In With arrays, why is it the case that a[5] == 5[a]? it is explained that the [] operator in a[5] is defined as *(a + 5) and because + is commutative, 5[a] means *(5 + a) and so the two expressions refer to the same memory location. Fine.
However, C also defines in 6.4.2.1 that an identifier cannot start with a digit. In 5[a] the array identifier is 5 which is not a valid identifier. Why does 5[a] not generate an error about an invalid identifier?