[ruby/uri] Re-allow consecutive, leading and trailing dots in EMAIL_REGEXP

Effectively reverts commit https://github.com/ruby/uri/commit/788274b180d6 and
https://github.com/ruby/uri/commit/0abac721d8fe.

EMAIL_REGEXP was mostly drawn from WHATWG HTML LS. This spec states that
it intentionally violates RFC 5322 to provide a practical regex for
validation.

> This requirement is a willful violation of RFC 5322, which defines a
> syntax for email addresses that is simultaneously too strict (before the
> "@" character), too vague (after the "@" character), and too lax
> (allowing comments, whitespace characters, and quoted strings in manners
> unfamiliar to most users) to be of practical use here.

The allowing of consecutive dot s(`a..a@`) and leading/trailing dots
(`.a@`, `a.@`) is not the only derivation from RFC 5322. If a truly RFC
5322-compliant regexp is needed, tt should be organized under a
different name, since too much departure from the original EMAIL_REGEXP
must be introduced.

https://github.com/ruby/uri/commit/c551d7020b

2 files changed, 11 insertions, 21 deletions

diff --git a/lib/uri/mailto.rb b/lib/uri/mailto.rb
index f747b79ec7..cb8024f301 100644
--- a/lib/uri/mailto.rb
+++ b/lib/uri/mailto.rb
@@ -52,11 +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
     # https://html.spec.whatwg.org/multipage/input.html#valid-e-mail-address
-    EMAIL_REGEXP = %r[\A#{
-      atext = %q[(?:[a-zA-Z0-9!\#$%&'*+\/=?^_`{|}~-]+)]
-    }(?:\.#{atext})*@#{
-      label = %q[(?:[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)]
-    }(?:\.#{label})*\z]
+    EMAIL_REGEXP = /\A[a-zA-Z0-9.!\#$%&'*+\/=?^_`{|}~-]+@[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:
 
     #
diff --git a/test/uri/test_mailto.rb b/test/uri/test_mailto.rb
index 59bb5ded09..6cd3352978 100644
--- a/test/uri/test_mailto.rb
+++ b/test/uri/test_mailto.rb
@@ -145,29 +145,23 @@ class URI::TestMailTo < Test::Unit::TestCase
     u.to = 'a@valid.com'
     assert_equal(u.to, 'a@valid.com')
 
-    # Invalid emails
-    assert_raise(URI::InvalidComponentError) do
-      u.to = '#1@mail.com'
-    end
+    # Intentionally allowed violations of RFC 5322
+    u.to = 'a..a@valid.com'
+    assert_equal(u.to, 'a..a@valid.com')
 
-    assert_raise(URI::InvalidComponentError) do
-      u.to = '@invalid.email'
-    end
+    u.to = 'hello.@valid.com'
+    assert_equal(u.to, 'hello.@valid.com')
 
-    assert_raise(URI::InvalidComponentError) do
-      u.to = '.hello@invalid.email'
-    end
-
-    assert_raise(URI::InvalidComponentError) do
-      u.to = 'hello.@invalid.email'
-    end
+    u.to = '.hello@valid.com'
+    assert_equal(u.to, '.hello@valid.com')
 
+    # Invalid emails
     assert_raise(URI::InvalidComponentError) do
-      u.to = 'n.@invalid.email'
+      u.to = '#1@mail.com'
     end
 
     assert_raise(URI::InvalidComponentError) do
-      u.to = 'n..t@invalid.email'
+      u.to = '@invalid.email'
     end
 
     # Invalid host emails