Regex Tester

What is a Regex Tester?

A regex tester lets you write, test, and debug regular expressions against sample text with instant visual feedback. Regular expressions (regex) are pattern-matching sequences used across virtually every programming language for text searching, validation, extraction, and replacement. They're powerful but notoriously tricky to get right — this tool shows matches in real-time as you type, helping you iterate quickly until your pattern works correctly. All processing happens locally in your browser using JavaScript's native RegExp engine.

How to Use This Tool

1. Enter your regex pattern between the delimiters. Set flags (g for global, m for multiline, i for case-insensitive) in the flags field.
2. Paste your test string in the text area below. Matches highlight immediately as you type in either field.
3. Review matches in the results panel — see the count, matched text, and verify your pattern catches exactly what you intend.

Common Use Cases for Developers

• Input validation: Test email, phone number, URL, and date patterns before implementing them in form validation logic.
• Log parsing: Build patterns to extract timestamps, error codes, IP addresses, or user IDs from application log files.
• Find and replace: Develop complex search patterns for IDE refactoring operations or sed/awk scripts.
• Data extraction: Write capture groups to pull structured data from unstructured text (scraping, CSV parsing, template matching).
• Route matching: Test URL path patterns for Express.js, Next.js, or API gateway route definitions.

Tips & FAQ

• Escape special characters: Characters like . * + ? ^ $ [ ] ( ) | \ have special meaning in regex. To match them literally, escape with a backslash: \. matches a period, \$ matches a dollar sign.
• Use the global flag (g): Without the g flag, regex stops after the first match. Add g to find all occurrences. Add m for multiline so ^ and $ match line boundaries, not just string start/end.
• Performance warning: Avoid catastrophic backtracking with nested quantifiers like (a+)+. If your regex takes too long, simplify or use atomic groups. Test with realistic input lengths to catch performance issues early.