Why is ""="" == "" true?

It came from a statement like this. I almost always quote regexes (whether they need it or not) and in a test, I wanted to use the regex "=".

Code:
if "%1" != "" (set regex=%1) else (echoerr tgrep: Missing regex & quit)

I changed it to this.

Code:
if *%1* != ** (set regex=%1) else (echoerr tgrep: Missing regex & quit)