Allow pipes that have a missing operand - if an operand is missing, it is replaced with a zeroLengthMatchState(), which always has a zero-length match

compile.go

@@ -666,10 +666,7 @@ func thompson(re []postfixNode) (Reg, error) {
 	// nothing), the output of shuntingYard() (and the input to thompson()) ends up being empty.
 	// In these cases, we will return an NFA with 1 state, with an assertion that is always true.
 	if len(re) == 0 {
-		start := newState()
-		start.content = newContents(EPSILON)
-		start.isEmpty = true
-		start.assert = ALWAYS_TRUE
+		start := zeroLengthMatchState()
 		nfa = append(nfa, &start)
 	}
 
@@ -852,8 +849,30 @@ func thompson(re []postfixNode) (Reg, error) {
 			s2 := question(s1)
 			nfa = append(nfa, s2)
 		case PIPE:
-			s1 := mustPop(&nfa)
-			s2 := mustPop(&nfa)
+			// A pipe operator doesn't actually need either operand to be present. If an operand isn't present,
+			// it is replaced with an implicit 'matchZeroLength' state (this is the same thing that we add at the top if our
+			// input has zero postfixNodes).
+			// Things to think about:
+			// 	'a|'
+			// 	'|a'
+			// 	'^a|'
+			// 	'^|a'
+			s1, err1 := pop(&nfa)
+			s2, err2 := pop(&nfa)
+			if err2 != nil || (s2.groupBegin && len(s2.transitions) == 0) { // Doesn't exist, or its just an LPAREN
+				if err2 == nil { // Roundabout way of saying that this node existed, but it was an LPAREN, so we append it back
+					nfa = append(nfa, s2)
+				}
+				tmp := zeroLengthMatchState()
+				s2 = &tmp
+			}
+			if err1 != nil || (s1.groupBegin && len(s1.transitions) == 0) { // Doesn't exist, or its just an LPAREN
+				if err1 == nil { // See above for explanation
+					nfa = append(nfa, s1)
+				}
+				tmp := zeroLengthMatchState()
+				s1 = &tmp
+			}
 			s3 := alternate(s1, s2)
 			nfa = append(nfa, s3)
 		}