c-opt-op-identifier-prefix is a variable defined in `cc-engine.el'.
Its value is

  • Automatically becomes buffer-local when set.

Regexp matching the token before the ones in
`c-overloadable-operators' when operators are specified in their
"identifier form". This typically matches "operator" in C++ where
operator functions are specified as e.g. "operator +". It's nil in
languages without operator functions or where the complete operator
identifier is listed in `c-overloadable-operators'.

This regexp is assumed to not match any non-operator identifier.