Simple regex failing test to determine a 3 digit number

Simple regex failing test to determine a 3 digit number

I have had a difficult time wrapping my head around regular expressions.
In the following code, I used a Regex to determine if the data passed was
a 1 to 3 digit number. The expression worked if the data started with a
number (ex. "200"), but also passed if the data had a letter not in the
first digit (ex. "3A5"). I managed to handle the error with the
INT32.TryParse() method, but it seems there should be an easier way.
if (LSK == MainWindow.LSK6R)
{
int ci;
int length = SP_Command.Length;
if (length > 3) return MainWindow.ENTRY_OUT_OF_RANGE; //Cannot
be greater than 999
String pattern = @"[0-9]{1,3}"; //RegEx pattern for
1 to 3 digit number
if (Regex.IsMatch(SP_Command, pattern)) //Does not check for
^A-Z. See below.
{
bool test = Int32.TryParse(SP_Command, out ci); //Trying
to parse A-Z. Only if
if (test) //it no
letter will it succeed
{
FlightPlan.CostIndex = ci; //Update
the flightplan CI
CI.Text = ci.ToString(); //Update
the Init page
}
else return MainWindow.FORMAT_ERROR; //It
contained a letter
}
else return MainWindow.FORMAT_ERROR; //It
didn't fit the RegEx
}