What does ?= mean in a regular expression?

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

(?=.*\d).

?= is a positive lookahead, a type of zero-width assertion. What it's saying is that the captured match must be followed by whatever is within the parentheses but that part isn't captured.

Your example means the match needs to be followed by zero or more characters and then a digit (but again that part isn't captured).

(?=pattern) is a zero-width positive lookahead assertion. For example, /\w+(?=\t)/ matches a word followed by a tab, without including the tab in $&.