The contents of {..} in your regex aren't interpolated. You actually need them to be (re)interpolated at the time of a possible match. You can do this with (??{ code }), which is a bit of an ugly hack... I have no idea if Parse::RecDescent will like these, presumably it just evals the regex so it may work. 
my $regex = qr/\[(\d{1,20})(??{  " {" . (20 - length $1) . "}"  })\]/;

while (<DATA>) {
  print /$regex/ ? "yes\n" : "no\n";
}

__DATA__
[12345678901234567890]
[123                 ]
[234223423      ]
[23409234329c        ]

[download]
I don't know of a good way to do this without extended regex features (or multiple regexes). If there were some way to do this in general, I'd have to get to work on some regex abuse a la Abigail. There have been a few times when something like this would have been handy!

blokhead

In reply to Re: Regex to match 20 chars of some digits followed by some spaces by blokhead
in thread Regex to match 20 chars of some digits followed by some spaces by leriksen

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.