C# Regular Expression. Uniqueness of the character

Question

How can I check the uniqueness of the specified character in the line with regular expression. For example I have a line with different symbols. If there is only one dot(.) in the line then it must match with expression. I tried to do like this

public static void GetLines(List<string> lines)
    {
        Regex rx = new Regex(@"^.*\..*");
        foreach (var item in lines)
        {
            if (rx.IsMatch(item))
            {
                Console.WriteLine(item);
            }
        }
    }

This prints only lines where exists the dot(.) character. I think that this method will work if I write the expression like this @"^.*\\.{1}.*" But it doesn't work.

Answer 1

You need to modify your regex and write it like this if you want it to match to input that only and only contains single dot character.

Regex rx = new Regex(@"^[^.]*\.[^.]*$");

Explanation:

• [^.]* ensures it matches any character zero or more times but not dot
• . followed by a dot character
• [^.]* again ensures it matches any character zero or more times but not dot

Similarly you can do it for any character just like dot.