fix escaping of exclude arg

[yufukui-m]

we need to more escape backslashes

irb(main):001:0> name = 'foobar'
=> "foobar"
irb(main):002:0> re = Regexp.new("\A#{Regexp.escape(name)}\z")
=> /Afoobarz/
irb(main):003:0> re.match('foobar')
=> nil
irb(main):004:0> re.match('Afoobarz')
=> #<MatchData "Afoobarz">

irb(main):005:0> re = Regexp.new("\\A#{Regexp.escape(name)}\\z")
=> /\Afoobar\z/
irb(main):006:0> re.match('foobar')
=> #<MatchData "foobar">