✅ Index expressions are only supported with constants - Avalonia ReactiveUI

IObservable<bool> okUsername = (IObservable<bool>)this.WhenAnyValue(
  x => x.Username,
  x => IValidate.IsValidUsername(x.Username));

public class InputValidation
{
    public bool IsValidUsername(string username)
    {
        if (string.IsNullOrEmpty(username))
        {
            return false;
        }

        string pattern = @"^[a-zA-Z0-9!@#$%^&*_-]{7,12}$";
        return Regex.IsMatch(username, pattern);
    }

IObservable<bool> okUsername = (IObservable<bool>)this.WhenAnyValue(
  x => x.Username,
  x => IValidate.IsValidUsername(x.Username));

public class InputValidation
{
    public bool IsValidUsername(string username)
    {
        if (string.IsNullOrEmpty(username))
        {
            return false;
        }

        string pattern = @"^[a-zA-Z0-9!@#$%^&*_-]{7,12}$";
        return Regex.IsMatch(username, pattern);
    }