xslt strings: 'substring-between'?

I want to be able to parse an url string, and return name value pairs:
page.cgi?T=boston&S=MA
I can test if the T parameter exists and has a value (not equal to &), but, how do I extract the contents between the = and the next &? e.g. "boston"?
What if there isn't a trailing & on the parameter (the last parameter in the url, such as S=MA above)? 

I have an <url> tag on each page which references the current document (such cgi environment variables) and I want to be able to extract it's contents without knowing beforehand what the contents may look like.

Has anyone assembled a list of string utility functions? I've seen a few in the XSLT Programmers reference, but none which will do what I want above.

Thanks,
-Steve


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