Position

Position[expr,pattern] gives a list of the positions at which objects matching pattern appear in expr. 

Position[expr,pattern,levelspec] finds only objects that appear on levels specified by levelspec. 

Position[expr,pattern,levelspec,n] gives the positions of the first n objects found. 

Position[pattern] represents an operator form of Position that can be applied to an expression.

Please visit the official Wolfram Language Reference for more details and examples on core symbols.