If you want to check whether the accountnumber is part of the filename is has to be

if( $filename =~ /$accountnumber/gi ) {
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and not the other way round. But it seems you have tested that already. You might want to add additional checks such as that there is not a digit before and after the account number or similar. Otherwise there could be a match if the account number is part of another number in the filename.

Also, if you are after digits only use \d not \w.

Whilst (\w+) will match on digits also, you're probably better using ^(\d+); you're also better putting the start marker outside the brackets. Note also that "\w" also accepts "_" characters, so if you know a filename is going to be a certain pattern you are best tying it down as tightly as possible.

Note that also you probably need to look at greedy and non-greedy matching - that initial (\w+) may be getting all the characters

I admit regexs still manage to beat me occasionally and one way of building up patterns is to start at the simplest and incrementally make it more complex.

1) $filename =~ /^(\d+)_/
2) $filename =~ /^(\d+)_(\d+)/
3) $filename =~ /^(\d+)_(\d+)_(\w+)/
....
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You can also use glob to find all files that match a certain pattern.

The file name could start with some letters as well, here is what I am thinking that could work, since I know that a true file name needs to be at least 7 chars long:

#!/usr/bin/perl

use strict;
use warnings;

my @files = qw( 000231263444_01_XY_20130110_061717.txt 17034513_01_WQ_
+20130511_053551.txt 12345670_01_XY_20130110_061717.txt BOA034513_01_W
+Q_20130511_053551.txt);

foreach my $file(@files) {
  
  $file =~/(^\w+)_(\w{1,2})_(\w{1,2})_(\w+)_(\w+)\.txt$/i;
  #my $accountnumber = $1;
  # test
  my $accountnumber = "BOA034513";
 
  if( ($file=~/$accountnumber/gi) && ($accountnumber=~/\w{7,}/g) ) {

    print "\n Found - *$accountnumber* - *$file*\n";

  }else{

    print "\n Not Found - *$accountnumber* - *$file*\n";

   }
   
 }
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Any example of how the same process could be done using "GLOB"?
Thanks!

Would I be correct in inferring from your code that the "account number" is always at the start of the filename and followed by an underscore? In that case, a simple index test would do (and I think should be pretty efficient). Something like this:

#!/usr/bin/perl

use strict;
use warnings;

my $filename = "000231263444_01_XY_20130110_061717.txt";
my $accountnumber = '000231263444';

if (index($filename, "${accountnumber}_") == 0) {
  print "Found\n";
} else {
  print "Not found\n";
}
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Yes, it will be at that start of the file name but no underscore will be part of the file name:

my $filename = "000231263444_01_XY_20130110_061717.txt";
#after getting it with reg exp.
my $accountnumber = '000231263444';
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Splitting is probably better for you than using a regex. Especially if you later want to do something with the other parts of the filename, too.

my ( $accountnumber, $part2, $part3, $part4, $part5, $extension ) = sp
+lit /[_.]/, $filename;
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