Validation for Model Object using Fluent Validation
public bool IsValid(string stringToValidate)
{
    var charString = stringToValidate.ToCharArray();
    if (Regex.IsMatch(charString[0].ToString(), &quot;[0-9]&quot;) || Regex.IsMatch(charString[1].ToString(), &quot;[0-9]&quot;))
    {
        return false;
    }
    return true;
}


Be aware that each char in a string can be reached by index as stringToValidate[0], so you don't need to call ToCharArray():
public bool IsValid(string stringToValidate)
{        
    if (Regex.IsMatch(stringToValidate[0].ToString(), &quot;[0-9]&quot;) || Regex.IsMatch(stringToValidate[1].ToString(), &quot;[0-9]&quot;))
    {
        return false;
    }
    return true;
}

But Char has a static method Char.IsLetter() that can be used instead of the Regex expression:
public bool IsValid(string stringToValidate)
{
     return stringToValidate.Length &gt; 1 &amp;&amp; Char.IsLetter(stringToValidate[0]) &amp;&amp; Char.IsLetter(stringToValidate[1]);
}

This is more consistent with modern unicode character sets, and it will also catch if the two first characters are special chars like '&amp;' or '!' etc. If you just want to ensure they are not numeric, you can use Char.IsDigit() instead.