You may wish to avoid back references.

/(A{5}|B{5}|C{5}|D{5})/
[download]

Or for your other case:

/(A{2,5}|B{2,5}|C{2,5}|D{2,4})/

/(A{2,5}|B{2,5}|C{2,5}|D{2,5})/
[download]

Update: fixed typo per Nkuvu