Replace, unexpected behavior

Questions
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1

Mathematica 10

Can anybody explain the following behavior:

In:  Replace[Times[a, b], Times[x__] :> Dot[x]]   // FullForm
Out: Dot[Times[a,b]]

Why is “Times” not eliminated?

2

This was a bit tricky. I had to read the Trace pretty closely to figure it out.

Times[x___] turns into just x___.

So your code becomes:

Replace[Times[a, b], x__ :> Dot[x]]

Now x___ matches the entire expression so x is matched to Times[a, b]. This evaluates to

Dot[Times[a, b]]

Which in turn becomes just:

Times[a, b]

The key to preventing things like this is to use HoldPattern:

Replace[Times[a, b], HoldPattern[Times[x___]] :> Dot[x]]

The lesson I think is that it’s dangerous to mix pattern matching with expressions which might evaluate on the patterns.