If you don't want to follow the advice of others concerning Path::* (and there are other modules to do similar things) and want to roll your own regex, be warned that regex be tricky.

For instance, the  qr/${user_input}_ABC_.*pd/ regex used here fails (i.e., accepts as valid) the xyzabc_ABC_hipdip filename. (There's also the problem that the logical sense of the test is wrong, but that's minor.)

Line-end anchors  \A \z can be useful here, as can  \Q \E metaquoting escape sequences to guard against regex metacharacters included in the  $user_input portion of the filename. E.g.:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $user_input = 'ab+c';
 my $excluded = qr{ \A \Q$user_input\E _ABC_ .* [.] pdf \z }xms;
 ;;
 FILE:
 foreach my $file (qw(
   ab+c_ABC_something.pdf  ab+c_ABC_anything.pdf  ab+c_ABC_.pdf
   xab+c_ABC_.pdf  ab+c_ABC_.pdfx  xab+c_ABC_.pdfx
   abc_ABC_something.pdf  abc_ABC_anything.pdf  abc_ABC_.pdf
   abc  _.pdf  xyzabc_ABC_hipdip  foobar
   )) {
   next FILE if $file =~ $excluded;
   print qq{'$file' is not excluded};
   }
"
'xab+c_ABC_.pdf' is not excluded
'ab+c_ABC_.pdfx' is not excluded
'xab+c_ABC_.pdfx' is not excluded
'abc_ABC_something.pdf' is not excluded
'abc_ABC_anything.pdf' is not excluded
'abc_ABC_.pdf' is not excluded
'abc' is not excluded
'_.pdf' is not excluded
'xyzabc_ABC_hipdip' is not excluded
'foobar' is not excluded
[download]

Note that the initialization of the  $user_input has been changed to  'ab+c' so as to include a regex metacharacter as an example of the effect of the  \Q \E escapes.

Update: The quotemeta built-in can be used instead of  \Q \E escapes:
    my $user_input = quotemeta 'ab+c';
    my $excluded = qr{ \A $user_input _ABC_ .* [.] pdf \z }xms;

Note also that thorough testing is wise in case you are using a regex approach; see Test::More and its ilk.

Please see perlre, perlretut, and perlrequick.

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