I was wanting to expand my answer to Splitting every 3 digits? and do everything in one substitute statement.

I restate the problem (different from the initial one): writing an integer by block of three digits separated by underscores. The custom is to start from the end because the goal is to make visible thousands, millions and so on.

The easy way: 

$_=1113333444455;
$_=reverse; s/(\d{3})/$1_/g; $_=reverse; s/^_//;
print;

[download]
I thought that would do it in one substitute statement: s/(\d{3})(?=(\d{3})+)$/$1_/g; No way.

So I simplify the statement and instrument it with a zero witdh assertion: 

  DB<17>  $_=1113333444455;  m/(\d{3})(?{ print $1 })(?=\d{3})$/
455

[download]
It acts like if the right anchor were just behind the (\d{3}). Weird. Or I just need some sleep.
I am using perl 5.6.1

-- stefp

In reply to bug in regexp engine? by stefp

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.