UAX #31 specifies a default syntax for identifiers
based on properties from the Unicode Character Database, UAX #44
.The general syntax is
<Identifier> := <Start> <Continue>* (<Medial> <Continue>+)*
where
<Start> has the XID_Start property,
<Continue> has the XID_Continue property, and
<Medial> is a list of characters permitted between continue characters
.UAX #31 also specifies some standard profiles,
including the Mathematical Compatibility Notation profile
.For C++ we apply the Mathematical Compatibility Notation profile,
and we add the character
U+005f low line, or
_,
to the set of permitted
<Start> characters
.The
<Medial> set is empty
.In the grammar used in UAX #31, this is
<Identifier> := <Start> <Continue>*
<Start> := XID_Start + ID_Compat_Math_Start + U+005f
<Continue> := <Start> + XID_Continue + ID_Compat_Math_Continue