Thanks to Eike for finding this:

The anchors ^ and $ now anchor at the selection boundary because the
only text the regex matcher gets passed is the selected text. This in
two paragraphs

 aaa bbb aaa bbb
 aaa bbb aaa bbb

when the selection spans from the second aaa to the third bbb, for
"^aaa" finds the second aaa, where previously it found the third aaa at
the real paragraph start.

This may not be expected by the user, because the behavior of ^ is
described as "Match at the beginning of a line" (or paragraph in our
case), which the previous implementation did.

(regression from 806ced87)

Unfortunately it's not obvious how to implement the same in
searchBackward().

Change-Id: I07f7a8476b672d9511fa74ca473c32eea427698f