My logic would be “capture from the beginning of the line, as many characters as possible, until a lookahead of 10 characters before the EOL is matched”. That translates into something like:

• FIND = ^.*(?=.{10}$)
  REPLACE = empty
  MODE = regular expression

Which worked for me

ifu oaisoi
dlhidadsli
OOLhiol oo

If this doesn’t work for you, you will have to clarify, after reading and taking to heart the following advice:

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Do you want regex search/replace help? Then please be patient and polite, show some effort, and be willing to learn; answer questions and requests for clarification that are made of you. All example text should be marked as plain text using the </> toolbar button or manual Markdown syntax. Screenshots can be pasted from the clipboard to your post using Ctrl+V to show graphical items, but any text should be included as literal text in your post so we can easily copy/paste your data. Show the data you have and the text you want to get from that data; include examples of things that should match and be transformed, and things that don’t match and should be left alone; show edge cases and make sure you examples are as varied as your real data. Show the regex you already tried, and why you thought it should work; tell us what’s wrong with what you do get… Read the official NPP Searching / Regex docs and the forum’s Regular Expression FAQ. If you follow these guidelines, you’re much more likely to get helpful replies that solve your problem in the shortest number of tries.