Offhand, I think it could be useful for ensuring your regex is doing what you want it to do. Perhaps I'd write a regex to validate a xxx-xxx-xxxx telephone num

Offhand, I think it could be useful for ensuring your regex is doing what you want it to do. Perhaps I'd write a regex to validate a xxx-xxx-xxxx telephone number: \d{3}\-\d{3}\-\d{3} Oops! I made a typo! That last 3 should be a 4. If I were tired, I could very easily see myself doing something like this. If I could put it into a tool and see 100 things which match it, I'd see very quickly that I messed something up.