How Can I Do That?

When you click on "[quick quote reply]", this type of element appears (without space):

[ QUOTE : xxxxxx : username ]

Now you write something, select text and click on the Quote button, a HTML element appears like this (without space):

[ quote ][ / quote ]

this "[/quote]" commands to insert every text written before it into the quotation.

As you see the first element ([ QUOTE : xxxxxx : username ]) has no ending "[/quote]" part, therefore it considers the second quotation's "[/quote]" as its ending part. As a result it inserts every text written after "[ QUOTE : xxxxxx : username ]" and before "[/quote]" into a single quotation.