Regex Tester

Writing a regular expression is mostly a loop: try a pattern, look at what it matched, adjust. Doing that against real text rather than in your head is the entire point of a tester, and seeing the capture groups separately is usually what tells you which part is wrong.

Every match is listed with its position and the contents of each numbered and named group, including the groups that did not participate — which are shown as such rather than silently omitted. That distinction matters when you are debugging an alternation, because a group being empty and a group not matching at all mean different things.

The replacement box uses the same references as JavaScript's own replace: dollar-one for a numbered group, dollar-angle-brackets for a named one, and dollar-ampersand for the whole match. What you see here is therefore exactly what your code will produce, rather than an approximation.

The unusual part is the safety net. A pattern with one repetition nested inside another, such as an inner plus wrapped in an outer plus, can take exponential time on certain inputs — the tab freezes, with no error and no way to stop it. This tester runs the match in a background thread and stops it after two seconds, then explains what happened. Patterns with that shape are flagged before you even run them.

Everything happens on your device, so the text you are testing against never leaves it.