Revert "remove regexp as nepomuk anyway does a substring match using contains()"

Nepomuk does not do substring match with contains. Active image-browser
now shows results for any subtring (even for one character).

This reverts commit b789c94d.

CCBUG: 296941
(cherry picked from commit 6947cfa0b8b6a12788741e10bc4116400d09079d)