Started working on converting to PCRE matching rules (prefer left branch of alternation)

regex/matching.go

@@ -2,6 +2,7 @@ package regex
 
 import (
 	"fmt"
+	"slices"
 	"sort"
 )
 
@@ -320,7 +321,7 @@ func findAllSubmatchHelper(start *nfaState, str []rune, offset int, numGroups in
 			}
 		}
 
-		currentStates, _ = uniqueAppend(currentStates, tempStates...)
+		currentStates = slices.Concat(currentStates, tempStates)
 		tempStates = nil
 
 		// Take any transitions corresponding to current character
@@ -329,11 +330,15 @@ func findAllSubmatchHelper(start *nfaState, str []rune, offset int, numGroups in
 		lastStateInList := false         // Whether or not a last state was in our list of states
 		var lastStatePtr *nfaState = nil // Pointer to the last-state, if it was found
 		lastLookaroundInList := false    // Whether or not a last state (that is a lookaround) was in our list of states
-		for _, state := range currentStates {
+		for numStatesMatched == 0 && lastStateInList == false {
+			if len(currentStates) == 0 {
+				break
+			}
+			state, _ := pop(&currentStates)
 			matches, numMatches := state.matchesFor(str, i)
 			if numMatches > 0 {
 				numStatesMatched++
-				tempStates = append(tempStates, matches...)
+				tempStates = append([]*nfaState(nil), matches...)
 				foundPath = true
 				for _, m := range matches {
 					if m.threadGroups == nil {
@@ -383,12 +388,17 @@ func findAllSubmatchHelper(start *nfaState, str []rune, offset int, numGroups in
 				lastStateInList = true
 			}
 		}
-		if lastStateInList { // A last-state was in the list of states. add the matchIndex to our MatchIndex list
+		if lastStateInList && numStatesMatched == 0 { // A last-state was in the list of states. add the matchIndex to our MatchIndex list
 			for j := 1; j < numGroups+1; j++ {
 				tempIndices[j] = lastStatePtr.threadGroups[j]
 			}
 			endIdx = i
 			tempIndices[0] = Group{startIdx, endIdx}
+			if tempIndices[0].StartIdx == tempIndices[0].EndIdx {
+				return true, tempIndices, tempIndices[0].EndIdx + 1
+			} else {
+				return true, tempIndices, tempIndices[0].EndIdx
+			}
 		}
 
 		// Check if we can find a zero-length match