Character position in .net web browser

We are striving to develop an application, where users will capture data from html document showing on .net web browser control.

Requirement is to store starting and ending character position along with the value, so that, later on, when user will click on the captured value, document will scroll and highlight value captured. However, value can be repeated in the HTML document, we have to find out the exact char position from where it was captured.

Can someone guide to deal with this peculiar problem?

Thanks.