Find function in text

Find function in text

I'm re-designing a mathematics program for students. The original program
was written in adobe flash. This time I am doing it in PHP and Javascript.
In the original program there are several xml files with hundreds of
mathematical questions and some of which are fractions. Those fractions
are written as: #(1,4). I want to make an function called # which can make
classic fractions.
The problem: there is normal text around the #(1,3) and it is not
separated with quotations. How can I let the browser know there is a
function in the text? Example:
<question>what is #(2,8) divided by #(3,4)</question>
I hope you know how to do this. Thanks.