@Eric Johnstone please do not duplicate discussions, it is against the forum rules: http://wolfr.am/READ-1ST

The disadvantage of Replace is that you can easily make complicated riddles like this one. Most experienced programmers don't solve these riddles - they just avoid them completely.

As convention, people often add a "$" to the front of variables that are "internal". So let's try that:

a = v;
a b /. $a_ $b_ -> $a + $b

This works. There's no confusion about what the "a" on the right hand side of the arrow means.

In general you want to avoid undescriptive variable names. That also avoids this problem.

Now to explain what happens. a = v; a b /. a_ b_ -> a + b

First a is replaced by v per its definition. So we effectively have:

 v b /. a_ b_ -> v + b

"Then the product is reordered alphabetically because "v b" is equal to "b v". So now we have "b v".

 b v /. a_ b_ -> v + b

By the substitution, we get b + b or 2b.

Thank you, Gianluca.

That has solved the problem. I have a long list of rules. Would it be better to always use delayed evaluation? This one is only that rule that seems to be problematic, but there might be other undiscovered gotchas hidden in the list.

Eric

ETA There were other problems. The delayed evaluation worked when applied to the complete list.