As i understand you want at least one letter and at least one special character ..then try this regex..hope this will help
if (System.Text.RegularExpressions.Regex.IsMatch("$#%hg#$Y%#$", @"^(?=.*[a-zA-Z])(?=.*[!@#$%^&*])"))
MessageBox.Show("Match found");
else
MessageBox.Show("Match not found");
Updated answer
for "Enter at least one uppercase and leter in uppercase and lower case & special character"
if (System.Text.RegularExpressions.Regex.IsMatch("$#%hg#$Y%#$", @"^(?=.*[a-z])(?=.*[A-Z])(?=.*[!@#$%^&*])"))
MessageBox.Show("Match found");
else
MessageBox.Show("Match not found");