you need to escape the '/' in <\/a>, since it would improperly terminate the regex otherwise.

or use something other than a forward slash as your regex delimiter, for example paired curly brackets like this

$mystring =~
   s{(\d{3,5})}
   {<b><a href='lookup.cgi?id=$1</a>}g;
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You often see matching against *nix paths using the default forward slash delimiter (and omitting the m as it is not required with / ... /) when using a different delimiter would be more readable.

if( $path =~ /\/path\/to\/file/ ) { ... }
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is, I feel, much harder to read than

if( $path =~ m{/path/to/file} ) { ... }
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Cheers,

JohnGG

As it stands the regex will match a minimum of 3 and maximum of 5 digits, but doesn't care if they are part of a large number of digits or are imbedded in text.

use strict;
use warnings;

while (<DATA>) {
    s|\b(\d{3,5})\b|<b><a href='lookup.cgi?id=$1</a></b>|g;
    print;
}

__DATA__
foo 300 bar bas.
foo300bar bas.
foo 123456 bar 1234 bas 56789.
foo bar bas.
1 foo 1 bar 2 bas bas.
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Prints:

foo <b><a href='lookup.cgi?id=300</a></b> bar bas.
foo300bar bas.
foo 123456 bar <b><a href='lookup.cgi?id=1234</a></b> bas <b><a href='
+lookup.cgi?id=56789</a></b>.
foo bar bas.
1 foo 1 bar 2 bas bas.
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Update: fixed bold tags and changed regex delimiters to avoid picket fence