string[] file = File.ReadAllLines(fullPath, Encoding.Default);
string pattern = @"(Index[ ]+[0-9]+){1}";
Regex r = new Regex(pattern, RegexOptions.IgnoreCase);       
foreach (string s in file)
{            
    string str = r.Replace(s, String.Empty);
    <...>
}

	int Func(string fullPath)
        {
            int count = 0;
            try
            {
                string[] file = File.ReadAllLines(fullPath, Encoding.Default);
                string pattern = @"(Index[ ]+[0-9]+){1}";
                Regex r = new Regex(pattern, RegexOptions.IgnoreCase);               
                for (int i = 0; i < file.Count(); i++)
                {
                    if (r.IsMatch(file[i]))
                    {
                        file[i] = r.Replace(file[i], String.Empty);
                        count++;
                    }                   
                }
                if (count > 0)
                {
                    File.WriteAllLines(fullPath, file, Encoding.Default);
                }
            }
            catch (Exception ex)
            {
                LastError = ex.Message;
                return -1;
            }
            return count;
        }