sc-citation-nonnested-root-regexp

sc-citation-nonnested-root-regexp is a variable defined in `supercite.el'.
Its value is

"[-._[:alnum:]]+"

Documentation:
Regexp describing the variable root part of a nested citation.
This should NOT have a leading `^' character. This variable is
related to `sc-citation-root-regexp' but whereas that variable
describes both nested and non-nested citation roots, this variable
describes only nested citation roots.

You can customize this variable.