Questions about regularity

<code>       function palindrome(str) {
         return str.replace(/[\W_]/g,'').toLowerCase() === str.replace(/[\W_]/g,'').toLowerCase().split('').reverse().join('');
        }
    
  palindrome("0_0 (: /-\ :) 0-0");
</code>

Why W does not match underscore_ Doesn’t
W match non-text and numbers?