use strict;
use warnings;

my $maxLen = 20;
my @tests = (
    ['N' . ('-' x ($maxLen + 20)) . 'B', 'N' . ('-' x 20) . ('B' x ($m
+axLen + 1))],
    ['-B' . ('-' x ($maxLen + 20)), '-B' . ('B' x $maxLen) . ('-' x 20
+)],
    ['-N-N--B--N', '-N-NBBBBBN'],
    ['N-N-B-', 'N-NBBB'],
    ['-B-N-B-N-B-N', '-BBNBBBNBBBN'],
    ['-B-N', '-BBN'],
    ['-B-', '-BB'],
    ['B-', 'BB'],
);

for my $test (@tests) {
    my ($org, $ref) = @$test;
    my $str = $org;
    
    $str =~ s/((?=N-+)(?:N)(?:-*?))(-{1,$maxLen}B)/$1 . ('B' x length 
+$2)/eg;
    $str =~ s/(B-{1,$maxLen})(?=-*(N|$))/'B' x length $1/eg;
    next if $str eq $ref;

    print "Couldn't handle: '$org'\n";
    print "       Expected: '$ref'\n";
    print "            Got: '$str'\n"
}
[download]

which generates no output indicating all is well.

The two regex sustitutions:

    $str =~ s/((?=N-+)(?:N)(?:-*?))(-{1,$maxLen}B)/$1 . ('B' x length 
+$2)/eg;
    $str =~ s/(B-{1,$maxLen})(?=-*(N|$))/'B' x length $1/eg;
[download]

do the work. The rest is test code. Change maxLen to the maximum number of characters to replace.