$ cat t2.rb
n = 5;
while n < 12 do
  s = "username@host"+".abcde"*n+"."
  start = Time.now();
  p /\A[a-zA-Z0-9.!\#$%&'*+\/=?^_`{|}~\-]+@[a-zA-Z0-9\-]+(?:\.[a-zA-Z0-9\-]+)*\z/ =~ s
  p s.length.to_s + ': ' + (Time.now() - start).to_s;
  n += 1
end

$ uname -a ; time ruby -v t2.rb
Darwin chamomile.local 16.4.0 Darwin Kernel Version 16.4.0: Thu Dec 22 22:53:21 PST 2016; root:xnu-3789.41.3~3/RELEASE_X86_64 x86_64 i386 MacBookPro12,1 Darwin
ruby 2.4.0p0 (2016-12-24 revision 57164) [x86_64-darwin16]
t2.rb:8: warning: ambiguous first argument; put parentheses or a space even after `/' operator
nil
"44: 3.7e-05"
nil
"50: 9.0e-06"
nil
"56: 7.0e-06"
nil
"62: 8.0e-06"
nil
"68: 8.0e-06"
nil
"74: 1.0e-05"
nil
"80: 8.0e-06"
ruby -v t2.rb  0.09s user 0.04s system 57% cpu 0.231 total