Built-in Mathematica Symbol

Cases

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

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

Level -1 consists of numbers, symbols and other objects that do not have subparts.

With the option setting Heads->True, Cases looks at heads of expressions, and their parts.

Cases traverses the parts of expr in a depth-first order, with leaves visited before roots.

Find cases that explicitly match integers:

Find cases that do not match integers:

Return the x from inside each f[x_] matched:

Find cases that explicitly match integers:

Find cases that do not match integers:

Return the x from inside each f[x_] matched:

Find all cases of lists of two elements:

Find the sum of every pair of elements:

Pick out elements that are not 0:

Pick out cases of integers down to level 2:

Use :> to evaluate only after explicit elements have been found: