[embed] shortcode right after another

While working on #2269433-7: Support URLs on a line by themselves in ckeditor/WYSIWYG → , I have noticed that the second [embed] shortcode does not get parsed and converted into embeded media when 2 of them are placed without any characters in between:

[embed]http://url1[/embed][embed]https://url2[/embed]

The regex optionally matches a character before and/or after the [embed] shortcode, and therefore prevents 2 of them from working as the first match 'eats' up the first character of the second shortcode:

define('OEMBED_PATTERN_EMBED_SHORTCODE', '/(.?)\[embed\b(.*?)\](.+?)\[\/embed\](.?)/s');

Does anyone know what's the (.?) are for? Do we need it? The shortcode would work even if they are one right after the other if we remove it. I haven't seen any documentation that say if this usage of shortcode should work, so I am wondering if we are going to leave it be and mark it as "by design" or fix the regex so it works?

Cheers!
sunnz