Question

What’s the easiest and fastest way to find a sub-string(template) in a string and replace it with something else following the template’s letter case (if all lower case – replace with lowercase, if all upper case – replace with uppercase, if begins with uppercase and so on…)

so if the substring is in curly braces

"{template}" becomes "replaced content"
"{TEMPLATE}" becomes "REPLACED CONTENT" and
"{Template}" becomes "Replaced content" but
"{tEMPLATE}" becomes "rEPLACED CONTENT"

Answer 1

Ended up doing that:

public static string ReplaceWithTemplate(this string original, string pattern, string replacement)
{
  var template = Regex.Match(original, pattern, RegexOptions.IgnoreCase).Value.Remove(0, 1);
  template = template.Remove(template.Length - 1);
  var chars = new List<char>();
  var isLetter = false;
  for (int i = 0; i < replacement.Length; i++)
  {
     if (i < (template.Length)) isLetter = Char.IsUpper(template[i]);
     chars.Add(Convert.ToChar(
                       isLetter ? Char.ToUpper(replacement[i]) 
                                : Char.ToLower(replacement[i])));
  }

  return new string(chars.ToArray());
}