At first I thought it might have to do with the bizarre static/dynamic scoping duality of the $1, $2, ... variables, since the postponed (??{CODE}) blocks may be compiled somewhere where they can't reach the correct $1, $2, .... But they also can't seem to access @- and @+, which I don't think have the same scoping properties.

And now I don't know what to think, because of the following example: I add an empty capturing group before the (??{CODE}) block in the outermost match, and it works (or at least seems to): 

# This is perl, v5.8.6 built for i386-linux-thread-multi
use Data::Dumper;

$rx = qr{ (.) (?{ print Dumper $1 }) }x;
"japhy" =~ m{    (??{ $rx }) }x;
## $VAR1 = undef;

$rx = qr{ (.) (?{ print Dumper $1 }) }x;
"japhy" =~ m{ () (??{ $rx }) }x;
## $VAR1 = 'j';

[download]
If you were to dump @- and @+ from inside the first example, you'd see that @- has two entries, but @+ has one. It's as if $1 was only partially "set up"..

Now I don't know if this "workaround" helps you or not. It could probably allow you to write the parser closer to how you originally envisioned. But it seems like a more fragile workaround than the one you have, and I'm not sure how much I trust it. I think I remember seeing similar weirdness with capturing parens somewhere else, but I can't find the reference at the moment.

Then there's the fact that doing this with a regex is silly, when you could write a RecDescent grammar in about 5 seconds.. but I know you know that ;)

blokhead

In reply to Re: Dynamic regex assertions, capturing groups, and parsers: joy and terror by blokhead
in thread Dynamic regex assertions, capturing groups, and parsers: joy and terror by japhy

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.