QA regex: How does the "Grouped search expression" function?

Based on the best documentation I can found, I have prepared a very simple file to test this function.

German: Seite 1 -> English: Page 2 

For this I have coded 2 regexes:

Source: Seite (\d+)

Target: Page (\d+)

I get the error message -> "Found in source but not in target"

Although the documentation mentions that a backreference should be used in the target, this would be Page \1, but this is not accepted in the regex editor.

What should be coded?