Why is the alphabet split into multiple ranges in this C code?

Question

The author of this code presumably had to support EBCDIC at some point, where the numeric values of the letters are non-contiguous (gaps exist between I, J and R, S, as you may have guessed).

It is worth noting that the C and C++ standards only guarantee that the characters 0 to 9 have contiguous numeric values for precisely this reason, so neither of these methods is strictly standard-conforming.

Looks like it attempts to cover both EBCDIC and ASCII. Your alternative method doesn't work for EBCDIC (it has false positives, but no false negatives)

C and C++ do require that '0'-'9' are contiguous.

Note that the standard library calls do know whether they run on ASCII, EBCDIC or other systems, so they're more portable and possibly more efficient.