Cases

Cases[{e1,e2,…},pattern] gives a list of the ei that match the pattern. 

Cases[{e1,…},pattern->rhs] gives a list of the values of rhs corresponding to the ei that match the pattern. 

Cases[expr,pattern,levelspec] gives a list of all parts of expr on levels specified by levelspec that match the pattern. 

Cases[expr,pattern->rhs,levelspec] gives the values of rhs that match the pattern. 

Cases[expr,pattern,levelspec,n] gives the first n parts in expr that match the pattern. 

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

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