The interviewer is not asking to solve a problem here, they're asking for a simple ability to follow instructions, hence the offer to use Google to find the correct answer.
You could make a very solid case for using "in" (it is 2-4x faster), but only after you've solved the task at hand, this is what is expected in interviews. Not knowing the interview meta makes an average Joe basically unhireable in this market.
The existence of a more conventional .find method does some damage to my original point. Oops.
The reason is the intent behind their question, which they don't vocalize.
This question means we are dealing with an extremely broad hiring funnel designed to fail people who can't FizzBuzz and need to keep answers at MVP level.
In other words, if you are asked to put out a fire use a bucket of sand, not a state-of-the-art fire extinguisher.
Maybe this was a very junior position, but I'm with the interviewer here. Using regex would be very questionable - and a solid case of 1171.