There are many problems with grading and hinting in regular expression using partial matching.

Let's suppose the worst case: we have a bunch of answers, several with 100% grade, several with partial grade and several with 0% grade.

Case 1: showing the correct part of student's response:
1.1. Should we choose an asnwer with maximum length of correct part or with minimum length of remaining characters before full match? That doesn't in any time mean the same answer.
1.2. Answers with grade 0% should be excluded from discovering correct part of student response, but what about partially correct answers? Should we show the closest answer possible or choose only from 100% answers?

Case 2: hinting
2.1. How about using partially correct answers there? Should hinting lead student to the partially correct (50% or something) answer if he is close to it, or hinting should use only 100% answers?

Case 3: grading
3.1. If a student gives response which is very near (it's easy to allow teacher to set threshold) to the answer, but isn't full match, should we give him a partial grade?