Ok, here's yet another fix to my regex. I noticed that  '.NET;.NET;' and similar are not counted properly. That's because the  (?<! \S) look-behind doesn't allow for a comma or semicolon. Easily fixed:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = '.net,.Net; .NET;.net, .NET  x.NET .NETx x.NET; x.NET,';
 ;;
 for my $kw (qw(.NET C C++)) {
   my $count = () = $s =~ m{ (?: (?<! \S) | (?<= [,;])) \Q$kw\E (?: (?
+! \S) | (?= [,;])) }xmsig;
   print qq{'$kw' $count};
   }
"
'.NET' 5
'C' 0
'C++' 0
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(So... How should  C;x C++,x .NET;x etc. be handled?)