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'