For example, its understanding of the single-quote character "'" is currently limited to that character only. It will set a token's type to APOSTROPHE only if the "'" was used.
In the patch attached, I added all the characters that ASCIIFoldingFilter would change into "'".
I'm not sure that this is the right approach so I didn't write a complete patch for all the other hardcoded characters used in jflex rules such as ".", "-" which have some variants in ASCIIFoldingFilter that could be used as well.
Maybe a better approach would be to make it possible to have an ASCIIFoldingFilter-like reader as a character filter that could be in inserted in front of StandardTokenizer ?