Best Way to Code

String between two Strings

string input = "Exemple of value between two string FirstString text I want to keep SecondString end of my string";
var match = Regex.Match(input, @"FirstString (.+?) SecondString ").Groups[1].Value;

All Number between two Strings

string input = "Exemple of value between two string FirstString 1234 I want to keep SecondString end of my string";
var match = Regex.Match(input, @"FirstString (\d*) SecondString ").Groups[1].Value;