first of all: " I would prefer a format which allows me to afterwards place it in a 'date field' via the Podio workflow, but this is not a requirement."
That isn't possible, cause a Podio Flow can't pull values from a calculation field.
To get the wanted substring you don't need a regex. This gets it:
But to popluate a date field with start/end date you need Globiflow. And if you use Globiflow for populating you don't need the calculation field cause you can do the string to date conversion in the flow.