BB-codes doesn't work inside "url" bb-code in preview

Steps to reproduce
1. Open posting form
2. Insert "bold" bb-code with some text:

[b]Insert your text here[/b]

3. Insert "link" bb-code with this as url text:

[url=htttp://ya.ru][b]Insert your text here[/b][/url]

4. Press "Preview" button

Actual result
Link is not bold

Expected result
Link should be bold