diff --git a/src/Assets.php b/src/Assets.php
index 0b84615..0959c6b 100644
--- a/src/Assets.php
+++ b/src/Assets.php
@@ -50,7 +50,7 @@ class Assets {
 	/**
 	 * Regular expression to match an external url
 	 */
-	const REGEX_EXTERNAL_URL = '/^((https?:)\/\/|data:)/i';
+	const REGEX_EXTERNAL_URL = '/^((https?:)?\/\/|data:)/i';
 
 	/**
 	 * File type detection options
diff --git a/tests/HelpersTest.php b/tests/HelpersTest.php
index 7bd1a5f..bb60388 100644
--- a/tests/HelpersTest.php
+++ b/tests/HelpersTest.php
@@ -41,6 +41,7 @@ public function testIsAbsoluteUrl() {
 
 		$this->assertTrue($assets->isAbsoluteUrl('http://example.com'));
 		$this->assertTrue($assets->isAbsoluteUrl('https://example.com'));
+		$this->assertTrue($assets->isAbsoluteUrl('//example.com'));
 		$this->assertTrue($assets->isAbsoluteUrl('data:some-encoded-data'));
 		$this->assertFalse($assets->isAbsoluteUrl('unknown://example.com'));
 		$this->assertFalse($assets->isAbsoluteUrl('http:/example.com'));