{catcode`_=13 gdef_{rule[-1pt]{0.75em}{1.0pt}}}
defpp#1{{catcode`_=13 scantokens{#1ignorespaces}}}

defaepp#1{{catcode`_=13 #1}}

aepp{hello_world}

! Missing $ inserted.
<inserted text>
                $

documentclass{article}
{catcode`_=13 gdef_{rule[-1pt]{0.75em}{1.0pt}}}
setlengthparindent{0pt}
begin{document}
{catcode`_=13 hello_world}
end{document}

{catcode`_=13 hello_world}