use warnings;
use strict;

my $text = qq{foo 1234 \n 1234 bar \n1234\n 1 22 333 55555 666666};
print qq{[[$text]] \n};

my $digits_4 = qr{ \b \d{4} \b }xms;
$text =~ s{ ($digits_4) }
          {<a href="script.pl?do=view&unit=$1"><b>$1</b></a>}xmsg;
print qq{[[$text]] \n};

##</code><code>##

c:\@Work\Perl\monks\htmanning>perl s_4_digits_1.pl
[[foo 1234
 1234 bar
1234
 1 22 333 55555 666666]]
[[foo <a href="script.pl?do=view&unit=1234"><b>1234</b></a>
 <a href="script.pl?do=view&unit=1234"><b>1234</b></a> bar
<a href="script.pl?do=view&unit=1234"><b>1234</b></a>
 1 22 333 55555 666666]]