Add function for validating ZIP codes (somewhat)

src/MD_STD_IN.php

@@ -392,6 +392,27 @@ public static function validate_isbn(string $input):string {
 
     }
 
+    /**
+     * Validates a ZIP code.
+     *
+     * @param string $input Input string.
+     *
+     * @return string
+     */
+    public static function validate_zip_code(string $input):string {
+
+        if (($input = trim($input)) === "") {
+            return "";
+        }
+
+        if (\mb_strlen($input) > 7) {
+            throw new MDgenericInvalidInputsException("ZIP code is too long");
+        }
+
+        return $input;
+
+    }
+
     /**
      * Returns an UTF8 version of a string.
      *