如何检查正则表达式是否永远匹配？

Question

I need a way to check if a regex can match 0 characters, and therefore matches infinitely. For example, the regex .* can match 0 characters which means that it will loop for ever, crashing my app.

In my app, the user inputs a regex, which can of course loop forever. How can I prevent this happening?

Answer 1

There is no such thing as a regex matching infinitely; thus being stuck in an infinite loop. As an example, the regex .* will stop immediately when there are 0 characters and will run for n iterations on any string of n characters.

A regex is a form of a finite state machine. A string, its input, is also always finite. There is no infinite in regular expressions.

Here is a the same question asked in a CS context: https://cs.stackexchange.com/questions/47835/can-a-regular-expression-be-infinite