regular expression to match firstname.lastname or just firstname with length limit

I need a regex to use in .net that will match a user name as firstname or firstname.lastname with a maxlength of 50 characters. The firstname case is easy but with firstname.lastname I can't figure out how to only allow 50 minus len(firstname.) in the lastname.

^([开发者_Python百科a-zA-Z]{1,50})|([a-zA-Z]+[.][a-zA-Z]+)$

will match the firstname case but the second case of firstname.lastname will allow infinite characters in the firstname and lastname.

Easily done. Here is a C# snippet with a commented regex:

if (Regex.IsMatch(userNameString, 
    @"# Match username with max length of 50 chars
    ^            # Anchor to start of string.
    (?!.{51})    # Assert length is 50 or less.
    [A-Za-z]+    # First name.
    (?:          # Optional second name.
      \.         # Required dot separator.
      [A-Za-z]+  # Last name.
    )?           # Last name is optional.
    $            # Anchor to end of string.
    ", 
    RegexOptions.IgnorePatternWhitespace)) {
    // Valid username
} else {
    // Invalid username
} 

^(?i)(?=(?:[a-z]\.?){1,50}$)[a-z]+(?:\.[a-z]+)?$

Will only match if number of [a-z]s is between 1 and 50.

If the total length can never be more than 50, you can use this instead:

^(?i)(?=.{1,50}$)[a-z]+(?:\.[a-z]+)?$

This would do it.

^(?:([a-zA-Z]{1,50})|(?=.{1,51}$)[a-zA-Z]+[.][a-zA-Z]+)$

Essentially, I added a lookaround saying match (without capturing) 1-51 chars and then the end of line. I used 51 because I'm assuming that the . should count in the length calculation.

Using a two-step validation would be so much easier and I don't see a reason to avoid it.