I have reviewed many string replacement Q&A but I have been unable to apply them to this case (MWE):

three = 3^"1";
2 three
three /. String -> Integer;
2 three
Replace[three, n : NumberString :> ToExpression[n], All];
FullForm[2 three]

(* Power[3,"1"]
2 3^(1)
2 3^(1)
Times[2,Power[3,"1"]] *)

So we see that each replacement actually leaves the String untouched.

The documentation on ReplaceAll etc. is however explicit that one can replace heads; String is a Head; why can it not be replace like Sin?

In[1]:= Sin[x]/.Sin->Cos
Out[1]= Cos[x]

Why can I not convert a string to an integer in this way, and how would I convert "three" above? (the expression I am interested in fixing up is of course a bit longer than this)


The actual expression of interest is the output of


which is


The next attempt:


gives: ToExpression::notstrbox: 128 3^(0)+64 3^(1)+32 3^(2)+8 3^(3)+4 3^(4)+2 3^(5)+3^(6) is not a string or a box. ToExpression can only interpret strings or boxes as Wolfram Language input.

Attending to the need to consider levels of application of Map, I then also tried

Map[ToExpression, thwPoly[m17], Infinity]

which gave: ToExpression::esntx: Could not parse -(1/139) as input, which is interesting as that is a calculated intermediate result/.

Success Using

Replace[thwPoly[m17], n_String :> FromDigits[n], All] 

gives -17, the correct value.

So, the conversion has been achieved - but I'm still not clear why head replacement doesn't work.

  • $\begingroup$ Note that after evaluation the form three = 3^"1" does not have head String. $\endgroup$ Feb 10, 2019 at 18:00
  • $\begingroup$ @HighPerformanceMark - yes, which is itself confusing ;) $\endgroup$ Feb 11, 2019 at 16:46

1 Answer 1

Replace[ 2 three, n_String :> FromDigits[n], All]


  • $\begingroup$ I could have sworn I also tried that, but am pleased that working explicitly from your example it does indeed work! $\endgroup$ Feb 11, 2019 at 16:45

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