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I would like to replace the following expression:

sp[p1, p1] + 2*sp[p1, p1]*sp[p2, p2] /. sp[p1, p1] -> sp[p2, p2]

BUT I do not want to replace the second term only the first. BTW, the sp stands for "scalar product" in my notation.

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    $\begingroup$ ReplacePart might be useful for this. $\endgroup$ – Daniel Lichtblau Mar 25 at 16:06
  • $\begingroup$ The above example is fairly easy. My actual expression is much more complicated thats why I need a rule to replace sp[p1,p1]->sp[p2,p2] but I should not replace the combination of the two scalarproducts. $\endgroup$ – NeAr Mar 25 at 16:08
  • $\begingroup$ Please edit the question to clarify this requirement. $\endgroup$ – Daniel Lichtblau Mar 25 at 19:46
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expr = sp[p1, p1] + 2*sp[p1, p1]*sp[p2, p2];

rules = {pat : HoldPattern[Times[___, _sp, ___, __sp, ___]] :> pat, 
        sp[p1, p1] -> sp[p2, p2]};

ReplaceAll[rules] @ expr
 sp[p2, p2] + 2 sp[p1, p1] sp[p2, p2]

ReplaceAll >> Details:

enter image description here

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One can use Replace with a level specification:

Replace[sp[p1, p1] + 2*sp[p1, p1]*sp[p2, p2], sp[p1, p1] -> sp[p2, p2], 1]

For ReplacePart mentioned in the comment by @DanielLichtblau:

ReplacePart[sp[p1, p1] + 2*sp[p1, p1]*sp[p2, p2], 1 -> sp[p2, p2]]
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