I already gone through some question in Stack Overflow regarding this but nothing helped much in my case.

I've tested this on Regex101, but for future reference, this was my "test-suite": ".txt" is a perfectly valid, if not odd, filename in windows.My file name should only contain these characters:'A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z,a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z,0,1,2,3,4,5,6,7,8,9,_,.,@,-,(,), ' and my extension only should accept files.My current function is pretty complicated and requires nested looping what I don't like. (files|file|Dateien|fichiers|bestanden|archivos|filer|tiedostot|pliki|soubory|elemei|ficheiros|arquivos|dosyalar|datoteke|fitxers|failid|fails|bylos|fajlovi|fitxategiak)$ const string pattern = "^(\W|_vti|_)|[^\w]|(files|file|Dateien|fichiers|bestanden|archivos|filer|tiedostot|pliki|soubory|elemei|ficheiros|arquivos|dosyalar|datoteke|fitxers|failid|fails|bylos|fajlovi|fitxategiak)$"; const string str = "Doc with & opr.docx"; var regexed Str = Regex. Add Or Update Item(String bstr Url, String bstr List Name, Boolean b Add, Boolean b System Update, Boolean b Preserve Item Version, Boolean b Preserve Item UIVersion, Boolean b Update No Version, Int32& pl ID, String& pbstr Guid, Guid pbstr New Doc Id, Boolean b Has New Doc Id, String bstr Version, Object& pvar Attachment Names, Object& pvar Attachment Contents, Object& pvar Properties, Boolean b Check Out, Boolean b Checkin, Boolean b Un Restricted Update In Progress, Boolean b Migration, Boolean b Publish, String bstr File Name, ISP2DSafe Array Writer p List Data Validation Callback, ISP2DSafe Array Writer p Restrict Insert Callback, ISP2DSafe Array Writer p Unique Field Callback) // Matches any string that starts with a non-word character // any string that starts with '_vti' // any string that starts with an _ (underscore) (apparently this is a 'word character') ^(\W|_vti|_) // Matches any characters that are non-word characters [^\w] // Matches all the illegal file endings listed in the support article.