Fix #989

[mmatera]

This is a more elegant way to fix #989 than #990

[rocky]

I think you have to use mark a lookahead, i.e. (?<= ...) (?=...) somehow.

[mmatera]

You are right! I hope now it works...

[mmatera]

So, the unique thing that deserves revision is the test in the docstring. @rocky, if you agree, I merge this.

[rocky]

If you ask my opinion, for an example it would need more explanation in the example.

For example to point out that this casually this looks similar to ReplaceAll[] : if /.x were used instead of /.2.

Otherwise I think it would be better done as a test in test/test_evaluation.py because a /.5 == 2 a isn't all that interesting in of itself.

Your choice though.

And feel free to merge whenever you like.