For literal (static) regexes there’s nothing to do — Perl will only compile them once.
if ($var =~ /foo|bar/) {
# ...
}
For regexes stored in variables you have a couple of options. You can use the qr// operator to build a regex object:
my $re = qr/foo|bar/;
if ($var =~ $re) {
# ...
}
This is handy if you want to use a regex in multiple places or pass it to subroutines.
If the regex pattern is in a string, you can use the /o option to promise Perl that it will never change:
my $pattern = 'foo|bar';
if ($var =~ /$pattern/o) {
# ...
}
It’s usually better to not do that, though. Perl is smart enough to know that the variable hasn’t changed and the regex doesn’t need to be recompiled. Specifying /o is probably a premature micro-optimization. It’s also a potential pitfall. If the variable has changed using /o would cause Perl to use the old regex anyway. That could lead to hard-to-diagnose bugs.