[Andreas]

Nice one
But I fear it's not fully compatible:

• What about complete phrases enclosed by quotes?
  I think your implementation will treat it as individual words.
• What about asterisk?
  I think this is a major problem. If I search for hack (with mySQL 4) it would not return rows containing hacks - your implementation would.
  If I search for hack* (with mySQL 4) it would return rows containing hack, hacks, etc. - your implementation would return nothing.
• What about negation?
  It seems like you don't support this at all, treating it as part of a word.
  Now if I search for ~vB3 (with mySQL 4) it would return rows that do not contain vB3, your implementation would return those only.
• What about sub-expressions?
• What if you do not put a plus/miuns in front of a word?
  It seems link in this case your implementation would strip off the first character and treat them as NOT words.

Although I think the performance wouldn't be good. Anyway, it's in interesting point to start with.