【C11】ctype

The header <ctype.h> declares several functions useful for classifying and mapping characters

isalnum
any character for which isalpha or isdigit is true
isapha
any character for which isupper or islower is true
isblank
any character that is a standard blank character
iscntrl
any control character
isdigit
any decimal-digit character
isgraph
any printing character except space ('')
islower
any character that is a lowercase letter
isprint
any printing character including space ('')
ispunct
any printing character that is one of a locale-specific set of punctuation characters
isspace
any character that is a standard white-space character
isupper
any character that is an uppercase letter
isxdigit
any hexadecimal-digit character
tolower
converts an uppercase letter to a corresponding lowercase letter
toupper
converts a lowercase letter to a corresponding uppercase letter