A regular expression that will allow a string with only one Capital Letter

Question

The string should be 6 - 20 characters in length. And it should contain 1 Capital letter.

I can do this in code using C#:

string  st = "SomeString"

Regex rg = new Regex("[A-Z]");
MatchCollection mc = rg.Matches(st);

Console.WriteLine("Total Capital Letters: " + mc.Count);

if (mc.Count > 1)
{
  return false;
}

But what i really want is a Regular expression that will match my string if it only contains one capital. The string can start with a common letter and should have only letters.

Answer 1

This one will match a string that contains lowercase letters, then a single capital letter, then more lowercase letters.

^[a-z]*[A-Z][a-z]*$

You can adjust the first and last parts to include other characters as necessary, depending on your required character domain.

For all strings of length 6 to 20, containing only letters and at most one capital letter, you can use lookaheads:

(?=^[a-zA-Z]{6,20}$)^[a-z]*[A-Z][a-z]*$