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Regex Tester & DebuggerPrivacy: All processing runs locally

Test, explain, and debug regular expressions with match highlighting, replace preview, and performance analysis.

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Test String
Highlighted Matches
Match Details
Enter a pattern and test string to see matches.
Replace Preview
Enter a replace pattern to see the transformed output.
Pattern Explanation
Enter a pattern to see a token-by-token explanation.
Common Patterns

How It Works

The Regex Tester executes your pattern using the browser's native JavaScript RegExp engine. All matching, replacement, and analysis runs locally — no data leaves your browser.

Flavor support: While execution uses JavaScript's engine, the tool provides compatibility notes for PCRE, Python, and .NET, flagging syntax differences (e.g., (?P<name>) in Python vs (?<name>) in JS, \p{L} support, lookbehind limitations).

Explanation: Each token in your regex is parsed and described in plain English — quantifiers, character classes, groups, anchors, lookaheads, and more.

Performance warnings: The tool detects patterns prone to catastrophic backtracking (nested quantifiers like (a+)+) and warns before they freeze your browser.

Replace preview: See the result of String.replace() in real-time with support for backreferences ($1, $<name>) and special tokens ($&, $`, $').

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