Get an array of numbers

I am making a process to get a certain lines from a text starting at the bottom using the below:
System.Text.RegularExpressions.Regex.Split(yourStr,"\n")(Strnumberoflines - X)
yourStr = string variable of text
Strnumberoflines = how many lines the text have
X = number of lines away from bottom

Lets say I need to get 10 lines.
I need to get X be 1 then 2, 3, 4, 5,10
How can I make such a process.