i think it would take less than 50 lines of perl or python to do it

That’s for sure :-)
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ok thanks for the elaborate answer.

cheers

Search and replace, even in the powerful regular expression mode, doesn’t have generic math built in, so with just Notepad++, it is not possible to do.

You could use Notepad++ to write up a utility in your favorite programming language, and have that script do the replacement. (i think it would take less than 50 lines of perl or python to do it, for example; probably much less: yeah, perl has a oneliner that would do it: perl -pi -e "m/X([+-]?\d[\d\.]+)/; my$x=$1; my$xx=$x-1; s/X$x/X$xx/g" FILENAME – and just change the formula for $xx if you want a different math transformation; python could probably do it in a few lines inside a for loop; 50 was an upper bound, and probably way too high)

Or, if you have the PythonScript or LuaScript or similar plugin for Notepad++, you could write a script for one of those plugins that runs inside the Notepad++ environment and can edit the active file directly inside Notepad++. if you’re willing to do that, install the plugin, read some docs, take a crack at it; if you’re having difficulty, show us what you tried, and we’ll probably be able to help you finish it off. But this isn’t a code-writing service, so there’s a lower chance that you’ll get someone to write it for you from scratch.

But really, the answer is that Notepad++ itself doesn’t do this.