Things you learn from looking at the HexFiend source code

static BOOL isHex(unichar c) { switch © { case '0' … '9': case 'a' … 'f': case 'A' … 'F': return YES; default: return NO; } }

Well that one is an eye-opener. I have often wished for the "range" shorthand provided here by the ... (three dots) operator. The thing is, I’d never seen this operator in C code before and my trusty C Pocket Reference only makes one mention of the operator as an "optional argument indicator". In its section on case labels it only says:

Every case label has the form case const:; where const is a constant integer expression.

How could I have missed such a useful thing? The Wikipedia page on page on C and C++ operators makes no reference to it, nor does the main article on C.

The 550-page C99 standard only makes references to ... as the "ellipsis punctuator" and describes how it is used to indicate an unspecified number of additional arguments.

Finally I found the answer: it’s a GNU GCC extension. As noted by the documentation:

Write spaces around the ..., for otherwise it may be parsed wrong when you use it with integer values.

Very useful indeed.