What is the proper way to check if a string is empty in Perl?

问题

I've just been using this code to check if a string is empty:

if ($str == "")
{
  // ...
}

And also the same with the not equals operator...

if ($str != "")
{
  // ...
}

This seems to work (I think), but I'm not sure it's the correct way, or if there are any unforeseen drawbacks. Something just doesn't feel right about it.

回答1:

For string comparisons in Perl, use eq or ne:

if ($str eq "")
{
  // ...
}

The == and != operators are numeric comparison operators. They will attempt to convert both operands to integers before comparing them.

See the perlop man page for more information.

回答2:

1. Due to the way that strings are stored in Perl, getting the length of a string is optimized.
   if (length $str) is a good way of checking that a string is non-empty.

2. If you're in a situation where you haven't already guarded against undef, then the catch-all for "non-empty" that won't warn is if (defined $str and length $str).

回答3:

As already mentioned by several people, eq is the right operator here.

If you use warnings; in your script, you'll get warnings about this (and many other useful things); I'd recommend use strict; as well.

回答4:

You probably want to use "eq" instead of "==". If you worry about some edge cases you may also want to check for undefined:

if (not defined $str) {

# this variable is undefined

}

回答5:

The very concept of a "proper" way to do anything, apart from using CPAN, is non existent in Perl.

Anyways those are numeric operators, you should use

if($foo eq "")

or

if(length($foo) == 0)

回答6:

To check for an empty string you could also do something as follows

if (!defined $val || $val eq '')
{
    # empty
}

来源：https://stackoverflow.com/questions/2045644/what-is-the-proper-way-to-check-if-a-string-is-empty-in-perl