Creating user input Search with 3rd party search engines

[phl]

hello,

I am trying to explore options on the various types of search engines
for searching user input. User input maybe stored in XML files. Does
anyone know if there are any good ready made search engines which will
automatically index the user input from the XML file(s), so I can
create a search functionality based on keywords.
I am aware that a DB solution is the obvious route. The purpose here
is cost vs benefit.
Thanks
-S