RegEx for numbers, letters & underscores only

I have this code:

public bool IsAlphaNumeric(String strToCheck)
{
  Regex objAlphaNumericPattern=new Regex("[^a-zA-Z0-9]");
  return !objAlphaNumericPattern.IsMatch(strToCheck); 
} 

how do I modify the Regex to also accept underscores? 
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