How to execute different functions automatically based on user defined ruleset

I am trying to make a parser webapp for documents and it will have multiple users..

I am confused on how to go for it. For e.g :

Below is the document template which a user will use to make his parsing rules.

this is a name of a guy who loves playing snowball.

Let's say the user wants to start extracting from the: name word.

The user saves the parsing rules.

Now what I want to know is that how to run the same parsing function automatically on every subsequent data received for the same user.

Should I save the function definition in database for the user and query the db for function everytime I receive the data of the same user or should I approach another way.

Basically the function is: Let the user define the parsing rules and the Web app will automatically parse the documents based on the rules set by the user.

I am going to use Nodejs as the framework of choice....

via ChanX