Need help matching a string if it ONLY contains approved substrings

I am having a hard time writing this regular expression.

Basically I need to block email delivery if the message recipient envelope contains an unapproved domain. My internet mail gateway (Brightmail) allows to create rules where I can specify a condition using Match/DoesNotMatch a regular expression.

For example I need only allow mail to yahoo.com, hotmail.com, and gmail.com.

If someone sends a message to some other domain, the message should be blocked.

If someone sends a message to yahoo.com AND some other domain, the message should be blocked

If someon开发者_StackOverflow中文版e sends a message to yahoo.com, the message should be allowed.

Is this possible?

If someone sends a message to yahoo.com and hotmail.com, the message should be allowed.

I am not a total noob with regular expression, but this one has me stumped.

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That sounds like its possible. It would be a token find where you have a series of OR operators between tokens.

I don't know if you have ever used this site, but it is a GODSEND when I am developing regex queries.

http://regexpal.com/

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I played with RegexPal and picked a combination that will satisfy my needs.

Basically this will match the substrings before the @ character for only those strings that ARE NOT the listed ones.

(?!(.@yahoo.com|.@hotmail.com|.@gmail.com)).@

or even this will work

(?!(.@yahoo.com|.@hotmail.com|.*@gmail.com))@

or this

@(?!(.@yahoo.com|.@hotmail.com|.*@gmail.com))

or even better

@(?!(.*(yahoo|hotmail|gmail).com))

The last one will just highlight the @ sign in the domain name strings that do Not belong.