Here's a regex for ssb-validate:
\.box(?:(?:[1-9A-F][0-9A-HJKMNP-TV-Z]{12})|(?:[1-9A-HJKMNP-TV-Z][0-9A-HJKMNP-TV-Z]{0,11}))?$
You can probably come up with a better one. Stuff that should match:

.boxF0123456789AB
.boxAH
.box10
.box1
.boxZZZ

Stuff that must not match:

.boxG0123456789AB
.box0
.box01
.boxU
.boxa