Regex Tester
See your regular expression's matches highlighted as you type.
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Build and debug regular expressions with instant feedback. Enter a pattern and flags, paste your test text, and see every match highlighted live along with a match count. It is ideal for validation rules, search patterns and learning regex. Everything runs in your browser.
Why testing beats guessing
Regular expressions are famously easy to write and hard to read, including your own from last month. The reliable way to build one is incrementally: write a small piece, see what it matches against real sample text, then extend it. Testing against actual data also surfaces the cases you did not think of, which is where regular expressions usually break in production.
The parts that cause most trouble
- Greedy quantifiers. By default .* takes as much as it can, which is rarely what you meant. Add a question mark to make it lazy.
- Escaping. Dots, question marks, plus signs and brackets all mean something, so a literal one needs a backslash.
- Anchors. Without ^ and $ you match anywhere in the string, not the whole of it.
- Flags. Global, case insensitive and multiline change the behaviour completely.
Know when not to use one
Regular expressions are the wrong tool for parsing HTML, and the reasons are structural rather than stylistic: nested tags are not something a regular language can describe. Validating email addresses fully is also a famous trap, since the correct expression is enormous and still wrong. For those jobs use a real parser, and keep regular expressions for pattern matching in flat text, which is what they are excellent at.
How to use Regex Tester
- 1
Enter your regex pattern and flags.
- 2
Paste the text to test against.
- 3
See matches highlighted instantly.
Frequently asked questions
It uses JavaScript's built-in regular-expression engine.