Add a method to ds:Reference to determine if it's an XPointer

Signed-off-by: Jaime Pérez <jaimepc@gmail.com>

Author: jaimeperez

src/XML/ds/Reference.php

@@ -171,6 +171,17 @@ private function setURI(?string $URI): void
     }
 
 
+    /**
+     * Determine whether this is an xpointer reference.
+     *
+     * @return bool
+     */
+    public function isXPointer(): bool
+    {
+        return !empty($this->URI) && preg_match('/^#xpointer\(.+\)$/', $this->URI);
+    }
+
+
     /**
      * Convert XML into a Reference element
      *