var fileNameOnly1 = Regex.Replace(fileNameOnly, @"[_d]", string.Empty);

4a_32
abcdef43252_43242

4a2
abcdef432523242

4a
abcdef43252

_d+

var fileNameOnly = "asdads_234asd";
var result = Regex.Replace(fileNameOnly, @"_d+", string.Empty);
Console.WriteLine(result);

asdadsasd

var fileNameOnly1 = fileNameOnly.Split('_')[0];

Regex.Replace(fileNameOnly, @"_d+", string.Empty);