There is an older plugin what might have compatibility issues with new DW versions.
The plugin page was not update for longer time but you may give it a try:
http://www.dokuwiki.org/plugin:searchtext
This plugin searches pages rendered as text instead of raw wiki.
This plugin displays search result snippets from text.
another one might be:
http://www.dokuwiki.org/plugin:sphinxsearch
DokuWiki Sphinx Search plugin replaces DokuWiki's built-in search functionality with the Sphinx Search Engine powered search which gives high-performance and more relevant search results