Added comments

regex/compile.go

@@ -966,9 +966,9 @@ func thompson(re []postfixNode) (Reg, error) {
 					s.groupNum = lparenNode.groupNum
 					to_add := concatenate(lparenNode, s)
 					nfa = append(nfa, to_add)
-				} else if middleNode.groupBegin && len(middleNode.transitions) == 0 { // The middle node is a lone lparen - something like '(())', and I'm looking at the first closing parentheses
-					nfa = append(nfa, lparenNode)
-					s.groupNum = middleNode.groupNum // In this case, the 'middle' node is actually a paren node
+				} else if middleNode.groupBegin && len(middleNode.transitions) == 0 { // The middle node is a lone lparen - something like '(())', and I'm looking at the first rparen
+					nfa = append(nfa, lparenNode)    // I shouldn't have popped this out, because it is not involved in the current capturing group
+					s.groupNum = middleNode.groupNum // In this case, the 'middle' node is actually an lparen
 					to_add := concatenate(middleNode, s)
 					nfa = append(nfa, to_add)
 				} else {