LaTeX: Access every character of an string variable

Simple Solution

This can be accomplished with the xstring package. In particular, the \StrMid{string}{#2}{#3} command allows you to take the substring of string from character positions #2 through #3.

\documentclass{article}
\usepackage{xstring}
\newcommand{\myText}[2]{\StrMid{2014/12/20}{#1}{#2}}

\begin{document}
\myText{1}{4}
\end{document}

Allowing for \myTest{}

However, the above solution always requires arguments for \myTest.

It seems you want \myText{} to give you the entire string 2014/12/20. So, we can make the first parameter optional (it defaults to the first position if not given in brackets), and the second parameter is the second position. We set up a conditional so that if the second position is empty, we just get the entire string.

\documentclass{article}
\usepackage{xstring}
\newcommand{\myText}[2][1]{
    \ifx\\#2\\
        {2014/12/20}
    \else
        \StrMid{2014/12/20}{#1}{#2}
    \fi
}

\begin{document}

\myText{}

\myText[1]{4} \quad
\myText[6]{7} \quad
\myText[9]{10} \quad

\myText{7} \quad
\myText[6]{10}
\end{document}

With the stringstrings package.

\documentclass{article}
\usepackage{stringstrings}
\begin{document}
\def\x{2014/12/20}
\substring{\x}{1}{4}\par
\substring{\x}{6}{7}\par
\substring{\x}{9}{$}\par% The $ implies the last character of the string.
\end{document}

---

Alternative with the listofitems package, set up here to parse the input string on the basis of a / separator.

\documentclass{article}
\usepackage{listofitems}
\begin{document}
\def\x{2014/12/20}
\setsepchar[.]{/}
\readlist\mydate{\x}
The year is \mydate[1]\par
The month is \mydate[2]\par
The day is \mydate[3]\par
\end{document}

A LuaLaTeX-based solution, which sets up a TeX macro called \substring that takes three arguments.

The first argument of the macro \substring is a string. It can be either a hard-coded string or a TeX macro that produces a string. E.g., if \myText is defined via \newcommand{\myText}{2014/12/20}, then \myText could used as the first argument of \substring.

The second and third arguments are the starting and ending indexes of the substring and are assumed to be integers. Please indicate if this assumption isn't valid.

% !TEX TS-program = lualatex
\documentclass{article}
\newcommand\substring[3]{%
    \directlua{ tex.sprint ( string.sub ( "#1", #2, #3 ) ) } }
\begin{document}
\substring{2014/12/20}{1}{4}
\end{document}