Allowed creation of empty non-capturing groups

compile.go

@@ -658,6 +658,21 @@ func shuntingYard(re string, flags ...ReFlag) ([]postfixNode, error) {
 func thompson(re []postfixNode) (Reg, error) {
 	nfa := make([]*State, 0) // Stack of states
 	numGroups := 0           // Number of capturing groups
+
+	// If thompson() receives an empty regex, then whatever was given to shuntingYard()
+	// was parsed away. This doesn't mean that the regex itself is empty.
+	// For example, it could have been '(?:)'. This is an empty non-capturing group. Since
+	// shuntingYard() doesn't include non-capturing groups in its output (and the group contains
+	// 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
+		nfa = append(nfa, &start)
+	}
+
 	for _, c := range re {
 		if c.nodetype == CHARACTER || c.nodetype == ASSERTION {
 			state := State{}