How to separate character and number part from string

Everyone and their mother will give you a solution using regex, so here's one that is not:

 // s is string of form ([A-Za-z])*([0-9])* ; char added
 int index = s.IndexOfAny(new char[] { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9' });
 string chars = s.Substring(0, index);
 int num = Int32.Parse(s.Substring(index));

Regular Expressions are best suited for this kind of work:

using System.Text.RegularExpressions;

Regex re = new Regex(@"([a-zA-Z]+)(\d+)");
Match result = re.Match(input);

string alphaPart = result.Groups[1].Value;
string numberPart = result.Groups[2].Value;

Use Linq to do this

string str = "OS234";

var digits = from c in str
             select c
             where Char.IsDigit(c);

var alphas = from c in str
             select c
             where !Char.IsDigit(c);