Regular Expression for Currency

I am trying to come up with a regular expression to validate some dynamic controls.  

I need to be able to validate against certain inputs, such as:

Text box can be empty - this is fine, display no error
Text box can have a whole number e.g. 100 - this is fine, display no error
Text box can have a decimal but must be 2 decimal places maximum (1 is allowed) - display error only if greater than 3 decimal places
Text box should not contain just a decimal point - otherwise display an error
Text box should not contain any other characters than digits and 1 decimal point - otherwise display an error

isValid = Regex.IsMatch(item.ItemAmount.ToString(), "^[0-9]+(.[0-9]{1,2})?$");

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The above is what I currently have but it doesn't seem to work, on validation it fails for 100, 100.20 etc.