It seems like the search engine does not allow searches for words with
less than four characters.

What if the search is for 'sed' or 'awk'?

Only works with 4 or more characters are indexed. This is a question of resources, searching for three letter works is too memory intensive.

If I search for "sed", it just tells me that it couldn't find it. It would be nice if there could a message telling me that the search needs to be more than 3 characters. Or maybe a help page with more information on how to effectively use the search (such as does it recognize quotes or does "+" have special meaning).

A list of exception could be made for words with less than four characters
that are unix commands, 'sed', 'awk', 'cp', 'df', 'tr', 'cut', 'ex', 'ps', etc.

We are stuck with the search engine that comes with vbulletin. If you don't like our search engine, why not use another one instead? As one example, this will search our site for "sed". reborg and Neo have spent a lot of time making our site easy for the crawlers used by the major search engines. And they are working closely with Yahoo and Google. Personally, I fall back to Google when I can't find something here.

Alternatively, you can suggest changes to vbulletin to the vbulletin developers on the vbulletin site. But it's not likely that will ever match Google's software and even if they did we would not have the computing resources required to run it.