You might frind a read through the Extended Patterns section of the perlre manual page. But here is a brief outline ...

/E(?!F)/ matches any E followed by anything other than an F. In a replacement, the advantage is that you don't have to deal with the matched bit. ie.

$a = "this is a test\n";

$a =~ s/t(?!e)/T/g;

print $a;

$b = "this is a test\n";

$b =~ s/t([^e])/T$1/g;

print $b;
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In your example, the code is looking for three digits, followed by "not a digit", but without having to mess about with replacing back in the "£not digit".

--
Steve Marvell