And tested :)

#!/usr/bin/perl --
use strict; use warnings;
use Data::Dump qw/ dd /;
use CGI qw/ escapeHTML /;
my $text = "Video1: http://www.youtube.com/watch?v=dTi4v3HveqE
Video2: [video]http://www.youtube.com/watch?v=dTi4v3HveqE[/video]";
$text =~ s{
(?:
    (^|\s|\>)          # $1
    (                  # $2
        (http|https)   # $3
        ://www.youtube.com/watch\?\S*v=
        ([a-z0-9-_%]+) # $4
        [&\w;=\+\-]*
    )
)
|
(?:
    \Q[video]\E
    (                  # $5
        (http|https)   # $6
        ://www.youtube.com/watch\?\S*v=
        ([a-z0-9-_%]+) # $7
        [&\w;=\+\-]*
    )
    \Q[/video]\E
)
}{
    my $ret = "";
    my $https = $3 || $6;
    my $vid = $4 || $7;
    $ret .= "\n<embed>\n";
    $ret .= escapeHTML($vid);
    $ret .= "\n</embed>\n";
    $ret;
}gmsxie;

dd $text;
print "\n\n$text\n";

__END__
"Video1:\n<embed>\ndTi4v3HveqE\n</embed>\n\nVideo2: \n<embed>\ndTi4v3H
+veqE\n</embed>\n"


Video1:
<embed>
dTi4v3HveqE
</embed>

Video2: 
<embed>
dTi4v3HveqE
</embed>
[download]