my @a = qw/a d/;
my $re = join '|',
    map { $_.quotemeta($a[$_]) } 0..$#a;
print $re, "\n";
$_ = '0a1d1y0z';
print s/($re)/xx/g, "\n";
print $_, "\n";
__END__
0a|1d
2
xxxx1y0z