Regex Tester

Testing a pattern step by step

Type your expression into the pattern box, set the flags, then paste or type sample text. Matches are highlighted immediately, with the count and a detailed list underneath. Start with the one-click examples if you want to see how a good pattern looks.

Reading the results

• Highlighted text shows every match, alternating two shades so neighbouring matches stay distinct. Empty matches appear as a thin marker.
• Match details list each match with its position, line and column, every numbered group and every named group. A group that did not take part shows as undefined.
• Replace preview applies your replacement to the whole text so you can check a search-and-replace before running it in code.

Flags worth knowing

1. g finds all matches, not just the first.
2. i ignores letter case.
3. m makes ^ and $ match at each line.
4. s lets the dot match newlines.
5. u turns on full Unicode handling, needed for \p{L} and emoji.
6. y is sticky: each match must start exactly where the last one ended.

Writing patterns that behave

• Anchor patterns with ^ and $ when validating a whole value, such as a form field.
• Prefer specific classes ([0-9], \d) over a greedy .*, which swallows more than you expect.
• Use lazy quantifiers (.*?) to stop at the first closing delimiter.
• Use non-capturing groups (?:...) when you only need grouping, to keep group numbers tidy.
• Escape the special characters . * + ? ( ) [ ] { } | \ ^ $ when you mean them literally.

Avoiding slow patterns

Nested repetition such as (a+)+ can trigger catastrophic backtracking, where the engine tries an exponential number of paths. Keep alternations distinct, avoid repeating a group that can itself match the same text, and test with long, almost-matching input. Never trust a regular expression alone to validate complex formats such as full email addresses; combine it with a sensible length limit and server-side checks.