测试了这个好像可以
// Regex match
RegexOptions options = RegexOptions.None;
Regex regex = new Regex(@"[0-9a-zA-Z][0-9a-zA-Z]*[0-9a-zA-Z]", options);
string input = @"a11111a111q243rfgq34rq24f222222222";
// Check for match
bool isMatch = regex.IsMatch(input);
if( isMatch )
{
// TODO: Do something with result
System.Windows.Forms.MessageBox.Show(input, "IsMatch");
}