$regexp = qr/^(get|post)\b/i; "GET / HTTP/1.1"=~m/$regexp/ and print "match\n";

##</code><code>##

$regexp = qr@get|post@;
"GET / HTTP/1.1"=~m@^$regexp\s+(\S+)@i and print "match\n";

##</code><code>##

$regexp = qr/(\d+)/;
while ($line=~/$regexp/g) { print "number $1 found\n"; }