Regex Tester & Match Visualizer

How to Use the Regular Expression Tester

1. Enter a Regex Pattern: Type your pattern between the slashes. Enclose sub-patterns in parentheses ( ) to create capture groups.
2. Configure Flags: Toggle flags: g for all matches, i for case-insensitivity, m for multiline anchors, and s for dotAll.
3. Inspect Live Matches: Matches are immediately highlighted with alternating colors. Review the Capture Groups Table to see the extracted tokens from each group.
4. Test Substitutions in Replace Mode: Switch to the Replace tab, specify your replacement string (e.g. $1-$2), and inspect the transformed output alongside the line-by-line diff.
5. Quick Reference: Click the "Regex Cheat Sheet" button anytime to look up character classes, quantifiers, anchors, and lookaround assertions.

Mastering Regular Expressions for Web Development

Regular expressions (regex) provide pattern-matching algorithms for string search, input validation, and text transformation. Whether verifying email formats in web forms, rewriting URL paths in Apache or Nginx, or scraping structured log data, regex is an indispensable tool in every developer's arsenal.

Understanding Greedy vs. Lazy Quantifiers:

• Greedy Quantifiers (*, +): Consume as much text as possible while still allowing the pattern to match. For example, <.*> matching against <b>bold</b> will match the entire string from the first < to the last >.
• Lazy Quantifiers (*?, +?): Stop matching as soon as the minimal condition is satisfied. For example, <.*?> will match <b> and </b> separately.

Key Features

• Real-Time Highlight Engine: Matches update instantly on every keystroke with distinct color badges.
• Capture Groups Inspector: Clear tabulated breakdown of indexed match positions and parenthesized capture groups.
• Find and Replace Diff: Real-time preview of token replacements with line-level change highlights.
• Built-in Syntax Cheat Sheet: Fast lookup for character classes, boundaries, quantifiers, and lookaround syntax.
• 100% Private and Local: All regex evaluation happens within your browser's V8 engine with zero network calls.

Privacy & Data Security

This Regex Tester processes all strings and patterns exclusively inside your browser. No strings are logged or sent to remote servers, making it safe for inspecting confidential customer data, auth tokens, database connections, and server error logs.

Frequently Asked Questions

Which regex flavor does this tool use?

This tool uses the standard ECMAScript (JavaScript) RegExp engine built into your browser. It supports all standard features including lookaround assertions (?=, ?!, ?<=, ?

What do the regex flags g, i, m, and s mean?

The 'g' (global) flag finds all occurrences rather than stopping after the first match. The 'i' (ignore case) flag makes matches case-insensitive. The 'm' (multiline) flag causes ^ and $ to match the beginning and end of each individual line instead of the whole string. The 's' (dotAll) flag allows the dot (.) character to match newline characters as well.

How does Replacement Mode work with capture groups?

In Replace Mode, you can use $1, $2, etc., in your replacement pattern to insert the text captured by the first, second, or subsequent parenthesized capture groups. Use $& to insert the entire matched string.

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