invalid scape sequence when trying to run jldoctest with filters as standard test

[Leandro Martínez]

I have a test with a filter:

```jldoctest; filter = r"(\d*)\.(\d{4})\d+" => s""

This filter works fine when generating the docs, for example with LiveServer.servedocs().

However, I want to run the doc tests as part of the standard test suite, and I add this to my runtests.jl file:

@testitem "Doctests" begin
    using Documenter: doctest
    doctest(CellListMap)
end

But, then, the filter does not work, throwing the error:

```jldoctest; filter = r"(\d*)\.(\d{4})\d+" => s""
#                         └┘ ── invalid escape sequence

[Leandro Martínez]

Sorry, there is something else wrong here.