I think the advice of others about using an (X|HT)ML parser to avoid certain "URLs" (whatever that means in your particular circumstances) is wisely given and well taken. However, if you can clearly define a "URL" in the context of your data (and that's a big if!), here's a neat approach that will work. (This needs Perl version 5.10+ regex extensions.) (Untested) 

my $digits_3_4 = qr{ \b \d{3,4} \b }xms;
my $url        = qr{ <a> ... </a>  }xms;

$text =~ s{ $url (*SKIP) (*FAIL) | ($digits_3_4) }
          {<a href="resident-info.pl?do_what=view&unit=$1"><b>$1</b></
+a>}xmsg;

[download]
Please see Special Backtracking Control Verbs for  (*SKIP) (*FAIL) in perlre.

Give a man a fish:  <%-{-{-{-<

In reply to Re: Replacing 3 and 4 digit numbers. by AnomalousMonk
in thread Replacing 3 and 4 digit numbers. by htmanning

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