Replace all whitespace characters

You want \s

Matches a single white space character, including space, tab, form feed, line feed.

Equivalent to

[ \f\n\r\t\v\u00a0\u1680\u2000-\u200a\u2028\u2029\u202f\u205f\u3000\ufeff]

in Firefox and [ \f\n\r\t\v] in IE.

str = str.replace(/\s/g, "X");

\s is a meta character that covers all white space. You don't need to make it case-insensitive — white space doesn't have case.

str.replace(/\s/g, "X")

We can also use this if we want to change all multiple joined blank spaces with a single character:

str.replace(/\s+/g,'X');

See it in action here: https://regex101.com/r/d9d53G/1

Explanation

/ \s+ / g

• \s+ matches any whitespace character (equal to [\r\n\t\f\v ])
• + Quantifier — Matches between one and unlimited times, as many times as possible, giving back as needed (greedy)

• Global pattern flags
  • g modifier: global. All matches (don't return after first match)

Have you tried the \s?

str.replace(/\s/g, "X");