Throw error if non-greedy operator is attempted

compile.go

@@ -480,6 +480,9 @@ func shuntingYard(re string, flags ...ReFlag) ([]postfixNode, error) {
 					if (c == '*' && outQueueFinalElement.nodetype == KLEENE) || (c == '+' && outQueueFinalElement.nodetype == PLUS) { // You cannot apply a quantifier to a quantifier in this way
 						return nil, fmt.Errorf("illegal use of token '%c'", c)
 					}
+					if c == '?' && slices.Contains([]NodeType{KLEENE, PLUS, QUESTION}, outQueueFinalElement.nodetype) {
+						return nil, fmt.Errorf("non-greedy operators not supported")
+					}
 					opStack = append(opStack, c)
 				}
 			}
@@ -1001,6 +1004,9 @@ func thompson(re []postfixNode) (Reg, error) {
 			if err != nil {
 				return Reg{}, fmt.Errorf("error applying kleene star")
 			}
+			if s1.isEmpty && s1.assert != NONE {
+				return Reg{}, fmt.Errorf("previous token is not quantifiable")
+			}
 			stateToAdd := kleene(*s1)
 			nfa = append(nfa, stateToAdd)
 		case PLUS: // a+ is equivalent to aa*