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The first example on the page for SequenceReplace is the following:

SequenceReplace[{a, b, x, x, a, c}, {a, e_} :> e]

(*b,x,x,c*)

Why do they use RuleDelayed here, instead of ->? I tried it with -> and got the same answer.

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    $\begingroup$ In general, when working with patterns, it's just safer. For ->, e is only bound by e_ if it is present in the rhs after evaluation. So for example, e = 1; SequenceReplace[{a, b, x, x, a, c}, {a, e_} -> e] gets you {1, x, x, 1}. So it's good practice to use :> just in case e is defined somewhere else, I think. $\endgroup$
    – thorimur
    Sep 5 '21 at 19:24
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I remember this was asked before, but can't find it now. i.e. when to use :> vs ->. As @thorimur mentions, it is related to when right side is evaluated.

If you have Wagner book, look at page 148, he gives a nice example showing when to use and what problem it can cause if you use -> instead of :>

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This book is now free to download. Thanks to this post Are you interested in purchasing David Wagner's "Power programming with Mathematica"?

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