doctest(Module; fix=true) produces weird results

[lkastner]

I made a minimal working example. My doctest looks similar to:

``` jldoctest label
julia> a = ("a", "b", "c");                       

julia> a

```

(I left the output empty on purpose to trigger the error.)
Running Documenter.jl to fix the doctest gives:

```jldoctest label
julia
("a", "b", "c")> a = ("a", "b", "c");

julia> a

```

It seems like some regex is replacing the first appearance of a with the command + correct output, instead of the actual command + (wrong) output.