block out certain words from search

I am lost again need the experts...
(I can't believe I can't find something like this a novice can rewrite into his code)

I have a simple form that searches a concordance:

<form method="post" action="result.php" name="keysearch" >

<h3>Enter single word at least 3 characters long.</h3>

<input type="text" name="search" value="" size="40" /><br />
<input type="submit" value="Display Results" />
</form>

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First part, I need to stop people from searching on anything less than 3 characters long and no spaces (i.e. 2+ words) in the search box.

Second part, I need to make sure that the following list is not allowed, something like:

$forbidden_word="she"||"they"||"them";

# if $forbidden_word matches the submitted word
if($_POST['keysearch']==$forbidden_word)
{
$message='The word: '.$forbiden_word.' is not allowed;
}
else

SEND FORM