You should explain this for better suggestions for your real problem.
So far, as it is still not clear, what exactly you try to do, I stay with guessing and take it as a given, that it is needed to work on the array of characters collect(str) and for this I propose this line:
cs=collect(str)
result=deleteat!(cs,findall(x -> x != nothing, match.(r,string.(cs))))
But I am also sure, that this is not what you should use, whatever real problem you try to solve.