my $formget = qr(10-[KQ][A-Z0-9]*(?!/))i;

The problem with this regex when used in a  /^$formget/ match is that it allows strings like  10-KA/ to match: if a  / is found at the end of  10-KA/ the regex can backtrack to  10-K and look ahead to  A which is not a  / character! BrowserUk's approach below gets around this by using an end-of-string anchor assertion to make sure that only non-/ characters follow the [KQ]. If anchoring were not possible, another way would be to use an 'atomic'  (?>pattern) group which will not allow backtracking into it:

c:\@Work\Perl\monks>perl -wMstrict -le
"my @tests = qw[
   10-K    10-KSB      10-K405    10-KSB405    10-Q
   10-K/B  10-KSB/ABC  10-K405/A  10-KSB405/A  10-Q/A  10-KA/
   ];
 ;;
 my $formget = qr{ (?i) 10- [KQ] [A-Z0-9]* (?! /) }xms;
 printf 'valid:  ';
 /^$formget/ and printf qq{'$_' } for @tests;
 print '';
 ;;
 my $formget2 = qr{ (?> (?i) 10- [KQ] [A-Z0-9]*) (?! /) }xms;
 printf 'valid2: ';
 /^$formget2/ and printf qq{'$_' } for @tests;
"
valid:  '10-K' '10-KSB' '10-K405' '10-KSB405' '10-Q' '10-KSB/ABC' '10-
+K405/A' '10-KSB405/A' '10-KA/'
valid2: '10-K' '10-KSB' '10-K405' '10-KSB405' '10-Q'
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