Variable keywords in syntax files

[Gerald Dennett]

Hi
I want to create a syntax file that will highlight the following:
<<n,Pm>> -- where n and m are any number
<Style:n> - where n is any number
If I select an html style syntax file it highlights the intial and terminal <> in each of these examples, and also highlights the word 'Style' (because that is also an html keyword.
I cannot see how to enter a regular expression as part of a keyword & I assume that the syntax start and syntax end are covering the initial and terminal <>, how do I get the second and penultimate characters in my first example.
TIA
Gerald