documentclass{article}
usepackage[utf8]{inputenc}
usepackage{color}
%custom command : shortcut for formating
newcommand{gr}[1]{{bf color{red}#1}}
newcommand{ro}[1]{{color{red}#1}}
newcommand{g}[1]{{bf #1}}
title{Latin' s Scansion}
begin{document}
maketitle
begin{tabular}{rlll}
734 & ro{h={a}s} g{={i}n}ro{t={e}r} / g{p={e}s}ro{t={e}s} // g{d=  {u}}ro{r={o}} /// g{Cu{a}}tu{o} gr{m={i}}lu{i}tu{e} gr{s={i}c}cum  &   SSSD  & hhhh \
end{tabular}
end{document}

documentclass{article}
usepackage[utf8]{inputenc}
usepackage{color}
%custom command : shortcut for formating
newcommand{gr}[1]{{bfseries color{red}#1}}
newcommand{ro}[1]{{color{red}#1}}
newcommand{g}[1]{{bfseries #1}}

begin{document}
begin{tabular}{rlll}
734 & ro{h={a}s} g{={i}n}ro{t={e}r} / g{p={e}s}ro{t={e}s} //
g{d=  {u}}ro{r={o}} /// g{Cu{a}}tu{o} gr{m={i}}lu{i}tu{e}
gr{s={i}c}cum  &   SSSD  & hhhh \
end{tabular}
end{document}

documentclass{article}
usepackage[utf8]{inputenc}
usepackage{color}
%custom command : shortcut for formating
newcommand{gr}[1]{{bfseries color{red}#1}}
newcommand{ro}[1]{{color{red}#1}}
newcommand{g}[1]{{bfseries #1}}

DeclareTextCompositeCommand{=}{OT1}{i}{=i}
DeclareTextCompositeCommand{u}{OT1}{i}{ui}

begin{document}

begin{tabular}{rlll}
734 & ro{h={a}s} g{={i}n}ro{t={e}r} / g{p={e}s}ro{t={e}s} //
g{d=  {u}}ro{r={o}} /// g{Cu{a}}tu{o} gr{m={i}}lu{i}tu{e}
gr{s={i}c}cum  &   SSSD  & hhhh \
end{tabular}

end{document}

documentclass{article}
usepackage[utf8]{inputenc}
usepackage{color}
%custom command : shortcut for formating
newcommand{gr}[1]{{bfseries color{red}#1}}
newcommand{ro}[1]{{color{red}#1}}
newcommand{g}[1]{{bfseries #1}}

newcommand*{LongShortAux}[2]{%
  begingroup
    sbox0{mdseries #1{}}%
    sbox2{mdseries #1{j}}% j is used instead of i to prevent endless loop
    ooalign{%
      hidewidthraisebox{dimexprht2-ht0relax}{mdseries#1{}}hidewidthcr
      hfill#2hfill
    }%
  endgroup
}

DeclareTextCompositeCommand{=}{OT1}{a}{LongShortAux{=}{a}}
DeclareTextCompositeCommand{=}{OT1}{e}{LongShortAux{=}{e}}
DeclareTextCompositeCommand{=}{OT1}{i}{LongShortAux{=}{i}}
DeclareTextCompositeCommand{=}{OT1}{o}{LongShortAux{=}{o}}
DeclareTextCompositeCommand{=}{OT1}{u}{LongShortAux{=}{u}}

DeclareTextCompositeCommand{u}{OT1}{a}{LongShortAux{u}{a}}
DeclareTextCompositeCommand{u}{OT1}{e}{LongShortAux{u}{e}}
DeclareTextCompositeCommand{u}{OT1}{i}{LongShortAux{u}{i}}
DeclareTextCompositeCommand{u}{OT1}{o}{LongShortAux{u}{o}}
DeclareTextCompositeCommand{u}{OT1}{u}{LongShortAux{u}{u}}

begin{document}
begin{tabular}{rlll}
734 & ro{h={a}s} g{={i}n}ro{t={e}r} / g{p={e}s}ro{t={e}s} //
g{d=  {u}}ro{r={o}} /// g{Cu{a}}tu{o} gr{m={i}}lu{i}tu{e}
gr{s={i}c}cum  &   SSSD  & hhhh \
end{tabular}
end{document}

documentclass{article}
usepackage[utf8]{inputenc}
usepackage{color}
usepackage{accsupp}
usepackage{stringenc}
usepackage{newunicodechar}

%custom command : shortcut for formating
newcommand{gr}[1]{{bfseries color{red}#1}}
newcommand{ro}[1]{{color{red}#1}}
newcommand{g}[1]{{bfseries #1}}

newcommand*{LongShortAux}[3]{%
  begingroup
    StringEncodingConvert{x}{detokenize{#3}}{utf8}{utf16be}%
    BeginAccSupp{%
      unicode,%
      method=plain,%
      ActualText=x
    }%
      sbox0{mdseries #1{}}%
      sbox2{mdseries #1{j}}%
      ooalign{%
        hidewidthraisebox{dimexprht2-ht0relax}{mdseries#1{}}hidewidth
        cr
        hfill#2hfill
      }%
    EndAccSupp{}%
  endgroup
}

makeatletter
newcommand*{DeclareLatinDuration}[3]{%
  DeclareTextCompositeCommand{#1}{f@encoding}{#2}{%
    LongShortAux{#1}{#2}{#3}%
  }%
  @ifundefined{u8:detokenize{#3}}{%
    newunicodechar{#3}{#1{#2}}%
  }{}%
}
makeatother

DeclareLatinDuration{=}{a}{ā}
DeclareLatinDuration{=}{e}{ē}
DeclareLatinDuration{=}{i}{ī}
DeclareLatinDuration{=}{o}{ō}
DeclareLatinDuration{=}{u}{ū}

DeclareLatinDuration{u}{a}{ă}
DeclareLatinDuration{u}{e}{ĕ}
DeclareLatinDuration{u}{i}{ĭ}
DeclareLatinDuration{u}{o}{ŏ}
DeclareLatinDuration{u}{u}{ŭ}

begin{document}
begin{tabular}{rlll}
734 & ro{hās} g{īn}ro{tēr} / g{pēs}ro{tēs} //
g{dū}ro{rō} /// g{Că}tŏ gr{mī}lĭtĕ
gr{sīc}cum  &   SSSD  & hhhh \
end{tabular}
end{document}

documentclass{article}
usepackage[utf8]{inputenc}
usepackage{color}

usepackage{newunicodechar}
%newunicodechar{ă}{u{a}} % not needed (already defined by the utf8 option)
newunicodechar{ĕ}{u{e}}
newunicodechar{ĭ}{u{i}}
newunicodechar{ŏ}{u{o}}
newunicodechar{ŭ}{u{u}}
newunicodechar{ā}{={a}}
newunicodechar{ē}{={e}}
newunicodechar{ī}{={i}}
newunicodechar{ō}{={o}}
newunicodechar{ū}{={u}}

%custom command: shortcut for formatting
newcommand{gr}[1]{textbf{color{red}#1}}
newcommand{ro}[1]{textcolor{red}{#1}}
newcommand{g}[1]{textbf{#1}}

begin{document}

title{Latin's Scansion}
author{Evelyne}
maketitle

begin{tabular}{rlll}
734 & ro{hās} g{īn}ro{tēr} / g{pēs}ro{tēs} // g{dū}ro{rō} ///
g{Că}tŏ gr{mī}lĭtĕ gr{sīc}cum  &   SSSD  & hhhh \
end{tabular}
end{document}