Regex Tester — Confirm the Pattern Matches What You Think
This runs a regular expression against sample text and shows what matched, where, and what each capture group holds. Regular expressions are unusually easy to write incorrectly in a way that still works on the example you had in mind, which is why testing against several inputs rather than one is the actual discipline.
When it helps
The failures are characteristic. Greedy quantifiers matching far more than intended, because the pattern was tested on a string containing one occurrence and deployed against strings containing several. An unescaped dot matching any character rather than a literal full stop. Anchors missing, so a pattern intended to validate a whole value happily matches a fragment of something much longer. Each looks correct in review and fails on real data.
Worth running automatically
There is a security dimension that deserves attention: a pattern with nested quantifiers can take exponential time on a crafted input, which turns a validation rule into a denial-of-service vector. Testing with deliberately awkward input is how you find that before someone else does. Where a regex validates user input in production, a scheduled check that submits pathological values and measures the response time is a genuine safeguard rather than a theoretical one.
What you get out of it
Confidence that a pattern matches what you intend across realistic input rather than the one example you had in mind — plus a way to find the catastrophic-backtracking case before it is exploited.