Rails validates NOT in regex - ruby-on-rails

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[ foo#gmail.com
bar#yahoo.com
BAZ#HOTMAIL.COM
qux#example.com
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)