How to force text to be Upper case except selective strings c#

Question

C#:

string mystring = "Hello World. & my name is < bob >. Thank You."
Console.Writeline(mystring.ToUpper())

I am trying to get all the text to be uppercase except--

&  <  > 

Because these are my encoding and the encoding wont work unless the text is lower case.

Answer 1

You may split the string with a space, turn all the items not starting with & to upper and just keep the rest as is, and then join back into a string:

string mystring = "Hello World. & my name is < bob >. Thank You.";
string result = string.Join(" ", mystring.Split(' ').Select(m => m.StartsWith("&") ? m : m.ToUpper()));

Another approach is to use a regex to match &, 1+ word chars and then a ;, and match and capture other 1+ word char chunks and only turn to upper case the contents in Group 1:

var result = System.Text.RegularExpressions.Regex.Replace(mystring, 
    @"&\w+;|(\w+)", m => 
           m.Groups[1].Success ? m.Groups[1].Value.ToUpper() :
           m.Value
);