diff --git a/CloudObjects/SDK/COIDParser.php b/CloudObjects/SDK/COIDParser.php
index 0fb4aae..45de308 100644
--- a/CloudObjects/SDK/COIDParser.php
+++ b/CloudObjects/SDK/COIDParser.php
@@ -20,7 +20,7 @@ class COIDParser {
     const COID_VERSIONED = 3;
     const COID_VERSION_WILDCARD = 4;
 
-    const REGEX_HOSTNAME = "/^([a-z0-9-]+\.)?[a-z0-9-]+\.[a-z]+$/";
+    const REGEX_HOSTNAME = "/^([a-z0-9-]+\.)*[a-z0-9-]+\.[a-z]+$/";
     const REGEX_SEGMENT = "/^[A-Za-z-_0-9\.]+$/";
     const REGEX_VERSION_WILDCARD = "/^((\^|~)(\d+\.)?\d|(\d+\.){1,2}\*)$/";
 
diff --git a/tests/OfflineTests/COIDParserTest.php b/tests/OfflineTests/COIDParserTest.php
index b1ff203..e539121 100644
--- a/tests/OfflineTests/COIDParserTest.php
+++ b/tests/OfflineTests/COIDParserTest.php
@@ -10,121 +10,139 @@ use ML\IRI\IRI;
 
 class COIDParserTest extends \PHPUnit\Framework\TestCase {
 
-  public function testRootCOID() {
-    $coid = new IRI('coid://example.com');
-    $this->assertEquals(COIDParser::COID_ROOT, COIDParser::getType($coid));
-  }
+    public function testRootCOID() {
+        $coid = new IRI('coid://example.com');
+        $this->assertEquals(COIDParser::COID_ROOT, COIDParser::getType($coid));
 
-  public function testInvalidRootCOID() {
-    $coid = new IRI('coid://example');
-    $this->assertEquals(COIDParser::COID_INVALID, COIDParser::getType($coid));
-    $coid = new IRI('coid://exämple.com');
-    $this->assertEquals(COIDParser::COID_INVALID, COIDParser::getType($coid));
-    $coid = new IRI('coid://ex&mple.com');
-    $this->assertEquals(COIDParser::COID_INVALID, COIDParser::getType($coid));
-  }
+        $coid = new IRI('coid://subdomain.example.com');
+        $this->assertEquals(COIDParser::COID_ROOT, COIDParser::getType($coid));
 
-  public function testInvalidCOID() {
-    $coid = new IRI('http://example.com');
-    $this->assertEquals(COIDParser::COID_INVALID, COIDParser::getType($coid));
-    $coid = new IRI('example.com');
-    $this->assertEquals(COIDParser::COID_INVALID, COIDParser::getType($coid));
-    $coid = new IRI('COID://example.com');
-    $this->assertEquals(COIDParser::COID_INVALID, COIDParser::getType($coid));
-    $coid = new IRI('Coid://example.com');
-    $this->assertEquals(COIDParser::COID_INVALID, COIDParser::getType($coid));
-    $coid = new IRI('coid://EXAMPLE.COM');
-    $this->assertEquals(COIDParser::COID_INVALID, COIDParser::getType($coid));
-    $coid = new IRI('coid://exAMPle.CoM');
-    $this->assertEquals(COIDParser::COID_INVALID, COIDParser::getType($coid));
-  }
+        $coid = new IRI('coid://anotherlevel.subdomain.example.com');
+        $this->assertEquals(COIDParser::COID_ROOT, COIDParser::getType($coid));
+    }
 
-  public function testUnversionedCOID() {
-    $coid = new IRI('coid://example.com/Example');
-    $this->assertEquals(COIDParser::COID_UNVERSIONED, COIDParser::getType($coid));
-  }
+    public function testInvalidRootCOID() {
+        $coid = new IRI('coid://example');
+        $this->assertEquals(COIDParser::COID_INVALID, COIDParser::getType($coid));
 
-  public function testInvalidUnversionedCOID() {
-    $coid = new IRI('coid://example.com/Exümple');
-    $this->assertEquals(COIDParser::COID_INVALID, COIDParser::getType($coid));
-    $coid = new IRI('coid://example.com/Examp%e');
-    $this->assertEquals(COIDParser::COID_INVALID, COIDParser::getType($coid));
-  }
+        $coid = new IRI('coid://exämple.com');
+        $this->assertEquals(COIDParser::COID_INVALID, COIDParser::getType($coid));
 
