Ah!! it seems that nobody has noticed the wise hdb's advice!
This is a very important thing while coding, Know your data! or the other face of the coin: Bad data ruins your day

Compare the time and genius needed to modify all previous answer to NOT consider, let's say, AX_1 and  ZZ_2 as adjacent.

With plain code add a feaure to the code is normally a trivial task: just grab two parts from regular expression and one control more in the if loop. 
use strict;
use warnings;

my @arr;

chomp $_ and push @arr,$_ for <DATA>;

my %adj;

for (0..$#arr){
    last if $_ == $#arr;
    my ($cur_num,$cur_code)= ($2,$1) if $arr[$_] =~/(\d*[A-Z])_(\d+)$/
+;
    my ($next_num,$next_code)= ($2,$1) if $arr[$_ + 1] =~/(\d*[A-Z])_(
+\d+)$/;
    
    if (($cur_num == $next_num - 1) and ($cur_code eq $next_code)  ){
                $adj{$arr[$_]} = undef;
                $adj{$arr[$_ + 1]} = undef;
    }
}

foreach (@arr){
  print "$_\n" if exists $adj{ $_ };
}
__DATA__
AX_1
ZZ_2
2L_33
2L_34
3L_45
3L_87
X_202
X_203
X_204

[download]


L*
There are no rules, there are no thumbs..
Reinvent the wheel, then learn The Wheel; may be one day you reinvent one of THE WHEELS.

In reply to Re: Adjacent numbers - plain way second by Discipulus
in thread Adjacent numbers by melissa_randel

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.