As Mofi writes, BBcodes are transformed into HTML by the forum/board software (often written in PHP). Each forum also have one or more style sheets that determines how the html should be presented (font, spacing, graphics etc).
So it isn't really possible to ask UE to supply a preview option for BBcode.
In some forum software you are even able to define your own BBcodes (like in phpBB) and they are stored in the phpBB application and a text editor like UE can't have any knowledge of this.
But if you for some reason (like working offline, without local PHP script interpreter) you need to see how your BBcode-tagged document looks like in a browser, then if you are a version 13 user
then check out the script attached:
It basically copies your BBcode-tagged document into a new temporary document. Runs through a number of predefined BBcode to HTML conversions (using Regular expressions). After the script has run, then just toggle View - Toggle Browser View
You have to modify the script to your specific purpose. See the function defineSupportedTranslations();
In this you will find pairs of // Check for bold text
You should add similar pairs for your BBcode use if some are missing.
Also some BBcodes - code, PHP, quote - have larger HTML replacements. These should be modified also. They reference styles in an external stylesheet.
This external stylesheet has to be referenced in the function named writeHTMLtop()
. Look for http://www.yourserver.com
The script is "as is". There's plenty of room for improvements. Use at own risk.