my $re_single = qr/
    (?:\A|[\r\n]) # either the start of text or newline
    (
        [^\r\n]* # any non-newlines
        (?:\z|(?:\r\n|\n\r|\r|\n)\z) # last newline or eof
    )
/xs;

my $re_multiple = qr/
    (?:\A|[\r\n]) # either the start of text or newline
    (
        (?:
            [^\r\n]*
            (?:\r\n|\n\r|\r|\n)
        ){0,$nless}
    ) # have to enclose above so it all goes into $1
    (
        [^\r\n]*
        (?:\z|(?:\r\n|\n\r|\r|\n)\z) # last nl or eof
    )
/xs;