Xtykutl

All in all the same as the answer by Steven B. Segletes, but expandable. Also almost everything as contents should be fine (except the really unlikely endabr@).

documentclass{article}



makeatletter

newcommandabr[1]

  {%

    abr@#1endabr@

  }

defabr@#1#2endabr@

  {%

    #1.%

    ifrelaxdetokenize{#2}relax

      @%

      expandafter@gobble

    else

      ,%

      expandafter@firstofone

    fi

    {abr@#2endabr@}%

  }

makeatother



begin{document}

Single letter:

abr{eg}



Multi letter: 

abr{{th}e}

end{document}

Here is the simplest form of my original approach. It can be placed in an edef. Its only drawback is that it can blow the stack if the argument is too long (maybe 256 characters??)

documentclass{article}

newcommandabr[1]{abraux#1relaxrelax}

defabraux#1#2relax{#1.ifxrelax#2relax@else,abraux#2relaxfi}

begin{document}

Here is abr{OED} abbreviation.



Here is abr{XO} abbreviation.

end{document}

If that really were an issue, here is an alternative that doesn't have that problem.

There seems to be a misunderstanding that this definition of abr cannot be placed into an edef. It can. Naturally, the expansion is not necessarily pretty, but it will yield the proper typesetting. The only proviso is that @MyOwnMacro is not used elsewhere in your document.

documentclass{article}

usepackage[T1]{fontenc}

makeatletter

let@MyOwnMacrorelax

newcommandabr[1]{abraux#1relaxrelax}

defabraux#1#2relax{%

  #1.ifxrelax#2relaxdef@MyOwnMacro{@}elsedef@MyOwnMacro{,abraux#2relax}fi%

  @MyOwnMacro%

}

makeatother

begin{document}

Here is abr{OED} abbreviation.



Here is abr{XO} abbreviation.



Can be edef'ed:



edeftmp{abr{OED}} detokenizeexpandafter{tmp}



expands to tmp

end{document}

Here's a LuaLaTeX-based solution.

• Cases such as abr{OED} or abr{IMF} work just as expected. If the acronym contains both uppercase and lowercase letters, dots are inserted only before the uppercase letters in the interior of the acronym. E.g., abr{MSc} generates M.,Sc., and abr{PhD} generates Ph.,D..




• It can handle mixed-case acronyms such as "PhD" directly -- no need to write abr{{Ph}D}.




• If a "slovenly abbreviation" ends a sentence, one should place the "." punctuation mark inside the argument of abr. The code takes care to insert a @ "space factor* directive before the final . character. This, in turn, informs LaTeX that that . character should be treated as ending a sentence.




• The code returns nothing if the argument of abr is either empty or expands to return nothing. E.g., defttt{} /abr{ttt}/ returns //. If the code encounters non-letter characters -- say, ( and ) -- no periods are inserted before or after them.




• The code is expandable in the sense that abr can be included in the argument of an edef directive.

documentclass{article}

usepackage{luacode}   % for 'luacode' environment



%% Lua-side code:

begin{luacode}

function abr ( s )

  n = string.len ( s )

  -- Do nothing unless "s" is non-empty.

  if n>0 then 

    s_mod = ""  -- initialize the string



    -- Process the first n-1 characters in "s"

    for i=1, n-1 do

      s12 = string.sub ( s  , i, i+1 )

      s1  = string.sub ( s12, 1, 1 )

      if string.match ( s12 , "%a%u" ) then

        s_mod = s_mod .. s1 .. ".\,"

      else

        s_mod = s_mod .. s1

      end

    end



    -- Process the final character in "s"

    s_n = string.sub ( s , n)

    if     string.match (s_n, "%.") then   -- "." char.

      s_mod = s_mod .. "\@." 

    elseif string.match (s_n, "%l") then   -- lowercase letter

      s_mod = s_mod .. s_n .. ".\hbox{}"

    elseif string.match (s_n, "%u") then   -- uppercase letter

      s_mod = s_mod .. s_n .. "."

    else                    -- Any other character:

      s_mod = s_mod .. s_n  -- don't add anything after 's_n'

    end

    -- Print the modified string

    tex.sprint ( s_mod )

  end

end



end{luacode}



%% LaTeX-side code: macro that calls the Lua function

newcommandabr[2]{directlua{abr("#1")}}



begin{document}

abr{OED}, abr{PhD}, abr{DPhil}, abr{MSc}, abr{()}



smallskip

% Two calls to "abr" with an empty argument (upon expansion):

.abr{}. quad 

defttt{} .abr{ttt}. 



bigskip

edeftmp{abr{MA}}   detokenizeexpandafter{tmp}



edeftmp{abr{MA.}}  detokenizeexpandafter{tmp}



expands to: tmp



smallskip

edeftmp{abr{MSc}}  detokenizeexpandafter{tmp}



edeftmp{abr{MSc.}} detokenizeexpandafter{tmp}



expands to: tmp



bigskip

Some tests of spacing after punctuation marks:



smallskip

a abr{PhD} candidate --- good



a Ph.,D. candidate   --- just to verify



smallskip

an abr{MSc} candidate --- good



an M.,Sc. candidate  --- just to verify



smallskip

She has a abr{PhD.} So do I. --- good



She has a Ph.,D@.  So do I. --- just to verify



smallskip

He has an abr{MSc.} So do I. --- good



He has an M.,Sc.    So do I.  --- just to verify



smallskip

Does he have an abr{MSc.}? Really?! --- good



Does he have an M.,Sc.?    Really?!  --- just to verify

end{document} 

Let TeX do the recursion:

documentclass{article}

usepackage{xparse}

usepackage{etoolbox}



robustify{,} % just in order it doesn't expand in edef



ExplSyntaxOn

NewExpandableDocumentCommand{abr}{m}

 {

  tl_map_function:fN { tl_range:nnn { #1 } { 1 } { -2 } } __toothrot_abr:n

  tl_range:nnn { #1 } { -1 } { -1 } .

 }

cs_generate_variant:Nn tl_map_function:nN { f }

cs_new:Nn __toothrot_abr:n { #1., }

ExplSyntaxOff



begin{document}



abr{OED}



abr{{Ph}D}



edeftest{abr{OED}}

texttt{meaningtest}



edeftest{abr{}}

texttt{meaningtest}



end{document}

If a part of the argument is braced, it is considered as a single item.

One might check whether the argument is empty in order to print nothing at all, but it doesn't seem so important a feature.

With tl_range:nnn { #1 } { 1 } { -2 } we extract all items but the last; tl_range:nnn { #1 } { -1 } { -1 } extracts the last item.

documentclass{article}

usepackage{xinttools}

newcommandabr[1]{xintListWithSep{.,}{#1}.}

begin{document}

I've seen it in the abr{OED}.



I got my abr{{Ph}D}.

end{document}

Updated (à la Mico, but without LuaLaTeX)

The syntax here is to use abr{PhD.} for example at end of a sentence, and abr{PhD} if not at end of a sentence.

documentclass{article}

usepackage{xinttools}

makeatletter

newcommandabr[1]

   {expandafter@gobbletworomannumeral0xintapplyunbracedabr@aux{#1}.@}

defabr@sep{.,}

defabr@aux#1{if.#1expandafterabr@end

               else

                if1ifnum`#1<`A 0fiifnum`#1>`Z 0fi1%

                   expandafterexpandafterexpandafterabr@sep

                fi

               fi#1}%

defabr@end.{ abr@@end}

defabr@@end.@{@.}

makeatother



begin{document}%ttfamily

I've seen it in the abr{OED}, and if located at end of a sentence

just insert a dot in the verb|abr| argument: abr{OED.} It ended a

sentence and in non-French spacing mode, TeX inserted the extra

space.



texttt{We can see it better with monospace font: abr{OED.} See?}



I got my abr{PhD} and even my abr{PhilD}, leniency ruled

in those days.



texttt{The dots are added in a smart way: abr{AaaaBbbbCccc.} But it is

assumed that the first letter is abr{Uppercased.} That's it.}



texttt{Notice that neitger abr{Aaaa} nor abr{AaA} trigger an end of

  sentence spacing after the dot, which is abr{Good.} Isn't it?}

end{document}

Again updated, for automatic end of sentence dot detection after abbreviation

Here an end of sentence dot will be detected automatically.

Of course we can't use @ifnextchar for that, as it swallows spaces.

I added some comments about expandability, which seems to have drawn great attention in other answers :).

documentclass{article}

usepackage{shortvrb}MakeShortVerb{|}

usepackage{xinttools}



makeatletter

protecteddefabrsep{.,}% maybe redefined even after edeffoo{abr{DPhil}}...

protecteddefabrend{futurelet@let@tokenabr@end}

defabr@end{ifx.@let@token@else.@fi}

newcommandabr[1]

   {expandafter@gobbleromannumeral0xintapplyunbracedabr@aux{#1}abrend}

defabr@aux#1{if1ifnum`#1<`A 0fiifnum`#1>`Z 0fi1%

                   expandafterabr@sep

               fi#1}%

defabr@sep{ abrsep}

makeatother



begin{document}%ttfamily

I've seen it in the abr{OED}, and if located at end of a sentence

it will detect it automatically: abr{OED}. There was no double dot.

Besides, TeX applied its end of sentence extra space.



texttt{We can see it better with monospace font: abr{OED}. See?}



texttt{We can see it better with monospace font: abr{OED}, See?}



I got a abr{MSc}, a abr{PhD} and even a abr{DPhil}. Leniency ruled

in those days. 



{The abbreviation dots are added in a smart way, after the last

  lowercase letter following an uppercase letter:

  abr{AaaaBbbbCccc}. But it is emph{assumed} that the first letter is

  abr{Uppercased}. That's it.}



texttt{Notice that neither abr{DPhil} nor abr{PhilD} get TeX to

  consider the inserted final dot as signaling  an end of

  sentence spacing after the dot, which is abr{Good}. Isn't it?}



About expandability, the correct way for LaTeX2e's users would be to use

|protected@edef|, not a naked |edef|; although nowadays some

LaTeX2e users have heard about |edef|, they might not know

about |protected@edef|, which requires a cumbersome extra

|makeatletter| for its usage. Anyway, none of that is described in

textsc{Lamport} book, so I wonder if LaTeX2e users are really

emph{allowed} into using |edef| to start with.



But as it seems they know about |edef|, we as macro programmers need

better to use the e-TeX's |protected| prefix and not the LaTeX2e

|DeclareRobustCommand|, because users will not do |protected@edef|.



This is what I have done here for a macro |abrsep| (why haven't we all

used |abbr| by the way?) which is deliberately |protected|,

allowing it to be redefined at location of use, long after some macro

will have been defined via |edeffoo{abr{ABCDEFGH}}|.



edeffoo{abr{ABCDEFGH}}texttt{stringfoo is meaningfoo}



edeffoo{abr{AaaBccCcc}}texttt{stringfoo is meaningfoo}



The |abrend| is also |protected|, anyway as its expansion will be

context dependent (it detects if a dot follows), it had to not expand

in the |edef|.

end{document}

Notice that my proposals v2 and v3 will work only with ascii uppercase letters, no diacritics.

documentclass{article}

newcommandabr[1]{abraux#1..}

defabraux#1#2#3{%

  #1%

  ifnum`#2>91relax% we have a lowercase letter following

    defnext{abraux#2#3}%

  else

    .ifx.#2defnext{@}else,defnext{abraux#2#3}fi

  fi

  next

}

begin{document}

    Here is abr{OED} abbreviation.



    Here is abr{XO} abbreviation.



    Here is abr{XOOED} abbreviation.



    abr{MSc}  



    abr{DPhil}

end{document}