1: How can I tell NP++ to search for multiple different text snippets at once? For example all instances of 45/23 31/89 and 21/77 should all be replaced with 66/55
(a|b|c) will match a or b or c, so for the exact three you said, it would be (45/23|31/89|21/77) as the FIND and 66/55 as the replace.
But if you’re not describing it well, and you just want any two-digit fractions of the form ##/## to be replaced with 66/55, then it would be \d\d/\d\d (where \d is regex-speak for “any digit”)
If neither of those is what you really want, you will need to give more examples of what should and shouldn’t match.
2: How can I tell NP++ to delete the first three lines and the last four lines (the last line is always empty) of a file?
First three lines: FIND \A(^.*?\R){3} will match the start of file and the first three lines, and replacing that with empty text will delete them.
Last four: FIND (^.*?\R){4}\Z will match four lines and the end-of-file, so replacing that with empty text will delete them.
Fancy: do both with the | to say “either/or”, so FIND = \A(^.*?\R){3}|(^.*?\R){4}\Z
(don’t have . matches newline enabled)
All of that syntax is described in the User Manual regex section, so if you want more details, you can load that page and use your browser to search for \A or \Z or similar; the concepts used were anchors (for beginning/end of file), multiplying operators (for doing N lines), and groups (so the multiplying operators apply to the entire-line matcher)
200 files
Regex will work in the Find in Files, which is how you make it apply to that many files. But try it in one open file first, to make sure it does exactly what you think. And for bulk operations, always keep a backup, in case things don’t go as you expected.
----
Useful References
Please Read Before Posting
Template for Search/Replace Questions
Formatting Forum Posts
Notepad++ Online User Manual: Searching/Regex
FAQ: Where to find other regular expressions (regex) documentation