One common problem is the word a s s
Solution
filter it out, then replace what was filtered if it is inside another word.
I know this is late...I'm new to PHP, but perhaps someone will benefit.
#lets try a dirty word filter here
#============
$checkstring = eregi_replace(" ass", " a", $checkstring);
#==========
#correct words mistakingly filtered
$checkstring = eregi_replace("aa", "assa", $checkstring);
$checkstring = eregi_replace("ae", "asse", $checkstring);
$checkstring = eregi_replace("ai", "assi", $checkstring);
$checkstring = eregi_replace("ao", "asso", $checkstring);
$checkstring = eregi_replace("au", "assu", $checkstring);
