How to find and replace a string without case sensitivity?

Question

JavaScript

I know how to search. But here's how to replace the original no

function replace (str) {
  var mask = 'сан'
  return str.replace(new RegExp(mask, 'gi'), '<strong> + mask +</strong>')
}

replace('Санкт-Петербург');

// <strong>сан</strong>кт-петербург
// а нужно // <strong>Cан</strong>кт-петербург

Answer 1

str.replace(RegExp(mask, 'gi'), '<strong>$&</strong>')