I don't get it? In a DFA there's no way to store "the value of a capture group" because each state may have multiple values for any capture group, depending on

I don't get it? In a DFA there's no way to store "the value of a capture group" because each state may have multiple values for any capture group, depending on how it was reached. WAIT, I think what you've built is NOT a DFA, but an NDFA! Your code has multiple states active at once, and it just tests them all! Ok, now it all makes sense. In a DFA, it's common for a state to have multiple values for capture groups. For example in /(a*)a*/, state 1 is "in the first loop, or in the second loop with \1={0,0}" and state 2 is "in the first loop, in the second loop with \1={0,0}, or in the second loop with \1={0,1}". So the second state has multiple simultaneous values for group 1; this is why tagged-DFAs usually need more "tags" than there are capture groups and that's why I was asking.