How to convert regex "x{m, n}" to NFA?

Question

The regex x{m, n} matches from m to n repetitions of the preceding x , attempting to match as many as possible.

I have a naive solution, but the number of nodes and edges depends on m and n , which is unacceptable when n is big.

So, is there any effective way to convert the regex to NFA ?

Answer 1

Unfortunately, NFAs don't "count" very well. You're essentially going to have to manually expand your regex to something Thompsons' construction can handle. eg

m{2,5} -> mm(m(m(m)?)?)?

Search for the function SimplifyRepeat here to see Google's implementation. See this page for more information on regex implementation in practice.