re_string_byte_at
c = re_string_byte_at (input, idx);
unsigned char c = re_string_byte_at (input, str_idx), d;
d = re_string_byte_at (input, str_idx + 1);
d = re_string_byte_at (input, str_idx + i);
re_string_byte_at (input, str_idx) == '\n') ||
re_string_byte_at (input, str_idx) == '\0'))
ch = re_string_byte_at (&mctx->input, idx);
? 0 : re_string_byte_at (&mctx.input, offset));