How to pass '\v\C[pattern]' into the built-in match() VimScript function

Question

I am attempting custom fold logic as my first VimScript.

What works for me: USING VERY MAGIC (\v) and explicit CASE SENSITIVE (\C), find python comments in foo.py that begin with hash, then Roman Numeral, a period, and finally .*$

"/\v\C^#(XC|XL|L?X{0,3})(IX|IV|V?I{0,3})\.\s.*$.@<="

I want to apply this pattern to tell Vim when to start a fold in my Python scripts. The same pattern that works in foo.py now stops working once I pass it to a function. I assign the pattern to a variable in match() in python.vim:

function! PythonFolds()
  let thisline = getline(v:lnum)

  let rom_num = "\v\C^#(XC|XL|L?X{0,3})(IX|IV|V?I{0,3})\.\s.*$.@<="
  if match(thisline, rom_num) >= 0
    return ">1"
  else
    return "=" 
  endif
endfunction

setlocal foldmethod=expr
setlocal foldexpr=PythonFolds()
setlocal foldcolumn=4

Basically when the above code is applied to a python.vim preference file, the folds are not applied. I have successfully tested simpler regex, which makes me believe the problem can be isolated as something I don't understand with regex/VimScript:

 if match(thisline, "^#") >= 0      " folds work as intended
   return ">1"

Additionally, the tutorials below suggest that \v and \C are both passable in VimScript.

• \v http://andrewradev.com/2011/05/08/vim-regexes/
• \C http://learnvimscriptthehardway.stevelosh.com/chapters/46.html

I am running Vim8 on Ubuntu 18.04.