Function digit

3rd/lua-5.4.3/src/lstrlib.c:1396–1396

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