#!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 underline  
                           # last element: more than 24 intermediate chars

for my $item(@testitems) {
  if ( $item =~ m/
     ^aaa_              # starts with "aaa_"
     [_A-Za-z0-9]{1,23} # test next 1,23 chars: underline, alphas or decimal_nums
     (?<=\D)            # Pos_LookBehind, Last char BEFORE last 4 (may be the 24th) 
                              # can't be a digit else we may find 5 digits at the end
     \d{4}$             # has ONLY - emphasis mine - four digits at the 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 may want only lc
NO match: aaa_fdsfs_000                         # only 3 digits at end of string
                                                     # (contrary to spec)
NO match: aaa_sdf_jdsh_01111                    # 5 digits at end of string
                                                     # (contrary to spec)
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 spec)
MATCHES:  aaa_gjh_0000
MATCHES:  aaa_ABCDEFGHI123456789ZYXW_1111       # < 24 chars before final 4 digits 
                                                      # (ok per spec)
NO match: aaa_ABCDEFGHI123456789ZYXWVUT_1111    # > 24 chars before final 4 digits 
                                                      # (contrary to spec)
done
=cut