diff --git a/lib/uri/mailto.rb b/lib/uri/mailto.rb
index 3499910787..497d9630d2 100644
--- a/lib/uri/mailto.rb
+++ b/lib/uri/mailto.rb
@@ -52,7 +52,7 @@ module URI
     HEADER_REGEXP  = /\A(?<hfield>(?:%\h\h|[!$'-.0-;@-Z_a-z~])*=(?:%\h\h|[!$'-.0-;@-Z_a-z~])*)(?:&\g<hfield>)*\z/
     # practical regexp for email address
     # http://www.whatwg.org/specs/web-apps/current-work/multipage/states-of-the-type-attribute.html#valid-e-mail-address
-    EMAIL_REGEXP = /\A[a-zA-Z0-9.!#$%&'*+\/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*\z/
+    EMAIL_REGEXP = /\A[a-zA-Z0-9.!\#$%&'*+\/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*\z/
     # :startdoc:
 
     #