VB.NET find the next block of text inside a textbox beginning at position X

given a large amount of text inside  a textbox (it is HTML code), I need to be able to do the following:

starting at character position X, find the next instance of a string.  WHen the string is found, return the new  position of the first character after the identified string.

I.E.  For lets say I have text:
The cat chased the mouse and the dog chased the cat.

Position = 5.  Find next instance of string "the"
Return the new position of the first character after the found string, and need code to exit if string is not found.

this would return "the" after the word "chased" and the new character position would be 18


Thanks for your help.