c# Regex question: only letters, numbers and a dot (2 to 20 chars) allowed -

I am wrestling with my regex. I want to allow only one letter and numbers and one point in one username, and 2 to 20 characters, so I thought about something

[0- 9a-zA-Z] {2,20}

But then 21 characters are also ok, and that is not what I want

I recommend that you make two checks - for content based on one more fact for length, you probably want a point on the name rather than any number Are there. I think the name like usernames and user.name are allowed.

It should get content (but this allows at least):

  ^ \ w + (\. \ W +)? $  

If you do not want underscores, you can enter the 0- 9A-GAA-ZE + + \ w + On. To understand, it will match any string in which there will be one or more letter characters, after which after one or more word characters after exactly 0 or 1. It should also match the beginning and end of the string, that is, no other character is allowed in the string.

Then you only need to get length with a simple length check.