Allow hyphen to be escaped inside character class

postfixNode.go

@@ -116,6 +116,13 @@ func newEscapedNode(c rune, inCharClass bool) (postfixNode, error) {
 	case 'v': // Vertical tab
 		toReturn.nodetype = CHARACTER
 		toReturn.contents = append(toReturn.contents, rune(11))
+	case '-': // Literal hyphen - only in character class
+		if inCharClass {
+			toReturn.nodetype = CHARACTER
+			toReturn.contents = append(toReturn.contents, '-')
+		} else {
+			return postfixNode{}, fmt.Errorf("Invalid escape character.")
+		}
 	default: // None of the above - append it as a regular character
 		if isNormalChar(c) { // Normal characters cannot be escaped
 			return postfixNode{}, fmt.Errorf("Invalid escape character.")