Stop checking Email with regular expressions!

Seriously, stop. This is a waste of time and effort. Look for a regular checker to check Email at Google, take a look at it - and you want to step back for a breath of fresh air. I remember one very famous quote:

Some people, when faced with a problem, think, "Oh, I will use regular expressions."
Now they have two problems.

Jaime Zawinski, regex.info

Here is a fairly common example of code from a Rails application containing some similarity to the authorization system:
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class User < ActiveRecord::Base
  #      from https://github.com/plataformatec/devise,
  #      Rails
  validates_format_of :email, :with => /\A[^@]+@([^@\.]+\.)+[^@\.]+\z/
end

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class User < ActiveRecord::Base
  validates_format_of :email, :with => /^(|(([A-Za-z0-9]+_+)|([A-Za-z0-9]+\-+)|([A-Za-z0-9]+\.+)|([A-Za-z0-9]+\++))*[A-Za-z0-9]+@((\w+\-+)|(\w+\.))*\w{1,63}\.[a-zA-Z]{2,6})$/i
end

:

class User < ActiveRecord::Base
  validates :email, :with => EmailAddressValidator
end

class EmailValidator < ActiveModel::Validator
  EMAIL_ADDRESS_QTEXT           = Regexp.new '[^\\x0d\\x22\\x5c\\x80-\\xff]', nil, 'n'
  EMAIL_ADDRESS_DTEXT           = Regexp.new '[^\\x0d\\x5b-\\x5d\\x80-\\xff]', nil, 'n'
  EMAIL_ADDRESS_ATOM            = Regexp.new '[^\\x00-\\x20\\x22\\x28\\x29\\x2c\\x2e\\x3a-\\x3c\\x3e\\x40\\x5b-\\x5d\\x7f-\\xff]+', nil, 'n'
  EMAIL_ADDRESS_QUOTED_PAIR     = Regexp.new '\\x5c[\\x00-\\x7f]', nil, 'n'
  EMAIL_ADDRESS_DOMAIN_LITERAL  = Regexp.new "\\x5b(?:#{EMAIL_ADDRESS_DTEXT}|#{EMAIL_ADDRESS_QUOTED_PAIR})*\\x5d", nil, 'n'
  EMAIL_ADDRESS_QUOTED_STRING   = Regexp.new "\\x22(?:#{EMAIL_ADDRESS_QTEXT}|#{EMAIL_ADDRESS_QUOTED_PAIR})*\\x22", nil, 'n'
  EMAIL_ADDRESS_DOMAIN_REF      = EMAIL_ADDRESS_ATOM
  EMAIL_ADDRESS_SUB_DOMAIN      = "(?:#{EMAIL_ADDRESS_DOMAIN_REF}|#{EMAIL_ADDRESS_DOMAIN_LITERAL})"
  EMAIL_ADDRESS_WORD            = "(?:#{EMAIL_ADDRESS_ATOM}|#{EMAIL_ADDRESS_QUOTED_STRING})"
  EMAIL_ADDRESS_DOMAIN          = "#{EMAIL_ADDRESS_SUB_DOMAIN}(?:\\x2e#{EMAIL_ADDRESS_SUB_DOMAIN})*"
  EMAIL_ADDRESS_LOCAL_PART      = "#{EMAIL_ADDRESS_WORD}(?:\\x2e#{EMAIL_ADDRESS_WORD})*"
  EMAIL_ADDRESS_SPEC            = "#{EMAIL_ADDRESS_LOCAL_PART}\\x40#{EMAIL_ADDRESS_DOMAIN}"
  EMAIL_ADDRESS_PATTERN         = Regexp.new "#{EMAIL_ADDRESS_SPEC}", nil, 'n'
  EMAIL_ADDRESS_EXACT_PATTERN   = Regexp.new "\\A#{EMAIL_ADDRESS_SPEC}\\z", nil, 'n'

  def validate(record)
    unless record.email =~ EMAIL_ADDRESS_EXACT_PATTERN
      record.errors[:email] << 'is invalid'
    end
  end
end

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class User < ActiveRecord::Base
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end

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. jetman!