Is there a way to specify that some given letters are always in bold?

Question

I am writing an article, which requires the letters 'a', 'e', 'i', 'o', 'u' are always showed in bold. Given the following TeX file:

\documentclass{article}

% What should be put here?

\begin{article}
You can say Hi as Hello.
\end{article}

The default result is: You can say Hi as Hello.

What I really want is: You can say Hi as Hello.

Answer 1

When you are using XeTeX then your task is exactly solved by XeTeX primitives \XeTeXcharclass and \XeTeXinterchartoks. Read the manual of XeTeX. For letters 'a', 'e', 'i', 'o', 'u' the setting looks like:

\newXeTeXintercharclass \mycharclassbf
\XeTeXcharclass `\a =\mycharclassbf
\XeTeXcharclass `\e =\mycharclassbf
\XeTeXcharclass `\i =\mycharclassbf
\XeTeXcharclass `\o =\mycharclassbf
\XeTeXcharclass `\u =\mycharclassbf

\XeTeXinterchartoks 0 \mycharclassbf = {\bgroup\bf}
\XeTeXinterchartoks \mycharclassbf 0 = {\egroup}   
\XeTeXinterchartoks 255 \mycharclassbf = {\bgroup\bf}
\XeTeXinterchartoks \mycharclassbf 255 = {\egroup}   

\XeTeXinterchartokenstate = 1

You can say Hi as hello.

\bye

Answer 2

Let me present a small snippet in LuaTeX. We pass an argument and selected letters to the Lua core, where the snippet decomposes it at a character level and wraps the preselected letters only by the \malbf command.

Words to be marked are tagged, I didn't expect a TeX command, I presumed plain text. I enclose a small example with English (requested letters by OP) and Czech text (all the graphemes).

We can also check out the output in the terminal, it writes:

H\malbf{e}ll\malbf{o} W\malbf{o}rld! Y\malbf{ou} c\malbf{a}n s\malbf{a}y H\malbf{i} \malbf{a}s H\malbf{e}ll\malbf{o}.
Č\malbf{au} d\malbf{ě}ck\malbf{a}, j\malbf{a}k s\malbf{e} d\malbf{a}ř\malbf{í}?

If we would like to see different strategy how to preserve ligatures and kerning pairs of the formatted letters (I remove all the }\malbf{ sequences in this example), please see Version 2 of this answer. I mentioned one other method of the possible strategies there (I used a variable for font switching). In this example, we've got Č\malbf{au} instead of Č\malbf{a}\malbf{u}, and Y\malbf{ou} instead of Y\malbf{o}\malbf{u}.

I enclose the source code of the snippet and a preview of the PDF file. We run lualatex mal-letters.tex.

% lualatex mal-letters.tex
\documentclass[a4paper]{article}
\pagestyle{empty}
\usepackage{xcolor}
\usepackage{luatextra} % luacode

\begin{luacode*}
function boldme () -- the core function
letters=tex.toks[1] -- some more/other letters?
text=tex.toks[0] -- text taken from TeX
towrite="" -- an empty string
markerbegin="\\malbf{" -- a command to be used as a marker
markerend="}"

for i=1,unicode.utf8.len(text) do -- decomposition of text
s=unicode.utf8.sub(text, i, i) -- actual character
  isthere=nil -- initial value, no, character is not among letters
  for j=1,unicode.utf8.len(letters) do -- test if it is
     if unicode.utf8.sub(letters, j, j)==s then isthere=1; break end
     end -- for, j, is character between selected letters?
  if isthere then -- to be bold, or not?
     towrite=towrite..markerbegin..s..markerend -- yes, it is to be empasized in bold
        else
     towrite=towrite..s -- no, that character is not among selected letters
     end -- if, isthere
end -- for, i, decomposition of input text  
  -- combine the commands together
  -- e.g. \malbf{a}\malbf{e} -> \malbf{ae}
  towrite=unicode.utf8.gsub(towrite, markerend..markerbegin, "")
  -- writing result to the terminal and to the document
  io.write(towrite) -- information shipped to the terminal
  tex.sprint(towrite) -- information shipped to the document
end -- function, boldme
\end{luacode*}
\def\malbf#1{\textbf{\color{red}#1}}
\def\markit#1{\toks0{#1}\directlua{boldme()}}
\def\tomark#1{\toks1{#1}}

\begin{document}
% Text in English, selected letters.
\tomark{aeiou}
Some text before. \markit{Hello World! You can say Hi as Hello.} More text after.

% An example with Czech graphemes.
\tomark{aáeéěiíoóuúůyý}
Text in the middle. \markit{Čau děcka, jak se daří?} End of story.
\end{document}