$text =~ s{
(?:
    (^|\s|\>)          # $1
    (                  # $2
        (http|https)   # $3
        ://www.youtube.com/watch\?\.*v=
        ([a-z0-9-_%]+) # $4
        [&\w;=\+\-]*
    )
)
|
(?:
    \Q[video]\E

    (^|\s|\>)          # $5
    (                  # $6
        (http|https)   # $7
        ://www.youtube.com/watch\?\.*v=
        ([a-z0-9-_%]+) # $8
        [&\w;=\+\-]*
    )
    \Q[/video]\E
)
}{
    my $ret = "";
    my $https = $3 || $7;
    my $vid = $4 || $8;
    $ret .= "<embed>";
    $ret .= escapeHTML($vid);
    $ret .= "</embed>";
    $ret;
}gmsxie;