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

The below expression will find the last number set in a filename before its extension (excluding dot (.)).

'\d+(?=\.\w+$)'

file4.txt will match 4.

file123.txt will match 123.

demo.3.js will match 3 and so on.