diff --git a/index.js b/index.js
index c5752f8..25dca66 100644
--- a/index.js
+++ b/index.js
@@ -6,11 +6,11 @@ module.exports = url => {
 	}
 
 	// Don't match Windows paths `c:\`
-	if (/^[a-z]:\\/.test(url)) {
+	if (/^[a-zA-Z]:\\/.test(url)) {
 		return false;
 	}
 
 	// Scheme: https://tools.ietf.org/html/rfc3986#section-3.1
 	// Absolute URL: https://tools.ietf.org/html/rfc3986#section-4.3
-	return /^[a-z][a-z\d+\-.]*:/.test(url);
+	return /^[a-zA-Z][a-zA-Z\d+\-.]*:/.test(url);
 };
diff --git a/test.js b/test.js
index 448c4ca..f68537b 100644
--- a/test.js
+++ b/test.js
@@ -4,6 +4,7 @@ import isAbsoluteUrl from '.';
 test('main', t => {
 	t.true(isAbsoluteUrl('http://sindresorhus.com'));
 	t.true(isAbsoluteUrl('https://sindresorhus.com'));
+	t.true(isAbsoluteUrl('httpS://sindresorhus.com'));
 	t.true(isAbsoluteUrl('file://sindresorhus.com'));
 	t.true(isAbsoluteUrl('mailto:someone@example.com'));
 	t.true(isAbsoluteUrl('data:text/plain;base64,SGVsbG8sIFdvcmxkIQ%3D%3D'));