Regex Tester
Test regular expressions with live matching, highlighting, and captured groups
What is Regex Tester?
Regex Tester is a free online tool for testing regular expressions with live matching, syntax highlighting, and captured group display. Regular expressions are powerful pattern-matching tools used in programming, text processing, data validation, and search operations. This tool lets you write a regex pattern, set flags, and test it against sample text in real time. Matches are highlighted directly in the test string, and captured groups are displayed separately for easy inspection. It supports all JavaScript regex features including lookahead, lookbehind, named groups, and Unicode properties. The tool is indispensable for developers, data analysts, and anyone who works with pattern matching.
How to Use
- Enter your regex pattern.
- Select the flags you need (g, i, m).
- Enter your test string and matches will be highlighted in real time.
Tips & Best Practices
- Use the 'g' flag for global matching to find all occurrences, not just the first one.
- Test your regex with edge cases including empty strings, special characters, and very long inputs.
- Use named capture groups (?<name>...) for more readable and maintainable patterns.
- Start with a simple pattern and gradually add complexity to debug issues more easily.
- Use the 'm' flag when working with multiline text where ^ and $ should match line boundaries.
Use Cases
Form Validation
Test email, phone number, and URL validation patterns before implementing in code.
Data Extraction
Build patterns to extract specific data like dates, prices, or IDs from text.
Log Analysis
Create patterns to filter and extract relevant information from log files.
Search and Replace
Test find-and-replace patterns before applying them to large codebases.
FAQ
What are regex flags?
Flags modify regex behavior: 'g' for global (find all matches), 'i' for case-insensitive, and 'm' for multiline (^ and $ match line boundaries).
Why is my regex not matching?
Common issues include forgetting to escape special characters (like . or *), not enabling the global flag, or incorrect character class usage.
What is a regular expression (Regex)?
A regular expression is a formal language for finding or replacing specific patterns in strings, supported by virtually all programming languages.
Is my data sent to any server?
No, regex testing runs locally in your browser's JavaScript engine and no data is transmitted externally.
What do the g, i, and m flags mean?
g (global) finds all matches, i (case-insensitive) ignores case, and m (multiline) makes ^ and $ match the start and end of each line.
When can regex performance become slow?
Nested quantifiers (e.g., (a+)+) can cause catastrophic backtracking, so it is best to keep patterns simple for long input strings.
Is my data collected?
No, all matching happens in your browser. No patterns or test strings are sent to any server.
What regex flavor does this tool use?
This tool uses JavaScript's built-in RegExp engine, which supports most modern regex features.
What are capturing groups?
Capturing groups (parentheses in a pattern) capture the matched text for extraction. Named groups use (?<name>...) syntax.
Why is my pattern matching too much?
Regex quantifiers are greedy by default. Add ? after *, +, or {} to make them lazy (matching as little as possible).
How do I match a literal dot or bracket?
Escape special characters with a backslash: \\. for a literal dot, \\[ for a literal bracket.
Can I test multiline patterns?
Yes, use the 'm' flag for multiline mode where ^ and $ match the start and end of each line, not just the entire string.