How to get an array index in LINQ

I get two strings as input from two separate sources (over which I have no control). The first string strInput will contain one 'X', the position of which should be equal to the value of an element in the pattern string. I need the index of the element in pattern.

I've got it working in regular code using 'for' loops but would like to use LINQ and/or lambdas to make it more succinct. The following returns the value 4, but what I need is the index, which in this case is '1'.

string strInput = "    X       ";           // String from form (this is 4 spaces followed by X followed by n spaces
string pattern = "0,4,6,8,11";              // String containing matching positions of X in string

string place = strInput.IndexOf("X").ToString();        // Get position of X
string[] positions = pattern.Split(',');

var thisIndex = from pos in positions
                where (pos == place)
                select pos.First();

foreach (var x in thisIndex)
{
    Console.WriteLine("This is the index of X: {0}", x);
    Console.ReadLine();
}

Any help would be appreciated.

Tags: C#, .NET, LINQ