Automatically replace environment align by equation+aligned combination

Question

I would like to automatically replace the align environment by an equation + aligned combination. Specifically, whenever \begin{align} occurs, it should automatically read it as \begin{equation}\begin{aligned}. Similarly, \end{align} should be replaced by \end{aligned}\end{equation}. Occurrences of \nonumber also need to be gobbled up and ignored.

\documentclass[a4paper]{article}
\usepackage{amsmath}

\begin{document}

Any align such as
\begin{align}
x & = y + z, \nonumber \\
\alpha &= \beta + \gamma,
\end{align}
should automatically be replaced by an equation + aligned combination, effectively becoming
\begin{equation}
\begin{aligned}
x & = y + z, \\
\alpha &= \beta + \gamma.
\end{aligned}
\end{equation}

\end{document}

Ideally, I would like to achieve this without having to perform a manual replaceall throughout my document. The simplest solution I see would be to create a new environment that achieves the required effects, but then I would still need to replace all existing uses of the align environment with the new environment.

I've not been able to find or create an implemention of this. The most related answer I've been able to find is: Modify eqnarray to match amsmath align.

Answer 1

Use environ.

\documentclass[a4paper]{article}
\usepackage{amsmath,environ}

\RenewEnviron{align}{%
  \let\nonumber\relax % this is local to this environment
  \let\notag\relax % this is local to this environment
  \let\tag\relaxtag % this is local to this environment
  \equation % start equation
  \!% see http://tex.stackexchange.com/questions/98482
  \aligned % start aligned
  \BODY % the contents
  \endaligned % end aligned
  \endequation % end equation
}
\def\relaxtag#1#{\relaxrelaxtag}
\def\relaxrelaxtag#1{}

\begin{document}

Any align such as
\begin{align}
x & = y + z, \nonumber \\
\alpha &= \beta + \gamma,
\end{align}
should automatically be replaced by an equation + aligned 
combination, effectively becoming
\begin{equation}
\begin{aligned}
x & = y + z, \\
\alpha &= \beta + \gamma.
\end{aligned}
\end{equation}

\end{document}

Answer 2

Just for completeness, here's a LuaLaTeX-based solution. It defines a Lua function called replace_align that does the string replacements. align environments are replaced with equation/aligned combinations, and align* environments are replaced with equation*/aligned combinations.

The Lua function is assigned to the process_input_buffer callback, which operates at a very early stage on the contents of the tex file, before the TeX-side of LuaTeX does any real processing.

% !TEX TS-program = lualatex
\documentclass{article}
\usepackage{amsmath} % for "align", "aligned", and "equation*" environments

%% Lua-side code
\usepackage{luacode}
\begin{luacode*}
in_align = false -- Set up a Boolean toggle variable 

function replace_align ( buff )
   if string.find ( buff , "\\begin{align" ) then
      buff = string.gsub ( buff , "\\begin{align(*?)}" ,
             "\\begin{equation".."%1".."}\\!\\begin{aligned}" )
      in_align = true     -- Set in_align to "true"
   elseif string.find ( buff , "\\end{align" ) then
      buff = string.gsub ( buff , "\\end{align(*?)}" ,
             "\\end{aligned}\\end{equation".."%1".."}" )
      in_align = false    -- Set in_align to "false"
   elseif in_align then  -- Gobble "\nonumber" and "\notag"
      buff = string.gsub ( buff , "\\nonumber" , "")
      buff = string.gsub ( buff , "\\notag" , "")
   end
   return ( buff )
end

luatexbase.add_to_callback ( "process_input_buffer", replace_align, "replace_align" )
\end{luacode*}

\begin{document}   
Any \verb+align+ environment, such as
\begin{align}
x      &= y + z,  \nonumber \\
\alpha &= \beta + \gamma,
\end{align}
should automatically be replaced by an \verb+equation/aligned+ combination, effectively becoming
\begin{equation}\begin{aligned}
x      &= y + z, \\
\alpha &= \beta + \gamma.
\end{aligned}\end{equation}    
\end{document}