Once in awhile Perl still surprises me.

This code is obviously broken:

my ($left, $right) = qw(abc def);
print "$left_$right\n";

[download]

The problem is that interpolation causes Perl to want to print a variable $left_ and $right, but we declared $left and $right. There are several ways to fix this, two of which are:

print "$left\_$right\n";
print "${left}$right\n";

[download]

...and of course you could just use the concatenation operator, but then I wouldn't have anything to puzzle over.

But observe the following:

my $string = "abcd";
if ($string =~ m/(ab)(cd)/) {
    print "$1_$2\n";
}

[download]

The output is ab_cd. So in this case Perl automatically treated that interpolation as "${1}_$2\n" without me telling it to do so. I've looked over The Gory details of parsing quoted constructs and haven't found an explanation. I assume that in the case of numbered regex variables, Perl decides that if the variable starts with a numeric digit, the identifier must end when there are no more numeric digits. Is this behavior reliable? Is it documented? Is it likely to ever change?

I'm asking because I found it in a code review and was sure it was broken until we talked it over and tested to verify the behavior was to parse "$1_$2\n" as $1 . '_' . $2 . "\n" even though we would prefer to disambiguate using \ or ${n}.

Update: I do see in perldata: Identifier-parsing:

Meanwhile, special identifiers don't follow the above rules; For the most part, all of the identifiers in this category have a special meaning given by Perl. Because they have special parsing rules, these generally can't be fully-qualified. They come in six forms (but don't use forms 5 and 6):

  1. A sigil, followed solely by digits matching \p{POSIX_Digit}, like $0, $1, or $10000.

I don't know that this is worded quite right, because $1_ could be construed as NOT being an identifier consisting solely of digits. But it's the closest thing I can find to an explanation. But I'll take that as answering my own question: Yes, it's intentional and documented behavior.


Dave

In reply to How Perl decides where a variable ends and text starts: Match variables in string interpolation by davido

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.