Regular expression to find function definitions in Python file

I have a python project with function definitions written in CamelCase style. I'm trying to write a script to convert them to snake_case style.

CaseFormatter class

import re


class CaseFormatter:
   def __init__(self, file_directory):
       self.first_cap_re = re.compile('(.)([A-Z][a-z]+)')
       self.all_cap_re = re.compile('([a-z0-9])([A-Z])')
       self.functions_dict = {}
       self.file_directory = file_directory
       self.file = open(file_directory, "r", encoding="UTF-8")
       self.file_content = self.file.read()
       self.names_dictionary = {}

   def convert_camel_case_to_snake_case(self, name):
        """this function convert a camel case name to a snake case name """
       s1 = self.first_cap_re.sub(r'\1_\2', name)
       self.names_dictionary[name] = self.all_cap_re.sub(r'\1_\2', s1).lower()

   def find_camel_case_functions(self):
      """this function finds camel case functions in a file """
       s1 = re.findall("^\s*def (\S+)\s*\(\s*\S+\s*(?:,\s*\S+)*\):$", self.file_content)
    return s1

   def replace_functions(self):
       # file_content = open(self.file_directory, "r", encoding="UTF-8").read()
       self.file_content = self.file_content.replace(";", "")
       for key, val in self.names_dictionary.items():
           self.file_content = self.file_content.replace(key, val)
           # print(self.file_content)
       self.file = open(self.file_directory, "w", encoding="UTF-8")
       print(self.file_content)
       self.file.write(self.file_content)

testing the CaseFormatter class

import os
from CaseFormatter import *

walk_dir = 'some dirctory'

print('walk_dir = ' + walk_dir)
print('walk_dir (absolute) = ' + os.path.abspath(walk_dir))

for root, subDirs, files in os.walk(walk_dir):
    print('--\nroot = ' + root)

for filename in files:
    file_path = os.path.join(root, filename)
    if filename.endswith('.py') and filename != "__init__.py":
        print('\t- file %s (full path: %s)' % (filename, file_path))
        case_formatter = CaseFormatter(file_path)
        # print(case_formatter.find_camel_case_functions())
        for function_name in case_formatter.find_camel_case_functions():
            case_formatter.convert_camel_case_to_snake_case(function_name)
        print(case_formatter.names_dictionary)
        case_formatter.replace_functions()

I found the RegEx to find function definitions here. When I tried it on my project it gave me no results, the RegEx didn't work as I think. As an example of one of the files in the project:

class UnvoweledPattern(object):

   String = ''
   Rules = []
   IDs = []

   def __init__(self, string, rules, ids):

       self.String = string
       self.Rules = rules
       self.IDs = ids
       pass

   def GetRootsStringsAndRules(self, string):
       if (string == None):
           string = self.String

    rootStrings = []
       rootRules = []
       for j in range(len(self.Rules)):
           rootRule = ''
           rootString = ''
           for k in range(len(self.Rules[j])):
               rootRule += self.Rules[j][k]
               if self.Rules[j][k].isdigit():
                   rootString += string[int(self.Rules[j][k]) - 1]
               else:
                   rootString += self.Rules[j][k]
           rootStrings.append(rootString)
           rootRules.append(rootRule)

       return [rootStrings, rootRules]