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  3. Regex Tester & Debugger
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Test Regular Expressions Instantly

Regular expressions are powerful but notoriously tricky to get right. This tester lets you write a pattern, supply test strings, and immediately see which parts match — with highlighted capture groups, match indices, and flag options. Build and debug patterns interactively instead of guessing and re-running your code.

Getting Started

  1. 1Enter your regular expression in the pattern field
  2. 2Set flags (g, i, m, s, u) as needed
  3. 3Type or paste test strings below
  4. 4View highlighted matches and capture group details in real time
  5. 5Adjust your pattern until it matches exactly what you expect

What You Get

  • Real-time match highlighting as you type
  • Capture group extraction with named group support
  • Flag toggles: global, case-insensitive, multiline, dotAll, unicode
  • Match index positions displayed for each match
  • Quick-reference cheat sheet for regex syntax
  • Multiple test strings for batch testing
  • Detailed error messages for invalid patterns

Understanding the Concepts

The tester evaluates your regex against the JavaScript regex engine in real time. Each match is highlighted in the test string, and named or numbered capture groups are listed separately so you can verify that your groups are extracting the right content. You can toggle flags like global (g), case-insensitive (i), multiline (m), dotAll (s), and unicode (u) to see how they affect matching. A quick-reference cheat sheet is available for common patterns and syntax.

Perfect For

  • Validating input patterns

    Build and test regex patterns for email addresses, phone numbers, URLs, or custom input formats before embedding them in your code.

  • Parsing log files

    Craft patterns to extract timestamps, error codes, or IP addresses from log entries, verifying matches against sample lines.

  • Search-and-replace preparation

    Test complex find-and-replace patterns with capture groups before running them across a codebase or document.

  • Learning regular expressions

    Experiment with regex syntax and instantly see what matches — a much faster feedback loop than modifying and re-running scripts.

Examples

  • Email validation

    Pattern: ^[\w.-]+@[\w.-]+\.[a-zA-Z]{2,}$ — matches standard email addresses.

  • Extract date components

    Pattern: (\d{4})-(\d{2})-(\d{2}) — captures year, month, and day from ISO dates.

Frequently Asked Questions

Which regex engine does this use?

It uses the JavaScript (ECMAScript) regex engine, which is what runs in all modern browsers and Node.js.

Can I test regex for Python or other languages?

Most basic patterns are cross-compatible. However, features like lookbehinds, possessive quantifiers, or atomic groups may differ between engines. This tool reflects JavaScript behavior.

What do the flags mean?

g = match all occurrences (not just the first), i = case-insensitive, m = ^ and $ match line boundaries, s = dot matches newlines, u = full Unicode matching.

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All processing happens directly in your browser. Your files never leave your device and are never uploaded to any server.