Developer Tools
Test and Debug a Regex Pattern
Regex Tester for testing and working with regular expressions.
Direct Answer
Use the Regex Tester when a regular expression is not matching what you expect, matches too much, or needs to be checked against real sample text before using it in code.
Common reasons a pattern misbehaves
Most regex bugs come from a small set of recurring causes that show up clearly once you test against real sample text.
- greedy quantifiers consuming more than intended
- missing anchors letting a pattern match a substring instead of the whole input
- capturing groups repeated inside a quantifier, overwriting earlier matches
- a regex flavor mismatch between where the pattern was written and where it's used
- the global flag's lastIndex state causing inconsistent results across repeated calls
What to check before shipping a pattern
A pattern that matches your first example isn't necessarily safe or correct for every input it'll see in production.
- test against edge cases: empty input, very long input, unexpected characters
- check performance on a long non-matching string to rule out catastrophic backtracking
- confirm which regex flavor (JavaScript, PCRE, Python, etc.) the target environment actually uses
- verify capturing group numbers or names match what your code expects
How to Use Regex Tester
- Paste or type your content into the input box.
- The transformed result appears instantly in the result panel below.
- Click Copy result to copy the output to your clipboard.
- Download .txt saves the transformed text as a file.
- Reset clears everything and lets you start over.
Reference
| Feature | Details |
|---|---|
| Purpose | Transform pasted text into a cleaner or different format. |
| Input | Text, lines, lists, or copied content. |
| Result | Transformed text ready to copy or download. |
| Best for | Cleanup, formatting, list preparation, and copy-paste workflows. |
| Category | Developer Tools |
| Works on | Desktop, tablet, and mobile browsers |
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Frequently Asked Questions
Why does my regex match too much text?
The pattern likely uses a greedy quantifier, a broad character class, or is missing anchors, so it keeps consuming characters as far as it can before backtracking just enough to satisfy the rest of the pattern. Testing against real sample text and tightening the pattern (or switching to a lazy quantifier) usually narrows the match to what you intended.
What is catastrophic backtracking?
It happens when nested or overlapping quantifiers - like (a+)+ - are tested against a long string that almost, but doesn't quite, match. The engine tries an exponential number of ways to split the string between the nested repetitions before failing, which can freeze a browser tab or hang a server process. Restructuring the pattern to remove the nested repetition is usually the fix.
What's the difference between greedy and lazy quantifiers?
By default, * and + are greedy and match as much text as possible before backtracking to satisfy the rest of the pattern. Adding a ? after them (like *? or +?) makes them lazy, matching as little as possible instead - this matters a lot when capturing text between delimiters, where .* vs .*? can grab completely different substrings.
What's the difference between ^ / $ and \b?
^ and $ anchor to the start and end of the string (or of each line, if the multiline flag is set). \b matches a zero-width word boundary - the transition between a word character and a non-word character - which is useful for matching a whole word regardless of where it sits within a line.
What does a non-capturing group (?:...) do?
It groups part of a pattern for quantifying or alternation purposes without creating a numbered capture group for it. This keeps your list of captured groups clean when you need the grouping behavior but don't actually need that submatch's value later.
Why does a regex that worked in Python fail in JavaScript?
Regex flavors aren't fully interchangeable. Named groups use different syntax ((?P<name>...) in Python vs (?<name>...) in JavaScript), variable-length lookbehind support differs between engines, and Unicode handling and certain flags behave differently - so a pattern copied between languages isn't guaranteed to work unchanged.
Why does test() give a different result each time with the same input?
If the regex has the global (g) flag, JavaScript's RegExp object keeps state in its lastIndex property, so each call to test() or exec() resumes searching from where the previous call left off. This produces alternating true/false results on repeated calls unless lastIndex is reset or a fresh regex instance is used.
What's the difference between a capturing group and a lookahead?
A capturing group like (foo) consumes and records its matched text as part of the overall match. A lookahead like (?=foo) or a negative lookahead (?!foo) only checks a condition ahead of the current position without consuming any characters, so it never appears in the matched text itself.