I can't make heads or tails of your descriptions and examples. If groups of digits are supposed to be surrounded with underscores (taking in account that N is a digit as well), then why does $correct end in R_345U_299M? I would expect it to end in R_345_U_299_M. Also, the group 69827 doesn't get underscores.

I can make a substitution that matches your descriptions, but that produces quite a different string of $string than $correct is. 

my $L = '[A-MO-Za-mo-z]';
my $D = '[0-9N]';

$_ = '423RY75Y69827EC67592C78657N965R345U299M';
s/($L?)($D+)\1/_${1}${2}${1}_/g;         
print $_, "\n";
__END__
_423_R_Y75Y__69827_E_C67592C__78657N965_R_345_U_299_M

[download]

In reply to Re: match if last char and first char are different excluding certain chars by Anonymous Monk
in thread match if last char and first char are different excluding certain chars by leocharre

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.