If you set $/ to a reference to the number of (virtual) grid columns, then open a filehandle to a reference to your string, you can then read each (virtual) grid row as a separate record and perform your matching on each distinct row-record. 

$ perl -Mstrict -Mwarnings -Mautodie -le '
    sub match_virtual_grid_rows {
        local $/ = \shift;
        open my $fh, "<", \shift;
        grep { /(.)\1\1/ } <$fh>;
    }

    my $virtual_grid_columns = 5;
    my $grid_string = "ABCBBBCBADBCCACDDDAC";
    print for match_virtual_grid_rows($virtual_grid_columns, $grid_str
+ing);
'
DDDAC

[download]

A couple of additional tests:

<Reveal this spoiler or all in this thread>

-- Ken

In reply to Re: Regex matching on grid alignment by kcott
in thread Regex matching on grid alignment by Anonymous Monk

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.