in reply to value returned on m// failure

  1. Try this. You will find it is wrong when you said m// returns undef for non-integers, in fact the return value is empty string. 
    $a = 123;
print $a =~ m/^\d+$/, "\n";

$a = "abc";
$ret = ($a =~ m/^\d+$/);
if (defined($ret)) {
    print "=$ret=\n";#print out ==
} else {
    print "undef";#not going this loop
}

    [download]
  2. Also try this, which tells you what is "true", what is "false" in Perl: 
    sub true_false {
    if (shift()) {
        print "true\n";
    } else {
        print "false\n";
    }
}

true_false("a");
true_false(1);
true_false("");
true_false(0);

if (undef) { #I don't want to use true_false for this case, so it is g
+uranteed that shift() does not play any magic
    print "true\n";
} else {
    print "false\n";
}

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  3. Go perlop, and look under m//, there is a clear answer for the return value from m//, under different situations. Not just the context, but also the modifiers changes the meaning of the return value from m//

Replies are listed 'Best First'.
Re: Re: value returned on m// failure
by parv (Parson) on Dec 31, 2002 at 01:33 UTC

    i am quite aware of what values are ture and what are false. explicitly, undef and empty string are false values. and the perlop says... 

           m/PATTERN/cgimosx
       /PATTERN/cgimosx
               Searches a string for a pattern match, and in
               scalar context returns true if it succeeds, false
               if it fails. ...

    ...it doesn't say anything about what the actual values are.

    i was asking why is that to express success number 1 is returned, yet a non-number is returned on failure. is it an implementation detail/decision such that empty string is easier/faster to return than the number 0?

[reply]

      It doesn't return 1 for success. It returns the number of matches (or substitutions for s/// or tr///). If you're not using the g option, it stops on the first match and returns 1. I'm not really up on perl internals but I would guess undef is represented as a null pointer whereas a zero would require creation of an SV.

      ---
print map { my ($m)=1<<hex($_)&11?' ':'';
$m.=substr('AHJPacehklnorstu',hex($_),1) }
split //,'2fde0abe76c36c914586c';

      [download]
[reply]
[d/l]

        right you are. i know that s/// & tr/// return the number of matched; never occured to me that m// behaves the same.

        i goofed when i wrote that number 1 (be it as string or numeric) was being returned on a regex match. it just happens that only one thing was being matched.

        thanks for the clue.

[reply]

      m// returns true or false in scalar context. Perl defines:

      NAMEValue as StringValue as NumberPerl Internal Variable
      true"1"1PL_sv_yes
      false""0PL_sv_no

      For evidence, see the source code module perl.c where PL_sv_yes and PL_sv_no are initialized. Also see the pp_match code in pp_hot.c where RETPUSHYES and RETPUSHNO are used to return a boolean value when invoked in scalar context without /g.

[reply]

        ah, so that is why bitwise AND operation w/ numeric** 1 yields 0 whenever following fails...

          (defined $var && $var =~ m/regex/) & 1

        [download]

        ...which otherwise would have produced an empty string. thanks mark.

        ** per perlop, an numeric operand implies numeric bitwise operation; per the table mark posted, false value is zero in numeric context.

[reply]
[d/l]
      Let me show you a piece of code first: 
      $_ = 123;
print 1 + m/^\d+$/, "\n"; # get 2
print "1" . m/^\d+$/, "\n"; #get 11

      [download]
      What does this piece of code tell us? It clearly tells us that, it is meaningless to ask the question whether m// returned a string or a number. The answer is only meaningful, when you use the returned value for a PURPOSE, either as a number, or as a string.

      This is the flexibility we get from Perl.

      Something not related, yet related:

      Before you look at it, the Schrodinger's cat is both dead and alive. Dead or alive only becomes determined after you looked at the cat.
[reply]
[d/l]

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