ToExpression Error when parsing MathML in XML

Question

My code for parsing MathML to be readable in Mathematica:

ToExpression @ ImportString @ ExportString[#, "XML"]& /@ Cases[
  xml, XMLElement["math", _, _],∞
] 

The MathML contents:

<math xmlns="http://www.w3.org/1998/Math/MathML">
        <apply>
          <ci> MM_e</ci>
          <ci> mem </ci>
          <ci> k </ci>
          <ci> sp_3 </ci>
          <ci> sp_1 </ci>
          <ci> Km </ci>
          </apply>
      </math>

I am getting this error when extracting MathML from a XML file. :

ToExpression::esntx: Could not parse FormBox[TagBox[RowBox[{MM_e,RowBox[{(,RowBox[{<<9>>}],)}]}],MathMLContentTag,AutoDelete->True],TraditionalForm] as input.

The math should display like this:

MM_e(mem,k,sp_3,sp_1,Km)

But it only displays somewhat correctly when I copy MathML content to Mathematica which then asks

"Do you want to paste the typeset expression?"

MM_e(mem,k,3 sp_,1 sp_,Km)

Although when using DisplayForm there are no errors but with other math content, it does not simplify unless I use Shift+Enter on the output.

DisplayForm @ ImportString @ ExportString[#, "XML"] & /@ Cases[ 
  xml, XMLElement["math", _, _], ∞
]

How can I fix this error without using DisplayForm?

Answer 1

The problem was with the underscore symbol. I fixed it by changing it to another symbol [LetterSpace] and Math displays perfectly.