Made lookarounds a little more efficient by only matching from (or to, in the case of lookbehind) the current index

nfa.go

@@ -118,15 +118,21 @@ func (s State) checkAssertion(str []rune, idx int) bool {
 	if s.isLookaround() {
 		// The process here is simple:
 		// 		1. Compile the regex stored in the state's contents.
-		// 		2. Run it on the test string.
+		// 		2. Run it on a subset of the test string, that ends after the current index in the string
 		// 		3. Based on the kind of lookaround (and the indices we get), determine what action to take.
 		startState := s.lookaroundNFA
-		matchIndices := findAllMatches(startState, str, startState.lookaroundNumCaptureGroups)
+		var strToMatch []rune
+		if s.assert == PLA || s.assert == NLA {
+			strToMatch = str[idx:]
+		} else {
+			strToMatch = str[:idx]
+		}
+		matchIndices := findAllMatches(startState, strToMatch, startState.lookaroundNumCaptureGroups)
 
 		numMatchesFound := 0
 		for _, matchIdx := range matchIndices {
-			if s.assert == PLA || s.assert == NLA { // Lookahead - return true (or false) if at least one match starts at the current index
+			if s.assert == PLA || s.assert == NLA { // Lookahead - return true (or false) if at least one match starts at 0. Zero is used because the test-string _starts_ from idx.
-				if matchIdx[0].startIdx == idx {
+				if matchIdx[0].startIdx == 0 {
 					numMatchesFound++
 				}
 			}