Regex should allow German Umlauts in C# - regex

Question

I am using following regular expression:
[RegularExpression(#"^[A-Za-z0-9äöüÄÖÜß]+(?:[\._-äöüÄÖÜß][A-Za-z0-9]+)*$", ErrorMessageResourceName = "Error_User_UsernameFormat", ErrorMessageResourceType = typeof(Properties.Resources))]
Now I want to improve it the way it will allow German Umlauts(äöüÄÖÜß).

Answer 1

The way you added German letters to your regex, it will only be possible to use German letters in the first word.
You need to put the letters into the last character class:
#"^[A-Za-z0-9äöüÄÖÜß]+(?:[._-][A-Za-z0-9äöüÄÖÜß]+)*$"
^^^^^^^
See the regex demo
Also, note that _-ä creates a range inside a character class that matches a lot more than just a _, - and ä (and does not even match - as it is not present in the range).
Note that if you validate on the server side only, and want to match any Unicode letters, you may also consider using
#"^[\p{L}0-9]+(?:[._-][\p{L}0-9]+)*$"
Where \p{L} matches any Unicode letter. Another way to write [\p{L}0-9] would be [^\W_], but in .NET, it would also match all Unicode digits while 0-9 will only match ASCII digits.

Answer 2

replace [A-Za-z0-9äöüÄÖÜß] with [\w]. \w already contains Umlauts.

Answer 3

This works better i just modified somebody else his code who posted it on Stackoverflow. this works good for German language encoding.
I just added this code (c >= 'Ä' && c <= 'ä') and now it is working more towards my needs. Not all German letters are supported you need to create your own (c >= 'Ö' && c <= 'ö') type to add the letters u are having a issue with.
public static string RemoveSpecialCharacters(this string str)
{
StringBuilder sb = new StringBuilder();
foreach (char c in str)
{
if ((c >= '0' && c <= '9') || (c >= 'Ö' && c <= 'ö') || (c >= 'Ü' && c <= 'ü') || (c >= 'Ä' && c <= 'ä') || (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z') || c == '.' || c == ' ')
{
sb.Append(c);
}
}
return clean(sb);
}