Separation of concerns

src/Filter/FilterParser.php

@@ -0,0 +1,15 @@
+<?php
+
+namespace Filter;
+
+class FilterParser
+{
+    public function parse(string $string): DocumentFilter
+    {
+        if (preg_match('/^.+=.*$/', $string)) {
+            [$prop, $term] = explode('=', $string, 2);
+            return new SpecificFilter(trim($prop), trim($term));
+        }
+        return new GenericFilter($string);
+    }
+}