Index and search a directory

I have a directory tree containing many text-files. I would like to index the full-text of all these files (ignoring files with certain file-extensions), so that I can quickly search thorugh all of them. I do not want to index my whole Home directory, or the whole system. I just want to index this particular directory. The index should update continuously, automatically detecting changes in the files inside. What tool can I use for this?