pdfLaTeX: Convert € (euro symbol) to command

It's trivial with Unicode TeX engines like XeTeX or LuaTeX:

\catcode€=\active
\newcommand*€[1]{\SI{...}{...}}

The same applies to the other TeX engines with 8-bit input encodings (for example, latin9).

But it is trickier with UTF-8 bytes as input, because the Euro symbol consists of three UTF-8 bytes. Three bytes cannot be one active byte, also the category codes are usually not "letter" (11), but "other" (12). Thus these UTF-8 bytes cannot be used inside command names.

The following example uses LaTeX's inputenc machinery to assign the UTF-8 byte sequence to a macro:

• If the macro is immediately followed by the left curly brace (without spaces in between), then the macro takes the number as parameter and passes it to \SI.

• If the macro is immediately followed by the left square bracket, then an optional argument together with the number is passed to \SI.

• Otherwise the normal euro sign is printed without using a parameter.

• \SI{9}{€} works.

• € is robust for \section and the table of contents.

• For hyperref's bookmarks, \texteuro can be used or an alternative for the bookmarks can be provided via \texorpdfstring. The euro symbol without arguments can be enabled by:

  \pdfstringdefDisableCommands{\let\EuroMacro\texteuro}
  

Full example:

\documentclass{article}

\usepackage[utf8]{inputenc}
\usepackage{newunicodechar}

\usepackage[T1]{fontenc}
\usepackage{textcomp}
\usepackage{lmodern}

\usepackage{ltxcmds}

\usepackage{siunitx}
\sisetup{output-decimal-marker={,}}
\DeclareSIUnit{\euro}{\texteuro}

\newcommand*{\sieuro}[2][]{\SI[{mode=text,#1}]{#2}{\euro}}

\makeatletter
\newcommand*{\EuroMacro}{}
\protected\def\EuroMacro{%
  \ltx@ifnextchar@nospace\bgroup\sieuro{%
    \ltx@ifnextchar[\sieuro\texteuro
  }%
}
\newunicodechar{€}{\EuroMacro}
\makeatother

\begin{document}
  €{12,34} and the currency is €.

  €[add-integer-zero]{.42}

  \sisetup{mode=text}
  \SI{9}{€}
\end{document}

A version with minimized number of packages in the TeX file. As variation, the euro symbol is generated by package eurosym:

\documentclass{article}

\usepackage[utf8]{inputenc}% UTF-8 input for non-Unicode TeX engines
\usepackage{eurosym}% euro sign
\usepackage{siunitx}

\sisetup{output-decimal-marker={,}}
\DeclareSIUnit{\Euro}{\euro}
\newcommand*{\sieuro}[2][]{\SI[{mode=text,#1}]{#2}{\Euro}}

\makeatletter
\newcommand*{\EuroMacro}{%
  \ifx\@let@token\bgroup
    \expandafter\sieuro
  \else
    \ifx\@let@token[%
      \expandafter\expandafter\expandafter\sieuro
    \else
      \expandafter\expandafter\expandafter\euro
    \fi
  \fi
}
\protected\def\EuroMacroAux{%
  \futurelet\@let@token\EuroMacro
}
\AtBeginDocument{%
  \DeclareUnicodeCharacter{20AC}{\EuroMacroAux}%
}
\makeatother

\begin{document}
  €{12,34} and the currency is €.

  €[add-integer-zero]{.42}

  \sisetup{mode=text}
  \SI{9}{€}
\end{document}

You can't set the category code of € in pdflatex, because it's three byte long. However, there are other methods.

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{textcomp}
\usepackage[right]{eurosym} % I can't stand textcomp euro symbol
\usepackage{newunicodechar}

\makeatletter
\newunicodechar{€}{\olivetree@euro}
\newcommand{\olivetree@euro}{\@ifnextchar\bgroup\EUR{\euro}}
\makeatother

\begin{document}

You owe me €{100.23}; please, pay in €.

\end{document}

If you want to use siunitx features for printing the number,

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{textcomp}
\usepackage[right]{eurosym} % I can't stand textcomp euro symbol
\usepackage{newunicodechar}
\usepackage{siunitx}

\makeatletter
\newunicodechar{€}{\olivetree@euro}
\newcommand{\olivetree@euro}{\@ifnextchar\bgroup\olivetree@euro@arg\euro}
\newcommand{\olivetree@euro@arg}[1]{\num{#1}\,\euro}
\makeatother

\begin{document}

You owe me €{100.23}; please, pay in €.

\end{document}

Some changes are needed for XeLaTeX or LuaLaTeX:

\documentclass{article}
\usepackage{fontspec}
\usepackage{newunicodechar}
\usepackage{siunitx}

\makeatletter
\newcommand{\olivetree@euro}{\@ifnextchar\bgroup\olivetree@euro@arg{€}}
\newcommand{\olivetree@euro@arg}[1]{\num{#1}\,€}
\newunicodechar{€}{\olivetree@euro}
\makeatother

\begin{document}

You owe me €{100.23}; please, pay in €.

\end{document}

\€ is also acceptable

Then, a simple approach:

\documentclass{article}
\usepackage{eurosym,siunitx}
\def\€#1{\SI{#1}{\mbox{\euro}}}
\begin{document}
\€{9x10,00}
\end{document}