The following minimal program results in a NoMatchError for line 2:

program simple
    character(len=10) :: foo = '\' // 'b'
end program simple

I believe this is correct Fortran--at least it compiles fine on Intel.

I'm still investigating myself. It's doesn't seem to be a problem with the regex pattern for the concat-op. I think it has something to do with the fact that backslashes in strings get automatically escaped to \\ in Python.

Interestingly, if I take out the line from the program block, i.e. just:

character(len=10) :: foo = '\' // 'b'

it works fine.