Taking a special substring

Hi everyone ! I'm still trying to do that simple parser with C#. I was wondering if there is an easy way to take a substring out of another. I'd need to take every characters that form a number and put them together and form my number.
Ex: 1234 + 5678
Would store 1234 and 5678 so I can reuse them later.

Ok, I've found out how to do it, but how do I tell the compilator that I will need an unknown number of strings ?
Here's the function's code:

Code:

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public static void GetNumbers( )
{
        string[] temp = new string[ 3 ] ;
        temp = Maths.IO.GetEquation( ).Split( ' ' ) ;
        for( int i = 0 ; i < temp.Length ; ++i )
                if( temp[i] != "+" && temp[i] != "-" && temp[i] != "*" && temp[i] != "/" )
                        Numbers.Add( temp[i] ) ;
}

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If you have an unknown quantity, you need a collection, not an array. You can use generic collections like ArrayList that store objects, or you can save yourself a lot of casting to and from string and use System.Collections.Specialized.StringCollection.