Maybe (?) something like this is what you want? 

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = 'ABCDEF';
 ;;
 my @triplets;
 $s =~ s{ (?= (...)) . }{ push @triplets, $1;  ''; }xmsge;
 ;;
 print qq{'$s'};
 printf qq{'$_'  } for @triplets;
"
'EF'
'ABC'  'BCD'  'CDE'  'DEF'

[download]

Update: Here's another  s/// solution. I'm not sure I like it so much: I'm always suspicious, perhaps without cause, of code embedded in a regex. In addition, the  @triplets array must be a package global (ideally local-ized) due to a bug in lexical management that wasn't fixed until Perl version 5.16 or 5.18 (I think — I don't have access to these versions and I'm too lazy to check the on-line docs). 

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = 'ABCDEF';
 ;;
 local our @triplets;
 $s =~ s{ (?= (...) (?{ push @triplets, $^N })) . }''xmsg;
 ;;
 print qq{'$s'};
 printf qq{'$_'  } for @triplets;
"
'EF'
'ABC'  'BCD'  'CDE'  'DEF'

[download]

In reply to Re^3: substitution in regular expression by AnomalousMonk
in thread substitution in regular expression by aeqr

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.