You do realize that @Alan-Kilborn already answered that question for you: the regex he showed worked for him. If it doesn’t work for you, you will have to provide more details , because that regex will work to delete the first line of the files. He even asked you to provide more details to explain how the given regex doesn’t work for you.
You really need to read this FAQ and see the referenes below.
That said, can be simplified: a large part of the original expression was only needed because the original post wanted to delete line#15, not line#1. To delete the first line is simpler: FIND = (?-s)\A(^.*$\R?), REPLACE is empty, and SEARCH MODE must be REGULAR EXPRESSION.
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Useful References Please Read Before Posting Template for Search/Replace Questions FAQ: Where to find regular expressions (regex) documentation Notepad++ Online User Manual: Searching/Regex