In general, you can't. There are several methods available to see if it looks reasonable, but there is no way to determine if an address is actually deliverable without actually attempting delivery.
Using regular expressions:
# Match basically blah@blah.blah if ( $addr =~ /^\S+\@\S+\.\S+$/ ) { print "Looks OK"; }
If you're doing any real work, you may wish to look to one of the modules available on http://search.cpan.org, such as:
Email::Address
Email::Valid
Email::Valid
makes it easy to determine if an email address is well-formed:
use Email::Valid; print ( Email::Valid->address( 'someone@gmail.com' ) ? 'Yes' : 'No' ); # prints Yes print ( Email::Valid->address( 'someone#gmail.com' ) ? 'Yes' : 'No' ); # prints No
Email::Valid
can also tell you why an address is invalid:
print "Invalid address: $Email::Valid::Details \n" unless Email::Valid->address( 'you#foo.bar' );
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