I should have been more clear perhaps: many regexes are capable of being verified with 100% certainty: https://en.m.wikipedia.org/wiki/Thompson%27s_construction

I should have been more clear perhaps: many regexes are capable of being verified with 100% certainty: https://en.m.wikipedia.org/wiki/Thompson%27s_construction https://en.m.wikipedia.org/wiki/Thompson%27s_construction But not all regexes (eg, those using PCRE extensions afaik) are amenable to such a treatment. Those you just tend to hope they work.