Cases
Extracts elements from an expression that match a pattern.
$ wo 'Cases[{a, b, a}, a]'
{a, a}
Cases with Except pattern
$ wo 'Cases[{1, 2, 3, 4, 5}, Except[3]]'
{1, 2, 4, 5}
Cases with Alternatives
$ wo 'Cases[{1, 2, 3, 4, 5}, Except[2 | 4]]'
{1, 3, 5}
Cases with level specification
$ wo 'Cases[{{1, 2}, {3, 4}}, _Integer, {2}]'
{1, 2, 3, 4}