Char edit in a text box

Hi, how do I edit a char for example "R" in a text box and set it to a value..But I want to set all the "R"'s to "102"(Int)

Textbox1.Text = "Red Roses"
Textbox2.Text should then be "102ed 102oses"

and if that is posible i want to set all the char to a value
Between each char should be "_"
Space = +
e = 01
s = 23
d = 1
o = 00

Red Roses then must be : "102_01_1+102_00_23_01_23"

Then I would like to do the opposite : "102_01_1+102_00_23_01_23" = Red Roses

Please and thanks