Any employer who permits the entry upon premises occupied by him or under his control of a laborer or an assistant hired by an employe or contractor, for the performance upon such premises of a part of such employer's regular business entrusted to that employe or contractor, shall be liable for the payment of compensation to such laborer or assistant unless such hiring employe or contractor, if primarily liable for the payment of such compensation, has secured the payment thereof as provided for in this act. Any employer or his insurer who shall become liable hereunder for such compensation may recover the amount thereof paid and any necessary expenses from another person if the latter is primarily liable therefor.

For purposes of this subsection (b), the term “contractor” shall have the meaning ascribed in section 105 of this act.¹