diff --git a/is/rules.go b/is/rules.go
index 3cad215..c6380a4 100644
--- a/is/rules.go
+++ b/is/rules.go
@@ -246,7 +246,7 @@ var (
 	// Domain regex source: https://stackoverflow.com/a/7933253
 	// Slightly modified: Removed 255 max length validation since Go regex does not
 	// support lookarounds. More info: https://stackoverflow.com/a/38935027
-	reDomain = regexp.MustCompile(`^(?:[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-z0-9])?\.)+(?:[a-zA-Z]{1,63}| xn--[a-z0-9]{1,59})$`)
+	reDomain = regexp.MustCompile(`^(?:[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?\.)+(?:[a-zA-Z]{1,63}| xn--[a-z0-9]{1,59})$`)
 )
 
 func isISBN(value string) bool {