I (and others in this thread) explicitely mentioned no using (?{}) and (??{}). Allowing those means you have the full power of Perl available. It would mean that anything you can do in Perl, you can to with a "regular expression", and the question "can I do foo with a regular expression" becomes a non-question.

And I would have used (?{}) differently anyway. Something like: 

/^(a+)(b+)(c+)$(?(?{length($1) == length($2) && length($2) == length($
+3)})|(*FAIL))/;

[download]
But that's just a fancy way of writing: 
/^(a+)(b+)(c+)$/ && length($1) == length($2) && length($2) == length($
+3);

[download]
And you can solve the "match a square" with: 
/^([0-9]+)$(?(?{sqrt($1) == int sqrt($1)})|(*FAIL))/;

[download]
Or in general, match "whatever" by: 
/^(.*)$(?(?{is_whatever($1)})|(*FAIL))/;

[download]
after writing the appropriate "is_whatever" function.

But that means the question no longer is "how can I do foo with a regexp", but "how can I do foo in Perl".

In reply to Re^5: check for square-number with a regex by JavaFan
in thread check for square-number with a regex by Ratazong

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.