Function digit

third-party/lua-5.5.0/src/lstrlib.c:1451–1451

** Read an integer numeral from string 'fmt' or return 'df' if ** there is no numeral */

** there is no numeral
*/
static int digit (int c) { return '0' <= c && c <= '9'; }

static size_t getnum (const char **fmt, size_t df) {
  if (!digit(**fmt))  /* no number? */