Get content inside string

A simple way to do this is to locate the position of the two parentheses and get the inner part, like this:

using static System.Console;

public class Program {
    public static void Main() {
        var texto = "ZACKSON MOREIRA MORGAN (zackson.morgan)";
        WriteLine(pegaTexto(texto));
        texto = "ZACKSON MOREIRA MORGAN (zackson.morgan";
        WriteLine(pegaTexto(texto));
        texto = "ZACKSON MOREIRA MORGAN (";
        WriteLine(pegaTexto(texto));
    }
    public static string pegaTexto(string texto) {
        texto += ")";
        texto = texto.Substring(texto.IndexOf("(") + 1);
        return texto.Substring(0, texto.IndexOf(")"));
    }
}

See running dotNetFiddle .

While the @bigown solution largely solves the issue, there is the regular expression version to solve the problem:

private static Regex regex = new Regex("^.*\((?<login>.*)\)$", RegexOptions.Compiled | RegexOptions.ExplicitCapture);

public static void Main()
{
    WriteLine(GetLogin("ZACKSON MOREIRA MORGAN (zackson.morgan)"));
    WriteLine(GetLogin("ZACKSON MOREIRA MORGAN (zackson.morgan"));
    WriteLine(GetLogin("ZACKSON MOREIRA MORGAN )"));
    WriteLine(GetLogin("ZACKSON MOREIRA MORGAN ("));
    WriteLine(GetLogin("ZACKSON MOREIRA MORGAN"));
}

private static string GetLogin(string input)
{               
    var res = regex.Match(input);
    if(res.Success)
        return res.Groups["login"].Value;
    else
        return "No matches found";
}

In this case, the (?<input>.*) syntax captures all characters between parentheses (defined by \( and \) ).

See working on dotNetFiddle .