-  public function testVersionedCOID() {
-    $coid = new IRI('coid://example.com/Example/1.0');
-    $this->assertEquals(COIDParser::COID_VERSIONED, COIDParser::getType($coid));
-    $coid = new IRI('coid://example.com/Example/alpha');
-    $this->assertEquals(COIDParser::COID_VERSIONED, COIDParser::getType($coid));
-  }
+        $coid = new IRI('coid://ex&mple.com');
+        $this->assertEquals(COIDParser::COID_INVALID, COIDParser::getType($coid));
+    }
 
-  public function testInvalidVersionedCOID() {
-    $coid = new IRI('coid://example.com/Example/1.$');
-    $this->assertEquals(COIDParser::COID_INVALID, COIDParser::getType($coid));
-  }
+    public function testInvalidCOID() {
+        $coid = new IRI('http://example.com');
+        $this->assertEquals(COIDParser::COID_INVALID, COIDParser::getType($coid));
 
-  public function testVersionWildcardCOID() {
-    $coid = new IRI('coid://example.com/Example/^1.0');
-    $this->assertEquals(COIDParser::COID_VERSION_WILDCARD, COIDParser::getType($coid));
-    $coid = new IRI('coid://example.com/Example/~1.0');
-    $this->assertEquals(COIDParser::COID_VERSION_WILDCARD, COIDParser::getType($coid));
-    $coid = new IRI('coid://example.com/Example/1.*');
-    $this->assertEquals(COIDParser::COID_VERSION_WILDCARD, COIDParser::getType($coid));
-  }
+        $coid = new IRI('example.com');
+        $this->assertEquals(COIDParser::COID_INVALID, COIDParser::getType($coid));
 
-  public function testInvalidVersionWildcardCOID() {
-    $coid = new IRI('coid://example.com/Example/^1.*');
-    $this->assertEquals(COIDParser::COID_INVALID, COIDParser::getType($coid));
-    $coid = new IRI('coid://example.com/Example/1.a.*');
-    $this->assertEquals(COIDParser::COID_INVALID, COIDParser::getType($coid));
-  }
+        $coid = new IRI('COID://example.com');
+        $this->assertEquals(COIDParser::COID_INVALID, COIDParser::getType($coid));
 
-  public function testIRICaseSensitivity() {
-    $coid1 = new IRI('coid://example.com/example/1.0');
-    $coid2 = new IRI('coid://example.com/Example/1.0');
-    $this->assertFalse($coid1->equals($coid2));
-  }
+        $coid = new IRI('Coid://example.com');
+        $this->assertEquals(COIDParser::COID_INVALID, COIDParser::getType($coid));
 
-  public function testRootFromString() {
-    $coid1 = new IRI('coid://example.com');
-    $coid2 = COIDParser::fromString('coid://example.com');
-    $coid3 = COIDParser::fromString('example.com');
-    $this->assertTrue($coid1->equals($coid2));
-    $this->assertTrue($coid1->equals($coid3));
-  }
+        $coid = new IRI('coid://EXAMPLE.COM');
+        $this->assertEquals(COIDParser::COID_INVALID, COIDParser::getType($coid));
+    
+        $coid = new IRI('coid://exAMPle.CoM');
+        $this->assertEquals(COIDParser::COID_INVALID, COIDParser::getType($coid));
+    }
 
-  public function testUnversionedFromString() {
-    $coid1 = new IRI('coid://example.com/Example');
-    $coid2 = COIDParser::fromString('coid://example.com/Example');
-    $coid3 = COIDParser::fromString('example.com/Example');
-    $this->assertTrue($coid1->equals($coid2));
-    $this->assertTrue($coid1->equals($coid3));
-  }
+    public function testUnversionedCOID() {
+        $coid = new IRI('coid://subdomain.example.com/Example');
+        $this->assertEquals(COIDParser::COID_UNVERSIONED, COIDParser::getType($coid));
+    }
 
-  public function testVersionedFromString() {
-    $coid1 = new IRI('coid://example.com/Example/1.0');
-    $coid2 = COIDParser::fromString('coid://example.com/Example/1.0');
-    $coid3 = COIDParser::fromString('example.com/Example/1.0');
-    $this->assertTrue($coid1->equals($coid2));
-    $this->assertTrue($coid1->equals($coid3));
-  }
+    public function testInvalidUnversionedCOID() {
+        $coid = new IRI('coid://example.com/Exümple');
+        $this->assertEquals(COIDParser::COID_INVALID, COIDParser::getType($coid));
 
-  public function testNormalizeRootFromString() {
-    $coid1 = new IRI('coid://example.com');
-    $coid2 = COIDParser::fromString('COID://example.com');
-    $coid3 = COIDParser::fromString('ExAmple.COM');
-    $this->assertTrue($coid1->equals($coid2));
-    $this->assertTrue($coid1->equals($coid3));
-  }
+        $coid = new IRI('coid://example.com/Examp%e');
+        $this->assertEquals(COIDParser::COID_INVALID, COIDParser::getType($coid));
+    }
 
