Last night I was doing a bit of light reading which just might be of help to you. In the book Mastering Algorithms with Perl, Chapter 9: Strings, p.399, there is an example of developing a top-down parser to translate a query into a regular expression. For example, it will take a query like:

abc and not (def or ghi)
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and turn it into:

/abc/ && ! ( /def/ || /ghi/ )
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The example is very thorough, and the finished code is very simple, so if you can get hold of the book I am sure you could adapt it to your needs.