How do you implement a good profanity filter?

Question

Many of us need to deal with user input, search queries, and situations where the input text can potentially contain profanity or undesirable language. Oftentimes this needs

Answer 1

Beware of localization issues: what is a swearword in one language might be a perfectly normal word in another.

One current example of this: ebay uses a dictionary approach to filter "bad words" from feedback. If you try to enter the german translation of "this was a perfect transaction" ("das war eine perfekte Transaktion"), ebay will reject the feedback due to bad words.

Why? Because the german word for "was" is "war", and "war" is in ebay dictionary of "bad words".

So beware of localisation issues.