Added question mark operator - 0 or 1

main.go

@@ -11,7 +11,7 @@ import (
 const CONCAT rune = '~'
 
 func isOperator(c rune) bool {
-	if c == '+' || c == '*' || c == '|' || c == CONCAT {
+	if c == '+' || c == '?' || c == '*' || c == '|' || c == CONCAT {
 		return true
 	}
 	return false
@@ -19,7 +19,7 @@ func isOperator(c rune) bool {
 
 /* priority returns the priority of the given operator */
 func priority(op rune) int {
-	precedence := []rune{'|', CONCAT, '+', '*'}
+	precedence := []rune{'|', CONCAT, '+', '*', '?'}
 	return slices.Index(precedence, op)
 }
 
@@ -43,7 +43,7 @@ func shuntingYard(re string) string {
 		re_postfix = append(re_postfix, re_runes[i])
 		if re_runes[i] != '(' && re_runes[i] != '|' {
 			if i < len(re_runes)-1 {
-				if re_runes[i+1] != '|' && re_runes[i+1] != '*' && re_runes[i+1] != '+' && re_runes[i+1] != ')' {
+				if re_runes[i+1] != '|' && re_runes[i+1] != '*' && re_runes[i+1] != '+' && re_runes[i+1] != '?' && re_runes[i+1] != ')' {
 					re_postfix = append(re_postfix, CONCAT)
 				}
 			}
@@ -153,6 +153,15 @@ func thompson(re string) *State {
 			s2 := kleene(*s1)
 			s1 = concatenate(s1, s2)
 			nfa = append(nfa, s1)
+		case '?': // ab? is equivalent to a(b|)
+			s1 := mustPop(&nfa)
+			s2 := &State{}
+			s2.transitions = make(map[int][]*State)
+			s2.content = EPSILON
+			s2.output = make([]*State, 0)
+			s2.isEmpty = true
+			s3 := alternate(s1, s2)
+			nfa = append(nfa, s3)
 		case '|':
 			s1 := mustPop(&nfa)
 			s2 := mustPop(&nfa)