perl -ne '}{ print $.' # count lines

##</code><code>##

perl -ne 'BEGIN{$_=shift for$B,$A,$p}/$p/and@l=(),do{<>for 1..$A},next;push@l,$_;print shift@l if$B<@l}{print@l' 3 5 pattern test.txt

##</code><code>##

sub BEGIN {
    ($_ = shift(@ARGV)) foreach (($B, $A, $p));
}
LINE: while (defined(($_ = <ARGV>))) {
    (/pat/ and ((@l = ()), do {
        <ARGV> foreach (1 .. $A)
    }, next));
    push(@l, $_);
    (($B < @l) and print(shift(@l)));
}
{
    print(@l);
}

##</code><code>##

keep 1
keep 2
keep 3
keep 4
delete 1
delete 2
delete 3
pattern
delete 1
delete 2
delete 3
delete 4
delete 5
keep 5
keep 6
keep 7
keep 8
keep 9
keep 10
keep 11
delete 1
delete 2
delete 3
pattern
delete 1
delete 2
delete 3
delete 4
delete 5
keep 12
keep 13
keep 14
keep 15
delete 1
delete 2
delete 3
pattern
delete 1
delete 2
delete 3
delete 4
delete 5
keep 16
keep 17
keep 18
keep 19
delete 1
delete 2
delete 3
pattern
delete 1
delete 2
delete 3
delete 4
delete 5
keep 20
keep 21

##</code><code>##

keep 1
keep 2
keep 3
keep 4
keep 5
keep 6
keep 7
keep 8
keep 9
keep 10
keep 11
keep 12
keep 13
keep 14
keep 15
keep 16
keep 17
keep 18
keep 19
keep 20
keep 21