Search engines on forums are generally not as sophisticate as compare to specialized search engine such as Google. This is because in order for users to search a key word, the search engine will first need to index those key words and store them in the database before hand. So for words that have very few characters (e.g. less than 3 characters), they will mixed up with other commonly used words (or characters) like "the", "are", "I", "a" etc. Due to these complexities involved, forums' search engine will never be able to match the dedicated search engines. So if you need to make an advanced search, it is better to use dedicated search engines like Google.