RegEx help needed

I have an array of a submitted text string (called $submittedTextString). And I have an array of common words (called $commonWordArray). I'm trying to use Regex to: 1. Remove any words from $submittedTextString that appear in $commonWordArray (about a hundred words). 2. Remove any numbers from $submittedTextString. 3. Remove any characters from $submittedTextString that aren't alphabetical. 4. Remove any duplicate words from $submittedTextString I have been wrestling RegEx using tutorials and the manual but I'm not getting it...could someone help me put together the regex I need to do the above? Thanks a lot! Ron