What does ?= mean in a regular expression?

Question

May I know what ?= means in a regular expression? For example, what is its significance in this expression:

(?=.*\\d).

Answer 1

If you want to mask a credit card number (execpt) the last 4 digits/characters, you could use ?= in your regex.

cc = "1234-5678"
cc.replace(/.(?=....)/g, '#');

cc = #####5678