Loosened restrictions for concatenation - It's okay if one of the

elements is missing

compile.go

@@ -828,9 +828,16 @@ func thompson(re []postfixNode) (Reg, error) {
 		switch c.nodetype {
 		case CONCATENATE:
 			s2 := mustPop(&nfa)
-			s1 := mustPop(&nfa)
-			s1 = concatenate(s1, s2)
-			nfa = append(nfa, s1)
+			// Relax the requirements for concatenation a little bit - If
+			// the second element is not found ie. the postfixNodes look
+			// like 'a~', then that's fine, we just skip the concatenation.
+			s1, err := pop(&nfa)
+			if err != nil {
+				nfa = append(nfa, s2)
+			} else {
+				s1 = concatenate(s1, s2)
+				nfa = append(nfa, s1)
+			}
 		case KLEENE: // Create a 0-state, concat the popped state after it, concat the 0-state after the popped state
 			s1 := mustPop(&nfa)
 			stateToAdd := kleene(*s1)