This is an irregular verb. It follows the conjugation pattern of the verb chegar.
Present:
A Ana rega as plantas todas as manhãs.
Ana waters the plants every morning.
Preterite:
O Miguel regou o jardim à tarde.
Miguel watered the garden in the afternoon.
Conditional:
A Clara regaria as flores se estivessem secas.
Clara would water the flowers if they looked dry.