Regex Tester

Frequently Asked Questions

Why does my pattern only find one match?

You're probably missing the g (global) flag. Without it, the engine stops after the first match. Add g to the flags field to find all matches.

What flags are supported?

Supported flags: g (global), i (case-insensitive), m (multiline), s (dotAll), u (Unicode), y (sticky), d (generate indices). Combine multiple flags like gi.

What is a capture group?

Parentheses in a regex create a capture group. The text matched by each group is extracted and reported separately, letting you pull out specific parts like the domain from an email address.

Is my text sent anywhere?

No. All matching runs entirely in your browser using the JavaScript RegExp engine. Your pattern and test text never leave your device.

What is catastrophic backtracking?

Certain patterns cause the engine to explore an exponentially large number of match paths. A pattern like (a+)+ on a string like 'aaaab' can freeze. Test performance-sensitive patterns carefully.

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How It Works

A regular expression (regex) is a pattern language for searching, matching, and extracting text. This tester takes a pattern, optional flags, and a block of sample text, then runs the pattern using the browser's native JavaScript RegExp engine and lists every match along with its starting position (zero-based index) and the contents of any capture groups.

With the g (global) flag, matchAll() finds every non-overlapping match. Without it, only the first match is returned. Syntax errors in the pattern produce a clear error message instead of silently failing. All matching runs entirely in your browser - the text is never sent to a server.

Use Cases

A quick regex tester eliminates the context-switch to a terminal or IDE during development:

Validating an email, URL, or phone number regex against representative edge cases before shipping

Testing log parsing patterns against a sample line before adding them to a log aggregation pipeline

Debugging an extraction regex that isn't matching what you expect, by seeing exactly which substrings match and at what position

Exploring how capture groups, non-capturing groups, and lookaheads change what gets matched

Quickly checking that a pattern handles Unicode, multiline, and dotAll edge cases with the appropriate flags

Tips

Enter the pattern body only, without surrounding slashes. The flags go in the separate flags field. A pattern like /\d+/g should be entered as pattern \d+ with flag g.

Escape special characters when you want to match them literally: \. matches a literal dot, while an unescaped . matches any character including a dot.

Use named capture groups ((?<name>...)) for self-documenting patterns; the group name makes the intent clear both in the regex and in code that uses the match result.

The s (dotAll) flag makes . match newline characters, which it doesn't by default. The m (multiline) flag makes ^ and $ match at the start and end of each line rather than the whole string.

Catastrophic backtracking can freeze a browser tab for patterns with nested quantifiers on overlapping character classes. Test complex patterns on short strings first to check performance.

FAQ

Why does my pattern only find one match?

You're probably missing the g (global) flag. Without it, the engine stops after the first match. Add g to the flags field to find all matches.

What flags are supported?

This tester passes flags directly to JavaScript's RegExp constructor. Supported flags are: g (global), i (case-insensitive), m (multiline), s (dotAll), u (Unicode), y (sticky), and d (generate indices). Combine multiple flags like gi.

What is a capture group?

Parentheses in a regex create a capture group. The text matched by each group is extracted and reported separately from the full match. Capture groups let you pull out specific parts - for example, the domain from an email address - without splitting or additional processing.

Is my text sent anywhere?

No. All matching runs entirely in your browser using the JavaScript RegExp engine. Your pattern and test text never leave your device.

What is catastrophic backtracking?

Certain regex patterns cause the engine to explore an exponentially large number of possible match paths when the input doesn't match. A pattern like (a+)+ on a string like 'aaaaaaab' can cause this. Prefer possessive quantifiers or atomic groups (not supported in JS) and test performance-sensitive patterns carefully.

Regex Tester

Frequently Asked Questions

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