From 08e01a1c817901f72481d966df84e6695c993d4f Mon Sep 17 00:00:00 2001
From: Aadhavan Srinivasan <aadhavan@twomorecents.org>
Date: Sat, 25 Jan 2025 13:09:41 -0500
Subject: [PATCH] Loosened restrictions for concatenation - It's okay if one of
 the elements is missing

---
 compile.go | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/compile.go b/compile.go
index 252d3b7..759a482 100644
--- a/compile.go
+++ b/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)