Asi nejako takto:

function match_bbcode ($matches)
{
	$bbcodes = array
	( 
		"b" => '<b>\1</b>',
		"i" => '<i>\1</i>',
		"u" => '<u>\1</u>',
		"img" => '<img src="\1" />',
		"url" => '<a href="\1">\1</a>'
	);
		
	return (isset ($bbcodes [$matches [1]])?
		str_replace ('\1', $matches [2], $bbcodes [$matches [1]]):
		$matches [0]);
}

$html_text = nl2br (preg_replace_callback ('/\[(\w+)\]([\s\S]*?)\[\/\1\]/', "match_bbcode", htmlspecialchars ($bb_text)));