How can I replace multiple whitespace with a single space in Perl?

Question

Why is this not working?

$data = "What    is the STATUS of your mind right now?";

$data =~ tr/ +/ /;

print $data;

Answer 1

Use $data =~ s/ +/ /; instead.

Explanation:

The tr is the translation operator. An important thing to note about this is that regex modifiers do not apply in a translation statement (excepting - which still indicates a range). So when you use
tr/ +/ / you're saying "Take every instance of the characters space and + and translate them to a space". In other words, the tr thinks of the space and + as individual characters, not a regular expression.

Demonstration:

$data = "What    is the STA++TUS of your mind right now?";

$data =~ tr/ +/ /;

print $data; #Prints "What    is the STA  TUS of your mind right now?"

Using s does what you're looking for, by saying "match any number of consecutive spaces (at least one instance) and substitute them with a single space". You may also want to use something like
s/ +/ /g; if there's more than one place you want the substitution to occur (g meaning to apply globally).

Answer 2

You can also use tr with the "squash" option, which will remove duplicate replaced characters. See perlop for details.

my $s = "foo      bar   fubb";
$s =~ tr/ //s;

Answer 3

Perl 5.10 has a new character class, \h, the stands for horizontal whitespace which is nice for this sort of thing:

 $s =~ s/\h+/ /g;