private void RegExp(string input)
{
	string s = input;
	Regex regexp = new Regex("^[0-9]*_[A-Za-z]*$");	
	if(regexp.IsMatch(input))
		Console.WriteLine("Matched");
	else
		Console.WriteLine("Not Match");
}

private void RegExp(string input)
{
	string s = input;
	Regex regexp = new Regex("^[0-9]*_[A-Za-z]*$");	
	if(regexp.IsMatch(input))
		Console.WriteLine("Matched");
	else
		Console.WriteLine("Not Match");
}

both the above answers are wrong....... Reg Exp should say..

new Regex("[0-9]*_[A-Za-z]*");	

^ and $ mean beginning and end of a line... and the question
is to select that particular combination no matter where
they are present...... need not be entire line.

Raghuram is right and his regex is 100% correct. The other 
2 answers also have a string which does nothing:

string s = input;

so it is not necessary.