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The regexp engine will match if it can find any way to. So what you're asking for is "root, followed by some number (possibly zero) of whitespace characters, followed by something that is not 'admin@somewhere.here'". So it matches with root, followed by zero spaces, followed by ' admin@somewhere.here' (with a leading space). Since the string ' admin@somewhere.here' isn't 'admin@somewhere.here' (without the space), the lookahead works. That's why you need the \s* inside the lookahead, making it "try to find spaces followed by admin@somewhere.here, and if you can, fail" instead of "look for spaces, but make sure it's not followed by admin@somewhere.here". Subtle, but important.

In reply to Re^3: On zero-width negative lookahead assertions by Eimi Metamorphoumai
in thread On zero-width negative lookahead assertions by bronto

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