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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 17:48:37 +0000
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cross@spitfire.i.gajendra.net (Dan Cross) writes:
> Rainer Weikusat  <rweikusat@talktalk.net> wrote:
>>cross@spitfire.i.gajendra.net (Dan Cross) writes:
>>> Rainer Weikusat  <rweikusat@talktalk.net> wrote:
>>>>cross@spitfire.i.gajendra.net (Dan Cross) writes:
>>>>> [snip]
>>>>> It's also not exactly right.  `[0-9]+` would match one or more
>>>>> characters; this possibly matches 0 (ie, if `p` pointed to
>>>>> something that wasn't a digit).
>>>>
>>>>The regex won't match any digits if there aren't any. In this case, the
>>>>match will fail. I didn't include the code for handling that because it
>>>>seemed pretty pointless for the example.
>>>
>>> That's rather the point though, isn't it?  The program snippet
>>> (modulo the promotion to signed int via the "usual arithmetic
>>> conversions" before the subtraction and comparison giving you
>>> unexpected values; nothing to do with whether `char` is signed
>>> or not) is a snippet that advances a pointer while it points to
>>> a digit, starting at the current pointer position; that is, it
>>> just increments a pointer over a run of digits.
>>
>>That's the core part of matching someting equivalent to the regex [0-9]+
>>and the only part of it is which is at least remotely interesting. 
>
> Not really, no.  The interesting thing in this case appears to
> be knowing whether or not the match succeeded, but you omited
> that part.

This of interest to you as it enables you to base an 'argumentation'
(sarcasm) on arbitrary assumptions you've chosen to make. It's not
something I consider interesting and it's besides the point of the
example I posted. 

>>> But that's not the same as a regex matcher, which has a semantic
>>> notion of success or failure.  I could run your snippet against
>>> a string such as, say, "ZZZZZZ" and it would "succeed" just as
>>> it would against an empty string or a string of one or more
>>> digits.
>>
>>Why do you believe that p being equivalent to the starting position
>>would be considered a "successful match", considering that this
>>obviously doesn't make any sense?
>
> Because absent any surrounding context, there's no indication
> that the source is even saved.

A text usually doesn't contain information about things which aren't
part of its content. I congratulate you to this rather obvious observation.

[...]

>>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.
>
> This is wrong in many ways.  Did you actually test that program?
>
> First of all, why `"string.h"` and not `<string.h>`?  Ok, that's
> not technically an error, but it's certainly unconventional, and
> raises questions that are ultimately a distraction.

Such as your paragraph above. 

> Second, suppose that `argc==0` (yes, this can happen under
> POSIX).

It can happen in case of some piece of functionally hostile software
intentionally creating such a situation. Tangential, irrelevant
point. If you break it, you get to keep the parts.

> Third, the loop: why `> 10`? Don't you mean `< 10`?  You are
> trying to match digits, not non-digits.

Mistake I made. The opposite of < 10 is > 9.

> Fourth, you exit with failure (`exit(1)`) if `!p` *and* if `!c`
> at the end, but `!c` there means you've reached the end of the
> string; which should be success.

Mistake you made: [0-9]+ matches if there's at least one digit in the
string. That's why the loop terminates once one was found. In this case,
c cannot be 0.