Help to write a custom validation script

I have a form field that is a measurement entered as a character string.

The proper format is inches followed by eighths of an inch (i.e. 13 3/8).

I would like to validate that:

1. the inches are a whole number between 1 and 30
2. the numerator of the fraction is between 0 and 7
3. the '/' exists
4. and the denominator is 8

How would I write that in a custom validation script?

Thank you in advance for your help!