G'day halak77,

Welcome to the Monastery.

"/^\s*[^#].*/"

[^#] is a character class containing all characters except '#'. What you really want is a character class containing only '#'.

The first line doesn't match: (^) - line start OK; (\s*) - match zero whitespace OK; ([^#]) - '#' is a '#' FAIL

The second line does match: (^) - line start OK; (\s*) - match zero whitespace OK; ([^#]) - '' is NOT a '#' OK

I'll leave you to continue through the remaining lines of data.

I'll also point out that the '.*' (at the end of the regex) is superfluous. It will always match: either none, one or some of any character matched by '.'. You're only interested in the front of the string, anyway.

Here's my test (pm_1124511_comment_removal.pl):

#!/usr/bin/env perl

use strict;
use warnings;

my $comment_line_re = qr{ \A \s* [#] }x;

while (<DATA>) {
    print "@{[/$comment_line_re/ ? 'IS' : 'NOT']} Comment: $_";
}

__DATA__
# This file is a comment and should not be used
 # This comment has one leading space
   # This comment has three leading spaces
/dev/mapper/centos-var_log /var/log                xfs     defaults   
+     1 2
 /dev/donstest  /expert         none    1 1 -rw,noglob # this is a fs
 #/dev/donstest22   /expert         none    1 1 -rw,noglob # this is a
+ fS
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Output:

$ pm_1124511_comment_removal.pl
IS Comment: # This file is a comment and should not be used
IS Comment:  # This comment has one leading space
IS Comment:    # This comment has three leading spaces
NOT Comment: /dev/mapper/centos-var_log /var/log                xfs   
+  defaults        1 2
NOT Comment:  /dev/donstest  /expert         none    1 1 -rw,noglob # 
+this is a fs
IS Comment:  #/dev/donstest22   /expert         none    1 1 -rw,noglob
+ # this is a fS
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