The sets of characters tested for by the isalnum, isalpha, iscntrl, islower, isprint, and isupper functions (§4.3.1)
| Function | Tests For |
| isalnum | Characters 0–9, A–Z, a–z ASCII 48–57, 65–90, 97–122 |
| isalpha | Characters A–Z, a–z ASCII 65–90, 97–122 |
| iscntrl | ASCII 0–31, 127 |
| islower | Characters a–z ASCII 97–122 |
| isprint | Characters A–Z, a–z, 0–9, punctuation, space ASCII 32–126 |
| isupper | Characters A–Z ASCII 65–90 |