Re: Regex: behaviour of ? after () atom

[Rich Felker <dalias@libc.org>]

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?

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.

Rich