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From: Rainer Weikusat <rweikusat@talktalk.net>
Newsgroups: comp.unix.shell,comp.unix.programmer,comp.lang.misc
Subject: Re: Command Languages Versus Programming Languages
Date: Fri, 22 Nov 2024 18:59:43 +0000
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scott@slp53.sl.home (Scott Lurndal) writes:
> Rainer Weikusat <rweikusat@talktalk.net> writes:

[...]

>>Something which would match [0-9]+ in its first argument (if any) would
>>be:
>>
>>#include "string.h"
>>#include "stdlib.h"
>>
>>int main(int argc, char **argv)
>>{
>>    char *p;
>>    unsigned c;
>>
>>    p = argv[1];
>>    if (!p) exit(1);
>>    while (c = *p, c && c - '0' > 10) ++p;
>>    if (!c) exit(1);
>>    return 0;
>>}
>>
>>but that's 14 lines of text, 13 of which have absolutely no relation to
>>the problem of recognizing a digit.
>
> Personally, I'd use:
>
> $ cat /tmp/a.c
> #include <stdint.h>
> #include <string.h>
>
> int
> main(int argc, const char **argv)
> {
>     char *cp;
>     uint64_t value;
>
>     if (argc < 2) return 1;
>
>     value = strtoull(argv[1], &cp, 10);
>     if ((cp == argv[1])
>      || (*cp != '\0')) {
>         return 1;
>     }
>    return 0;
> }

This will accept a string of digits whose numerical value is <=
ULLONG_MAX, ie, it's basically ^[0-9]+$ with unobvious length and
content limits.

return !strstr(argv[1], "0123456789");

would be a better approximation, just a much more complicated algorithm
than necessary. Even in strictly conforming ISO-C "digitness" of a
character can be determined by a simple calculation instead of some kind
of search loop.