penguinfuz has asked for the wisdom of the Perl Monks concerning the following question:

Whenever I use a specific regular expression for exclusion, I still get a value returned. Here is a snippet... 
while(($name,$pass,$uid,$gid,$quota,$comment,$gcos,$dir,$shell) = getp
+went()) {
  if(!($shell =~ m/false/ || m/null/ || m//)) {
  print blah...
  }
}

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The users with "false" and "" shells are excluded, but "null" shells still get printed; However, the exclusion works if I populate the variables by using split... 
...
while(<PW>) {
  ($name,$pass,$uid,$gid,$quota,$comment,$gcos,$dir,$shell) = split(/:
+/);
   print blah...
}

[download]
Can anyone offer me a bit of insight as to what's happening here?

Replies are listed 'Best First'.
Re: regex and getpwent()...
by japhy (Canon) on May 28, 2001 at 05:01 UTC
    Your first regex is done on $shell, but the second two are done on $_. I think you want to take an approach like: 
    if ($shell ne "" and $shell !~ /false|null/) {
  # it's ok
}

    [download]
    You could, of course, keep the other two regexes separate: 
    if ($shell ne "" and $shell !~ /false/ and $shell !~ /null/) {
  # it's ok
}

    [download]
    Also, the pattern // does NOT match empty strings -- it uses the last matched pattern again: 
    "foo" =~ /foo/;
print grep //, qw( bar foobar );  # foobar

    [download]
    To match empty strings, either use /^$/ or /^\z/, depending on your definition of empty.

    japhy -- Perl and Regex Hacker
[reply]
[d/l]
[select]
Re: regex and getpwent()...
by bobione (Pilgrim) on May 28, 2001 at 05:10 UTC
    I am not sure that m// match empty ("") value. It matches everything.

    BobiOne KenoBi ;)

[reply]
      It only matches everything until you've matched anything. (Think that's cryptic?) 
      for (qw( a b403 c<<>_# d84930 e89293->><{}W )) {
  print if //;
}

"foo" =~ /foo/;

for (qw( a b403 c<<>_# d84930 e89293->><{}W )) {
  print if //;
}

      [download]


      japhy -- Perl and Regex Hacker
[reply]
[d/l]
        Ouch! That's quite nasty looking...
        Looks like I really need to get my head around regular expressions a bit better! 8)

        Thanks for the help.
[reply]

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