Whether or not c is a punctuation character. That includes all ASCII characters which are not control characters, letters, digits, or whitespace.
assert( isPunctuation('.')); assert( isPunctuation(',')); assert( isPunctuation(':')); assert( isPunctuation('!')); assert( isPunctuation('#')); assert( isPunctuation('~')); assert( isPunctuation('+')); assert( isPunctuation('_')); assert(!isPunctuation('1')); assert(!isPunctuation('a')); assert(!isPunctuation(' ')); assert(!isPunctuation('\n')); assert(!isPunctuation('\0')); // N.B.: Non-ASCII Unicode punctuation characters are not recognized. assert(!isPunctuation('\u2012')); // (U+2012 = en-dash)