Question

I’m trying to create a Rails 3 validation that will ensure that people are not using one of the common free email addresses.

My thought was something like this ….

validates_format_of :email, :with => /^((?!gmail).*)$|^((?!yahoo).*)$|^((?!hotmail).*)$/

or

validates_exclusion_of :email, :in => %w( gmail. GMAIL. hotmail. HOTMAIL. live. LIVE. aol. AOL. ), :message => "You must use your corporate email address."

But neither works properly. Any ideas?

Answer 1

Basically you’ve written a regex that matches anything. Let’s break it down.

/
  ^(            # [ beginning of string
    (?!gmail)   #   followed by anything other than "gmail"
    .           #   followed by any one character
  )$            #   followed by the end the of string
  |             # ] OR [
  ^(            #   beginning of the string
    (?!yahoo)   #   followed by anything other than "yahoo"
    .           #   followed by any one character
  )$            #   followed by the end of the string
  |             # ] OR [
  ^(            #   beginning of the string
    (?!hotmail) #   followed by anything other than "hotmail"
    .*          #   followed by any or no characters 
  )$            #   followed by the end the of string
/               # ]

When you think about it you’ll realize that the only strings that won’t match are ones that start with “gmail,” “yahoo,” and “hotmail”–all at the same time, which is impossible.

What you really want is something like this:

/
  .+@                      # one or more characters followed by @
  (?!                      # followed by anything other than...
    (gmail|yahoo|hotmail)  # [ one of these strings
    \.                     #   followed by a literal dot
  )                        # ]
  .+                       # followed by one or more characters
  $                        # and the end of the string
/i                         # case insensitive

Put it together and you have:

expr = /.+@(?!(gmail|yahoo|hotmail)\.).+$/i

test_cases = %w[ [e-mail]
                 [e-mail]
                 [e-mail]
                 [e-mail]
                 quux
               ]

test_cases.map {|addr| expr =~ addr }
# => [nil, nil, nil, 0, nil]
#    (nil means no match, 0 means there was a match starting at character 0)