You're misreading the regex. \u200c is a single whitespace character. http://www.fileformat.info/info/unicode/char/200c/index.htm

But that's a weird character to put in a comment line! I don't get how this would happen accidentally.

You really can't think this way when accepting input from users.

What difference does it make if it got in there accidentally or on purpose?

Stackoverflow is a programmers site, you must expect that a programmmer might go, "Hmm, they're trimming whitespace, wonder what happens if I put 20,000 unicode whitespace characters in there instead of normal whitespace"?

Runaway automatic search-and-replace? There's no way to distinguish intent.

Runaway search and replace won't put a single 200 width whitespace character AFAICT

That's the meaning of "runaway" - Notepad++ had a search&replace that went into a somewhat random, long and uninterruptible loop if you were replacing using some types of regex and searhing forward in the file - you had to search backwards.