Re: Regex: behaviour of ? after () atom

[Steffen Nurpmeso <steffen@sdaoden.eu>]

Rich Felker wrote in <20180907153302.GM1878@brightrain.aerifal.cx>:
 |On Fri, Sep 07, 2018 at 05:25:17PM +0200, Steffen Nurpmeso wrote:
 |> Rich Felker wrote in <20180907151821.GL1878@brightrain.aerifal.cx>:
 |>|On Fri, Sep 07, 2018 at 03:38:05PM +0200, Steffen Nurpmeso wrote:
 |>|> Hello.
 |>|> 
 |>|> In perl this is
 |>|> 
 |>|>   $x="print 1 2";
 |>|>   if($x =~ /^(:[[:space:]]+)?([^[:space:]]+)(.*)$/){ 
 |>|>     print "<$0> -> <$1> <$2> <$3>\n"
 |>|>}
 |>|> 
 |>|> and the result is
 |>|> 
 |>|>   </tmp/t.pl> -> <> <print> < 1 2>
 |>|> 
 |>|> Now the same on AlpineLinux edge and musl-1.1.19-r10 with the MUA
 |>|> i maintain, which uses the normal regex stuff and calls it via
 |>|> 
 |>|>   echo eins=$3
 |>|>          vput vexpr i regex "${3}" \
 |>|>             '^(:[[:space:]]+)?([^[:space:]]+)(.*)$'  \
 |>|>             '<\$0> -> <\$1> <\$2> <\$3>'
 |>|>   echo i=$i
 |>|> 
 |>|> which in C code does 
 |>|> 
 |>|>       if((reflrv = regcomp(&re, argv[2], reflrv))){
 |>|>           ...
 |>|>          goto jestr;
 |>|>}
 |>|>   fprintf(stderr, "GOING for <%s> -> <%s> %u\n",
 |>|>   argv[1],argv[2],n_NELEM(rema));
 |>|>       reflrv = regexec(&re, argv[1], n_NELEM(rema), rema, 0);
 |>|> 
 |>|> and overall prints
 |>|> 
 |>|>   eins=print 1 2
 |>|>   GOING for <print 1 2> -> <^(:[[:space:]]+)?([^[:space:]]+)(.*)$> 17
 |>|>   i=<print 1 2> -> <> <> <>
 |>|> 
 |>|> It works correctly if i remove the ()? atom, so i thought i should
 |>|> report that.
 |>|
 |>|What is the value of the flags argument you passed to regcomp?
 |>|
 |> 
 |> REG_EXTENDED, optional REG_ICASE:
 |> 
 |>       reflrv = REG_EXTENDED;
 |>       if(f & a_ICASE)
 |>          reflrv |= REG_ICASE;
 |>       if((reflrv = regcomp(&re, argv[2], reflrv))){
 |
 |OK, it looks like that should work, and seemed to work here when I
 |passed the regex to grep -E linked with musl's regex. Can you provide
 |a minimal self-contained C program to demonstrate the issue you're
 |having?

Happy user that i am, here something for tests/:

  #include <stdio.h>
  #include <regex.h>
  int main(void){
          regmatch_t rema[1 + 21];
          regex_t re;
          int i;
          
          i = REG_EXTENDED;
          if((i = regcomp(&re, "^(:[[:space:]]+)?([^[:space:]]+)(.*)$", i)))
                  return 2;
          i = regexec(&re, "print 1 2", 21, rema, 0);
          regfree(&re);
          if(i == REG_NOMATCH)
                  return 3;
          for(i = 1; i < 21 && rema[i].rm_so != -1; ++i)
                  ;
          return (i == 3) ? 0 : 4;
  }       

i is 1 here.

 |BTW which "()?" are you talking about? The whole first parenthesized
 |subsexpression and the ? after it? I wouldn't call that an atom, but
 |nothing seems wrong with it.

I have read regex(7) first just in case something intellectual had
to be said.  Otherwise i am all for Finnish tango.

--steffen
|
|Der Kragenbaer,                The moon bear,
|der holt sich munter           he cheerfully and one by one
|einen nach dem anderen runter  wa.ks himself off
|(By Robert Gernhardt)