Return a new error instead of rethrowing a non-existent error

regex/compile.go

@@ -627,7 +627,7 @@ func shuntingYard(re string, flags ...ReFlag) ([]postfixNode, error) {
 								case "word": // Word characters
 									nodeToAdd, _ = newEscapedNode('w', true) // This isn't going to error, so I suppress it
 								default:
-									return nil, err
+									return nil, fmt.Errorf("invalid POSIX character class")
 								}
 								chars = append(chars, nodeToAdd)
 								i = temp_i + len(posixClass) + 2 // Skip over the class name, the closing colon and the closing bracket