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nLEyETn

Regular Expressions Question

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Can anybody explain to me why these give different results: /&(?!amp;|nbsp;)/g /&((?!amp;)|(?!nbsp;))/g The first one works correctly, but I don't understand why the second one gives different results.

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How do you use them? The difference here is that the second one has three subexpressions, while the first one only has one.

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The first matches "&" when it is followed by not ("amp;" or "nbsp;").
The second matches "&" when it is followed by (not "amp;") or (not "nbsp;").

That means the second will match every "&", because it's impossible to be followed by both "amp;" and by "nbsp;" - it will always succeed either the 'not followed by "amp;"' or the 'not followed by "nbsp;"' test (or both), and the '|' just means the whole expression will match if either of those cases matches. So it's equivalent to /&/g, which isn't very useful, whereas the first will correctly match any "&" that isn't part of " " or "&".

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Thank you very much for the explanation, it really helped my understanding of it . I don't have much experience with regexp, but I'm trying to learn more now; it just takes some getting used to I guess.

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