Dear Student,
Planaria has unique capability of regeneration due to presence of neoblast cells. These cells divide by mitosis. These cells can produce any cell planaria needs. So every portion of planaria has capability to regenerate into whole body.
It is not reproduction actually neoblast cells are similar to stem cells while has capability to generate whole body. It is different from reproduction as it can develop whole organism as well as only lost body part while reproduction only develops whole new organism.
Regards