fixdetach.cs

string pattern = @"_detached.*";
string replacement = "";
Regex rgx = new Regex(pattern);
fileName = rgx.Replace(fileName, replacement);
if (fileName.EndsWith(".rvt")) fileName = fileName.Replace(".rvt", String.Empty);
Source_File_Rvt = filename;

fixdetach.py

import re
test = ["dadaa__detached_1","dadaa__detached_3","dadaa__detached_2"]
# pattern regex to match the string with _detached and remove all _detached and all characters after it
pattern = re.compile(r'_detached.*')
# replace the string with empty string
test = [pattern.sub('', i) for i in test]
# print the list
print(test)