How to create an alias of Cyrillic character with leading backslash?

Question

RUN: xelatex book.tex

\documentclass[a5paper,12pt]{book}
\usepackage[X2,T2A,OT2]{fontenc}
\usepackage[english,russian]{babel}
\usepackage{marvosym}
\usepackage{fontspec}
\usepackage{xunicode,xltxtra,url,parskip}
\usepackage[usenames,dvipsnames]{xcolor}

\usepackage{supertabular}           
\usepackage{titlesec}
\newpagestyle{mystyle}{\setfoot[\thepage][][]{}{}{\thepage}}
\pagestyle{mystyle}

% \defaultfontfeatures{Mapping=tex-text}
% \setmainfont{OldStandard}[
%              Extension=.otf,
%              UprightFont=*-Regular,
%              ItalicFont=*-Italic,
%              BoldItalicFont =*-BoldItalic,
%              BoldFont=*-Bold
% ]
\newcommand{\И}{{\fontencoding{X2}\selectfont\CYRII}} % І
\newcommand{\и}{{\fontencoding{X2}\selectfont\cyrii}} % і
\newcommand{\Е}{{\fontencoding{X2}\selectfont\CYRYAT}} % Ѣ
\newcommand{\е}{{\fontencoding{X2}\selectfont\cyryat}} % ѣ
\newcommand{\Ф}{{\fontencoding{X2}\selectfont\CYROTLD}} % Ө
\newcommand{\ф}{{\fontencoding{X2}\selectfont\cyrotld}} % ө
\newcommand{\Ы}{{\fontencoding{X2}\selectfont\CYRIZH}} % Ѵ
\newcommand{\ы}{{\fontencoding{X2}\selectfont\cyrizh}} % ѵ

\usepackage[absolute]{textpos}
\begin{document}

\И \и \Е \е \Ф \ф \Ы \ы         % IT WORKS with trailing space!
Нар\еч\ие                       % Нарѣчіе, ERROR: Undefined control sequence.

\end{document}

Why aliases \И \и, and etc do not work without trailing space? Is it possible to create such kind of alias?

Answer 1

You're looking for complications. Here's a working version, provided you use a font with the required glyphs.

\documentclass[a5paper,12pt]{book}
\usepackage[english,russian]{babel}
\usepackage{fontspec}

\setmainfont{OldStandard}[
  Extension=.otf,
  UprightFont=*-Regular,
  ItalicFont=*-Italic,
  BoldItalicFont =*-BoldItalic,
  BoldFont=*-Bold
]

\begin{document}

И и Е е Ф ф Ы ы 

Наречие

Нарѣчіе

\end{document}

The package fontenc may be used, but in very special cases. Don't load xunicode nor xltxtra.

If you are not able to easily input the ancient characters, you could do something like

\newcommand{\И}{І}

but then \И should be followed by a space, unless you change the category code of И (which might be dangerous). If you like to be bold,

\newcommand{\И}{І} \catcode`И=12

(and similarly for the other characters) might do.

I'd prefer a different approach, namely to define \? that looks for the following character and takes the appropriate action. The list is easily extendable to other combinations.

\documentclass[a5paper,12pt]{book}
\usepackage[english,russian]{babel}
\usepackage{fontspec}

\setmainfont{OldStandard}[
  Extension=.otf,
  UprightFont=*-Regular,
  ItalicFont=*-Italic,
  BoldItalicFont =*-BoldItalic,
  BoldFont=*-Bold
]

\ExplSyntaxOn
\NewDocumentCommand{\?}{m}
 {
  \str_case:nnF { #1 }
   {
    {И}{І} {и}{і}
    {Е}{Ѣ} {е}{ѣ}
    {Ф}{Ѳ} {ф}{ѳ}
    {Ы}{Ѵ} {ы}{ѵ}
   }
   {??}
 }
\ExplSyntaxOff

\begin{document}

\?И\?и\?Е\?е\?Ф\?ф\?Ы\?ы

Наречие

Нарѣчіе

Нар\?еч\?ие

\end{document}