Regex working in JS but not in a tester?

A friend made me a Regex for my calculator (/^0+\B/, “”) which stops the user from inputting 00. I´m new to Regex and I´m trying to understand them. However, when I enter the regex here:

It doesn´t seem to match 00. So what´s going on?

I believe you need to enter only the part between forward slashes (/) to check it on regex101