Write a Python regular expression to extract all dates in the format "DD/MM/YYYY" from a given string. Assume the day (DD) ranges from 01 to 31, month (MM) ranges from 01 to 12, and year (YYYY) ranges from 1900 to 2099.
The project started on 10/04/2022 and is expected to end by 15/08/2023. There was an initial discussion on 01/01/2020.