Difference between Plus[x_, y_] and Plus[_, _] [duplicate]

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Why did MatchQ[a + b + c, Plus[x_, y_] is considered true?

In my opinion, it should only consider 2 elements within Plus, such as MatchQ[a + b, Plus[_, _]] true.

Why is this MatchQ[a + b, Plus[_, _]] false?

Where did I get wrong? Please point out.

asked Feb 29, 2020 at 12:38

kile

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$\begingroup$ @LeonidShifrin Adding HoldPattern is simply not working. And How can I make MatchQ[a + b, Plus[_, _]] true? $\endgroup$
– kile
Feb 29, 2020 at 13:02
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$\begingroup$ MatchQ[a + b, HoldPattern[Plus[_, _]]] works for me. $\endgroup$
– Leonid Shifrin
Feb 29, 2020 at 13:06
$\begingroup$ @LeonidShifrin I thought it was like this. MatchQ[(a + b + c), Plus[_, _, _]] // HoldPattern I am new to HoldPattern. Sorry for this. $\endgroup$
– kile
Feb 29, 2020 at 13:08
$\begingroup$ HoldPattern is wrapped around the pattern which you don't want to evaluate. It may be a good idea for you to read more on the language core / basics. You can search this site for resources, there are plenty of recommendations. $\endgroup$
– Leonid Shifrin
Feb 29, 2020 at 13:11
$\begingroup$ See the examples in the docs for HoldPattern $\endgroup$
– Michael E2
Feb 29, 2020 at 15:22

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