-  public function testNormalizeNonRootFromString() {
-    $coid1 = new IRI('coid://example.com/Example');
-    $coid2 = COIDParser::fromString('COID://example.com/Example');
-    $coid3 = COIDParser::fromString('ExAmple.COM/Example');
-    $coid4 = COIDParser::fromString('ExAmple.COM/EXample');
-    $this->assertTrue($coid1->equals($coid2));
-    $this->assertTrue($coid1->equals($coid3));
-    $this->assertFalse($coid1->equals($coid4));
-  }
+    public function testVersionedCOID() {
+        $coid = new IRI('coid://anotherlevel.subdomain.example.com/Example/1.0');
+        $this->assertEquals(COIDParser::COID_VERSIONED, COIDParser::getType($coid));
+
+        $coid = new IRI('coid://subdomain.example.com/Example/alpha');
+        $this->assertEquals(COIDParser::COID_VERSIONED, COIDParser::getType($coid));
+    }
+
+    public function testInvalidVersionedCOID() {
+        $coid = new IRI('coid://example.com/Example/1.$');
+        $this->assertEquals(COIDParser::COID_INVALID, COIDParser::getType($coid));
+    }
+
+    public function testVersionWildcardCOID() {
+        $coid = new IRI('coid://example.com/Example/^1.0');
+        $this->assertEquals(COIDParser::COID_VERSION_WILDCARD, COIDParser::getType($coid));
+
+        $coid = new IRI('coid://example.com/Example/~1.0');
+        $this->assertEquals(COIDParser::COID_VERSION_WILDCARD, COIDParser::getType($coid));
+
+        $coid = new IRI('coid://example.com/Example/1.*');
+        $this->assertEquals(COIDParser::COID_VERSION_WILDCARD, COIDParser::getType($coid));
+    }
+
+    public function testInvalidVersionWildcardCOID() {
+        $coid = new IRI('coid://example.com/Example/^1.*');
+        $this->assertEquals(COIDParser::COID_INVALID, COIDParser::getType($coid));
+
+        $coid = new IRI('coid://example.com/Example/1.a.*');
+        $this->assertEquals(COIDParser::COID_INVALID, COIDParser::getType($coid));
+    }
+
+    public function testIRICaseSensitivity() {
+        $coid1 = new IRI('coid://example.com/example/1.0');
+        $coid2 = new IRI('coid://example.com/Example/1.0');
+        $this->assertFalse($coid1->equals($coid2));
+    }
+
+    public function testRootFromString() {
+        $coid1 = new IRI('coid://example.com');
+        $coid2 = COIDParser::fromString('coid://example.com');
+        $coid3 = COIDParser::fromString('example.com');
+        $this->assertTrue($coid1->equals($coid2));
+        $this->assertTrue($coid1->equals($coid3));
+    }
+
+    public function testUnversionedFromString() {
+        $coid1 = new IRI('coid://example.com/Example');
+        $coid2 = COIDParser::fromString('coid://example.com/Example');
+        $coid3 = COIDParser::fromString('example.com/Example');
+        $this->assertTrue($coid1->equals($coid2));
+        $this->assertTrue($coid1->equals($coid3));
+    }
+
+    public function testVersionedFromString() {
+        $coid1 = new IRI('coid://example.com/Example/1.0');
+        $coid2 = COIDParser::fromString('coid://example.com/Example/1.0');
+        $coid3 = COIDParser::fromString('example.com/Example/1.0');
+        $this->assertTrue($coid1->equals($coid2));
+        $this->assertTrue($coid1->equals($coid3));
+    }
+
+    public function testNormalizeRootFromString() {
+        $coid1 = new IRI('coid://example.com');
+        $coid2 = COIDParser::fromString('COID://example.com');
+        $coid3 = COIDParser::fromString('ExAmple.COM');
+        $this->assertTrue($coid1->equals($coid2));
+        $this->assertTrue($coid1->equals($coid3));
+    }
+
+    public function testNormalizeNonRootFromString() {
+        $coid1 = new IRI('coid://example.com/Example');
+        $coid2 = COIDParser::fromString('COID://example.com/Example');
+        $coid3 = COIDParser::fromString('ExAmple.COM/Example');
+        $coid4 = COIDParser::fromString('ExAmple.COM/EXample');
+        $this->assertTrue($coid1->equals($coid2));
+        $this->assertTrue($coid1->equals($coid3));
+        $this->assertFalse($coid1->equals($coid4));
+    }
 
 }