Regex Tester

How It Works

The Regex Tester evaluates JavaScript regular expressions live as you type, using the browser’s native RegExp engine — the same engine that runs in your code, so what you see here is exactly what you will get in production. Type your pattern in the regex field, set flags (g for global, i for case-insensitive, m for multiline, s for dotAll, u for Unicode, y for sticky), and paste a sample string into the test area. Every match is highlighted in the test text and listed below with its position and any captured groups. Errors in your pattern (unbalanced parentheses, invalid escape sequences, lookbehind syntax errors) are surfaced inline with the JavaScript engine’s exact error message. Standard regex constructs are supported: character classes, anchors (^, $, \b), quantifiers (greedy, lazy, possessive via lookarounds), backreferences (\1, \k), named capture groups, lookaheads, lookbehinds, and Unicode property escapes such as `\p{L}` (with the u flag). Flag combinations behave exactly as the ECMAScript spec defines, which avoids the surprises that come with porting patterns between engines.

Worked Example

Test \b(\d{4})-(\d{2})-(\d{2})\b against “Released 2024-03-15, patched 2024-04-02”: the highlighter marks both dates, the stats cards report 2 matches with 3 capture groups each, and the group panel lists year, month and day separately. Enter the replacement $3/$2/$1 and the preview shows the dates flipped to 15/03/2024 format. The snippet generator then emits the same pattern as ready-to-paste Python, JavaScript or sed — with the replacement syntax correctly translated to \3/\2/\1 where the language requires backslashes.

Use Cases

• Prototyping input-validation patterns before adding them to code
• Extracting structured data — dates, emails, IDs — from raw text
• Learning regex syntax interactively with instant visual feedback
• Testing edge cases for log-parsing rules