Cleaning illegal characters from varchar field

[Nathan Gross]

Hi;
[Mysql 4x] In a table where I get data from another program, I have
many records (about 1000) that have illegal chars in a [unique]
indexed varchar field. I would like to clean the illegal characters
out and leave the rest of the data intact.
1. Is there a utility to do this?
1b) Or an UPDATE command?

Problem is that I do not even know which characters are illegal.
For the future, I can have my Java program check before the inserts.

2. What are the illegal characters?

Thank you.
-nat