full Java flag semantics from a successful preview.

Java source code adds another layer of escaping: a regex typed directly into this page is the pattern text, whereas a Java string literal may need doubled backslashes so the compiler passes the intended backslash to the regex engine. Oracle's Pattern API documentation describes Java syntax and string-literal escaping. The ECMAScript specification describes the browser engine's own syntax and flags. Both were checked October 2, 2026.

Test a pattern and read its output

1. Enter the pattern itself

   Type the regex without Java source-code quotation marks. For instance, ([A-Z]+)-([0-9]+) can find identifiers such as A-204 and capture the letter prefix and digits separately.

2. Add representative sample text

   Paste a short excerpt into the text field. Include examples that should match and nearby examples that should not, such as A-204, B-9, and lower-3.

3. Choose match or replacement

   Match mode shows highlighted hits and their groups. In Replace mode, enter replacement text, then leave Replace All enabled to preview every hit or turn it off to preview only the first.

4. Adjust settings and inspect details

   Toggle one setting at a time, then review the match count, positions, lengths, and group values. An invalid-pattern result means the browser engine rejected the current pattern. A transformed-pattern indicator means comments or literal preprocessing changed the entered pattern before execution.

Common questions about compatibility

Verify Java behavior in the target runtime

• A match here does not establish that the same pattern, flags, capture behavior, or replacement syntax will behave the same way in Java. Test important cases with the application's target JDK and its actual Pattern and Matcher calls.
• This preview reports results for the sample you provide; it does not prove that a pattern handles every production input. Include positive, negative, empty, multiline, and relevant Unicode cases in your own checks.
• Long text combined with a complex pattern can make browser interaction slow. Start with a small sample and avoid putting secrets, credentials, personal details, or unpublished data into shareable examples; the page's result-sharing feature includes the entered parameters.