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my $digits_4 = qr{ (?<!-)\b[0-9]{4}\b(?!-) }xms;
$text =~ s{ ($digits_4) }
          {<a href="apartments.pl?do_what=view&unit=$1"><b>$1</b></a>}
+xmsg;

$unitslist_4 = "1001,1002,1003,1004,1101,1102,1103,1104,1201,1202,1203
+,1204,1301,1302,1303,1304,1401,1402,1403,1404,
1501,1502,1503,1504,1601,1602,1603,1604,1701,1702,1703,1704,1801,1802,
+1803,1804,1901,1902,1903,1904,2001,2002,2003,2004,
...
$text =~ s{ ($digits_4) }
          {<a href="apartments.pl?do_what=view&unit=$1"><b>$1</b></a>}
+xmsg;
}