Not only is the above badly formatted, it occured to me to test additional possibilities; so (with apologies), revised, cleaned up, etc. : 
#!C:/perl/bin -w

my @testitems = qw( aaa_gjh1_dfgdf_0009 
                    aaa_gjh_0000 
                    aaa_gjh1_DFgdf_0009
                    aaa_fdsfs_000
                    aaa_sdf_jdsh_01111 
                    aaa_gjh1_dfgdf_0009 
                    aaa_gjhi0000
                    aaa_gjh1_df<:df_0009 
                    aaa_gjh_0000 
                    aaa_ABCDEFGHI123456789ZYXW_1111
                    aaa_ABCDEFGHI123456789ZYXWVUT_1111);
                           # next to last: 23 before the last underlin
+e  
                           # last element: more than 24 intermediate c
+hars

for my $item(@testitems) {
  if ( $item =~ m/
     ^aaa_              # starts with "aaa_"
     [_A-Za-z0-9]{1,23} # test next 1,23 chars: underline, alphas or d
+ecimal_nums
     (?<=\D)            # Pos_LookBehind, Last char BEFORE last 4 (may
+ be the 24th) 
                              # can't be a digit else we may find 5 di
+gits at the end
     \d{4}$             # has ONLY - emphasis mine - four digits at th
+e end
    /x ) {
            print "MATCHES:  $item\n";
    } else {
    print "NO match: $item\n";
    }
}

print "done\n";

=head1 OUTPUT:
MATCHES:  aaa_gjh1_dfgdf_0009
MATCHES:  aaa_gjh_0000
MATCHES:  aaa_gjh1_DFgdf_0009                   # caps are alpha; OP m
+ay want only lc
NO match: aaa_fdsfs_000                         # only 3 digits at end
+ of string
                                                     # (contrary to sp
+ec)
NO match: aaa_sdf_jdsh_01111                    # 5 digits at end of s
+tring
                                                     # (contrary to sp
+ec)
MATCHES:  aaa_gjh1_dfgdf_0009
MATCHES:  aaa_gjhi0000                          # no underscore before
+ final 4 digits
                                                     # (ok per spec)
NO match: aaa_gjh1_df<:df_0009                  # includes symbol and 
+punct 
                                                     # (contrary to sp
+ec)
MATCHES:  aaa_gjh_0000
MATCHES:  aaa_ABCDEFGHI123456789ZYXW_1111       # < 24 chars before fi
+nal 4 digits 
                                                      # (ok per spec)
NO match: aaa_ABCDEFGHI123456789ZYXWVUT_1111    # > 24 chars before fi
+nal 4 digits 
                                                      # (contrary to s
+pec)
done
=cut

[download]

But see grinder's elegant (and earlier) node, below!

In reply to Re^3: Number of digits at the end of the string by ww
in thread Number of digits at the end of the string by isha

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.