Attempt to fix articles.

Your noun "matched by the Search pattern text" is unclear. 
It would be better worded like this: "Nested pattern can be used when all of the text matched by the Search pattern does not have to be used to format the link."
And best like this: "Nested pattern can be used when only part of the text matched by the Search pattern should be used to format the link."
I have also changed "a" to "the" in this case as the nested pattern affects formatting of "the link" that will be created on the text found by the search pattern above. Once we've passed finding some text to make "a link" we use nested pattern to modify "the link" to use only part of the text.