Change styling of current sentence RichTextBox in WPF/Silverlight

If anybody can point me in the right direction on this it would be great. Using

mainRichTextBox.CaretPosition

I'm wanting to get hold of the current sentence and change the styling on it. Now I know I could easily get hold of the actual sentence by searching for the previous "." etc. But once I have hold of this sentence, or its index 开发者_StackOverflow社区bounds does anybody have a suggestion of how I could modify the text.

I don't want to use a

TextSelection text = richTextBox.Selection; 

Thanks is advance.