Question

Is it possible to create a regex that matches all strings with five a’s and five b’s?

Like aaaaabbbbb or ababababab or aabbaabbab.

I imagine it would require polynomial time for a deterministic engine.

Are there other matching languages which would enable such matching?

Update:

I wanted to use the kind of expression for searching, so I changed the one purposed to (?=b*ab*){5}(?=a*ba*){5}([ab]{10}) and it works nicely! 🙂
I’m still uncertain with respects to the performance of an expression like that. But I guess I can just look up lookahead expressions.

I’m still curious to which other kinds of patterns, that are simple to explain but hard to regex, are out there?

Answer 1

You can use lookahead assertions:

^(?=(?:[^a]*a){5}[^a]*$)(?=(?:[^b]*b){5}[^b]*$)