I like your regex solution! I learned a lot from it.

notes/reminders for my benefit:

1. replace returns a copy of the given string with modifications according to two parameters, target-to-change and replacement values.
2. .replace method accepts a call back function that takes the result from matches as arguments. so when the regex finds a match it can perform the task of converting each match toUpperCase.
3. logic of your regex: match any lower case character that appears at the start of a string or directly after a whitespace. - I like it!

I like how simple this is.
As a beginner i'm still writing out the long version and explicitly declaring statements, so this is very useful to see the shorthand versions of the same thing.

note for my benefit:
learnings from yours:

1. brackets not necessary for simple conditional test.
2. '!' operator understood as 'not true' is the same as '=== false'.