I tried this under both PHP4 (4.4.4) and PHP5 (5.1.6) on Gentoo and it worked both times. I did notice that the highlighting of the search results wasn't right. I happened to choose three pages from another test and placed the "openssl-0.x.xa" immediately after some digits containing 0's and 1's. The 0's in the number were highlighted.
http://wiki.jalakai.co.uk/dokuwiki_0/doku.php/?do=search&id=%22openssl-0.9.7a%22
http://wiki.jalakai.co.uk/dokuwiki_0/doku.php/?do=search&id=%22openssl-0%22
Tobias, can you provide a link to your test page(s)?