algebaric operations on a RegEx?

[matt.price-H217xnMUJC0sA/PxXw9srA at public.gmane.org]

hi all,

I want to quickly play with some numeric values in an xml file (xbmc's  
Fontxml files, of which there are often a number in any given skin).   
what I need to do is find lines like:
<size>13</size> and augment or decrement the values by a set number.   
so i'd like a command addsize, like this:

addsize 4 font.xml

that would change  the above to <size>17</size>

someone out there actually understand how to use perl, and willing to  
show me how to do this?  thanks much,

matt


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