String replacement

I have a string with markers in it. I want to replace the section of text with a formated section of text. For example: Original string = "This is a string. <CAP>This part should be capitalized.</CAP>" I want to use the "<CAP>" and "</CAP>" substrings as markers. I then want to take the text in the middle, pass it to a function, and replace the entire substring, including markers. Using the above example, I would pass "This part should be capitalized." to a function, in this case strtoupper, and replace it. The result would be: New string = "This is a string. THIS PART SHOULD BE CAPITALIZED." I figured out how to find the markers, but could you suggestions beyond that. Thanks. Peter