manipulating data in text file

I’m looking into doing a new project.  I have some postscript files with a defined header in them.  I need to scan the header looking for the %%Start tag then count over 4 space delimited fields, read in a string and replace it with another string.  that’s all 🙂  I’ve started scanning thru the documention but can’t make much headway.  Has anyone done something like this and if so can they point me in the right direction.  I’m guessing that a tagged VRL can get me to the %%Start field then I would scan the next 125 bytes one character at a time to get the the field I want.  That seems rather ugly and I’m hoping someone else has gone down this path before.

Thanks,

John Theune