Fixed regular expression queries not working anymore.

This was caused by RegexpQuery (new to Lucene 4) supporting a different
set of regexes than RegexQuery. Specifically, ^ (match beginning) and $
(match end) are not supported, but are implied.
Updated DocIndexerOpenSonar to index <event/> tags.
Fixed bug in Kwic.toConcordance() with matches of length 0.