$_ = 'foo(';

$regex = 'FOO(';
$modifiers = 'i';

# a bit of error checking never goes astray
die "Illegal modifier!" unless $modifiers =~ m/^[ismxo]\z/;

# escape everything except the essential regex char and \w \s
# this is a security measure as discussed below
$regex =~ s/([^\w\s\\|()[\]{}^\$*+?.])/\\$1/g;

# generate the regex with error checking
$regex = eval "qr/$regex/$modifiers";
die "Eval failed $@\n" if $@;

# test it
print "match" if /$regex/;

##</code><code>##

$regex =~ s/([^\w\s\\|()[\]{}^\$*+?.])/\\$1/g;