my $digits_4 = qr{ \b \d{4} \b }xms;


$text =~ s{ ($digits_4) }
          {<a href="resident-info.pl?do_what=view&unit=$1"><b>$1</b></a>}xmsg;
          
my $digits_3 = qr{ \b \d{3} \b }xms;
$text =~ s{ ($digits_3) }
          {<a href="resident-info.pl?do_what=view&unit=$1"><b>$1</b></a>}xmsg;