Built-in Mathematica Symbol

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[expr, pattern] tests all the subparts of expr in turn to try and find ones that match pattern.

Position returns a list of positions in a form suitable for use in Extract, ReplacePart and MapAt. The form is different from the one used in Part.

The default level specification for Position is {0, Infinity}, with Heads->True.

Position[list, pattern, {1}, Heads->False] finds positions only of objects that appear as complete elements of list.

With the default option setting Heads->True, Position includes heads of expressions, and their parts.

Position traverses expressions in depth-first order, yielding lists of indices in lexicographic order.

Find the positions at which b occurs:

Find all positions at which x to any power appears:

Find only those down to level 2: