Store the current string pointer as a 'thread variable' (allows us to simulate backtracking)

regex/nfa.go

@@ -45,6 +45,7 @@ type nfaState struct {
 	// The following properties depend on the current match - I should think about resetting them for every match.
 	zeroMatchFound bool    // Whether or not the state has been used for a zero-length match - only relevant for zero states
 	threadGroups   []Group // Assuming that a state is part of a 'thread' in the matching process, this array stores the indices of capturing groups in the current thread. As matches are found for this state, its groups will be copied over.
+	threadSP       int     // The string pointer of the thread - where it is in the input string
 }
 
 // Clones the NFA starting from the given state.
@@ -120,6 +121,7 @@ func resetThreadsHelper(state *nfaState, visitedMap map[*nfaState]bool) {
 	}
 	// Assuming it hasn't been visited
 	state.threadGroups = nil
+	state.threadSP = 0
 	visitedMap[state] = true
 	for _, v := range state.transitions {
 		for _, nextState := range v {