Improve performance of URI::MailTo::EMAIL_REGEXP

Fix the performance regression at #172 for valid emails.

``` yml
prelude: |
  require 'uri/mailto'
  n = 1000
  re = URI::MailTo::EMAIL_REGEXP
benchmark:
  n.t..t.: re.match?("n.t..t.@docomo.ne.jp")
  example: re.match?("example@example.info")
```

|         |released| 788274b| c5974f0|    this|
|:--------|-------:|-------:|-------:|-------:|
|n.t..t.  |  3.795M|  4.864M|  4.993M|  8.739M|
|         |       -|   1.28x|   1.32x|   2.30x|
|example  |  3.911M|  3.740M|  2.838M|  3.880M|
|         |   1.38x|   1.32x|       -|   1.37x|

Author: nobu

lib/uri/mailto.rb

@@ -52,7 +52,11 @@ class MailTo < Generic
     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 = /\A(?!\.)(?!.*\.{2})[a-zA-Z0-9.!\#$%&'*+\/=?^_`{|}~-]+(?<!\.)@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*\z/
+    EMAIL_REGEXP = %r[\A#{
+      atext = %q[(?:[a-zA-Z0-9!\#$%&'*+\/=?^_`{|}~-]+)]
+    }(?:\.#{atext})*@#{
+      label = %q[(?:[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)]
+    }(?:\.#{label})*\z]
     # :startdoc:
 
     #

test/uri/test_mailto.rb

@@ -210,6 +210,22 @@ def test_check_to
     end
   end
 
+  def test_email_regexp
+    re = URI::MailTo::EMAIL_REGEXP
+
+    rate = 1000
+    longlabel = '.' + 'invalid'.ljust(63, 'd')
+    endlabel = ''
+    pre = ->(n) {'a@invalid' + longlabel*(n*rate) + endlabel}
+    assert_linear_performance(1..10, pre: pre) do |to|
+      re =~ to or flunk
+    end
+    endlabel = '.' + 'email'.rjust(64, 'd')
+    assert_linear_performance(1..10, pre: pre) do |to|
+      re =~ to and flunk
+    end
+  end
+
   def test_to_s
     u = URI::MailTo.build([nil, 'subject=Ruby'])