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From: Steffen Nurpmeso <steffen@sdaoden.eu>
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Subject: Regex: behaviour of ? after () atom
Date: Fri, 07 Sep 2018 15:38:05 +0200
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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.
Ciao,

--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)