To help us communicate without going around in circles, please read and try to follow:

• I know what I mean. Why don't you?
• Short, Self-Contained, Correct Example

Here is my test program t1.pl:

use strict;
use warnings;

# Small standalone test program derived from [id://11153804]
# @lines contains some test lines derived from:
# https://www.sec.gov/Archives/edgar/data/831001/000095010323011632/00
+00950103-23-011632.txt
my @lines = (
   '<FILENAME>dp198076_424b2-us2342673.htm',
   '<FILENAME>dp198076_exfilingfees.htm',
   '<FILENAME>dp198076_oopswrongextension.htmfred',
);

foreach my $line (@lines) {
   print "line:$line:\n";
   if ($line =~ m/<FILENAME>(.*\.htm)/) {
      print "   matched line: filename='$1'\n";
   }
   else {
      print "   did not match line\n";
   }
}
[download]

When I run this program, this is what I see:

line:<FILENAME>dp198076_424b2-us2342673.htm:
   matched line: filename='dp198076_424b2-us2342673.htm'
line:<FILENAME>dp198076_exfilingfees.htm:
   matched line: filename='dp198076_exfilingfees.htm'
line:<FILENAME>dp198076_oopswrongextension.htmfred:
   matched line: filename='dp198076_oopswrongextension.htm'
[download]

When I change this line above from:

   if ($line =~ m/<FILENAME>(.*\.htm)/) {
[download]

to:

   if ($line =~ m/<FILENAME>(.*\.htm)\b/) {
[download]

when I run this program I see instead:

line:<FILENAME>dp198076_424b2-us2342673.htm:
   matched line: filename='dp198076_424b2-us2342673.htm'
line:<FILENAME>dp198076_exfilingfees.htm:
   matched line: filename='dp198076_exfilingfees.htm'
line:<FILENAME>dp198076_oopswrongextension.htmfred:
   did not match line
[download]

If that is not what you were asking about, please clarify your question by posting a Short, Self-Contained, Correct Example that we can run.