Can I said the word boundary is any character in class \w?(e,g, space, tab, newline?)

Not really, no. To illustrate the difference:

my $string = 'run rabbit run';
print "Matched with word boundary\n" if $string =~ /\brun\b/;
print "Matched with non-word char\n" if $string =~ /\Wrun\W/;
[download]

Update: I (and the above code) assume you meant 'class \W' rather than 'class \w.' If you meant 'class \s', the answer is the same: