Details

This message is generated when the first argument in an Optional pattern is not an expression based on Blank.
Optional[p,v] can also be entered using the notation p:v.
Off[message] switches off the message; On[message] switches it on. For example: Off[General::optb].

Examples

Optional pattern uses __ (BlankSequence) rather than _ (Blank):

Wolfram Language code: {{1}, {2}, {}} /. {Optional[__, 0]} :> p

This shows a valid Optional pattern:

Wolfram Language code: {{1}, {2}, {}} /. {Optional[p_, 0]} :> p

This pattern can also be entered using shorter notation:

Wolfram Language code: {{1}, {2}, {}} /. {p_ : 0} :> p

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