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Asked: 2026-06-02T23:14:14+00:00

I’m trying to create a regex that strips out all lines that don’t start

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I’m trying to create a regex that strips out all lines that don’t start with a certain prefix. Using this tip I came up with this:

(?:(?!\nLAST-MODIFIED:[^\n]+\n).*)

This doesn’t seem to work properly. What am I doing wrong? And yes, I’m aware of the fact that this will always strip the first line, whether it is prefixed with LAST-MODIFIED: or not. But I’m positive that the first line will never be a LAST-MODIFIED: line, so it should be safe. My suggested fix would be (?:(?!(\n|^)LAST-MODIFIED:[^\n]+\n).*) if I wanted to be perfectly safe.

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1 Answer

  1. Editorial Team

    Will this do what you need it to?

    (^(?!LAST-MODIFIED)[^\n]*)+

    Explanation:

    • ^ – match the beginning of a line
    • (?!LAST-MODIFIED) – check that it doesn’t begin with last-modified
    • [^\n]* – if the above is true, match everything until you find a line break
    • + – repeat
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