How do you match one of two words in a regular expression?
Try:
/(@|at)/
This means either @ or at but not both. It's also captured in a group, so you can later access the exact match through a backreference if you want to.
/(?:@|at)/
mmyers' answer will perform a paren capture; mine won't. Which you should use depends on whether you want the paren capture.
if that's only 2 things you want to capture, no need regex
if ( strpos($string,"@")!==FALSE || strpos($string,"at") !==FALSE ) {
# do your thing
}