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Zero or one

Matching "zero or one" of something is helpful sometimes. String
patterns have the usual ? regex, but _, __ and ___ aren't quite the
same for function definitions and MatchQ[ ]. For these occasions,
there's the following bit of syntactic sugar.

The pattern

  ___ ? (Length[{#}] == 0&)

is one way to say "zero of something". Wrapped into a function, we get
something like

  optional[expr_] := expr | ___ ? (Length[{#}] == 0&);

as a way of saying "zero or one of expr". Examples:

  MatchQ[{1,3}, {1, optional[2], 3}]

  MatchQ[{1,2, 3}, {1, optional[2], 3}]

  MatchQ[{1, 2, 2, 3}, {1, optional[2], 3}]

  MatchQ[{1, "foo", 3}, {1, optional[_String | _Integer], 3}]

  MatchQ[{1, "foo", "bar", 3}, {1, optional[_String | _Integer], 3}]

Generalizations over a sequence of expressions, or to count between
exactly m and n repetitions of an expression, are also possible.


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