Word filters are one of the hardest things to do on a forum, players will always find a way around it.
#1. Make a list of all banned words.
Example: "assume" contains "ass"
#2. Check words as a whole.
Example: "A55" is easy but might not be found.
#3. Use some external API.
This gets tricky quick, I would need to figure out the API, then fully trust it with the content that I give it.
Best Option:
Unlike spam and caps, language violations are very hard for a computer to make decisions on, they don't speak english. The best thing is to leave it up to the staff whole have been speaking english for many years and have the mind to quickly look at a giant block of text and know within seconds whether it breaks any rules.
- Crafty