What are the valid characters to use in labels? I thought that only alphabetic characters were valid, but I tested
! and they work. What are the restrictions to these names?
ASCII character that are not special for TeX are acceptable, including punctuation characters; don't use accented characters when the package inputenc is loaded.
Also some of the special characters can be employed:
Warning. Some characters might give problems when babel is loaded along with varioref (for example the colon