Here, I use tokcycle to process the tokens of the original input and add a , after each character token, unless it is inside a <...> grouping. Then, I can use listofitems to read this comma-separated list, storing each of the items in an array \mystring. This array is accessible by the index of the item.
If you need to access the actual tokens of each array element, the call to \mystring[...] needs to be twice-expanded.
\documentclass{article}
\usepackage{listofitems,tokcycle}
\newcounter{nestlevel}
\Characterdirective{\ifx<#1\stepcounter{nestlevel}\else
\ifx>#1\addtocounter{nestlevel}{-1}\addcytoks{,}\else
\addcytoks{#1}\ifnum\thenestlevel=0\relax\addcytoks{,}\fi\fi\fi}
\ignoreemptyitems
\gdef\getmystring#1{%
\setcounter{nestlevel}{0}%
\tokcyclexpress{#1}%
\expandafter\readlist\expandafter\mystring\expandafter{\the\cytoks}}
\begin{document}
\getmystring{abcdef<ghi>jkl<mn>opqrs}
The number of strings is \mystringlen.
\mystring[3], \mystring[6], \mystring[7], \mystring[11], \mystring[16]
\end{document}
If one were to allow brace grouping {...} of the input stream, rather than angle-bracket grouping <...>, the construction would be a bit simpler, to get the same result:
\documentclass{article}
\usepackage{listofitems,tokcycle}
\stripgroupingtrue
\ignoreemptyitems
\newcommand\getmystring[1]{%
\tokcycle{\addcytoks{##1,}}{\addcytoks{##1,}}{}{}{#1}%
\expandafter\readlist\expandafter\mystring\expandafter{\the\cytoks}}
\begin{document}
\getmystring{abcdef{ghi}jkl{mn}opqrs}
The number of strings is \mystringlen.
\mystring[3], \mystring[6], \mystring[7], \mystring[11], \mystring[16]
\end{document}
SUPPLEMENT
The OP comments that he would like to use this approach where the input is provided, not directly, but by way of a \defed string. Here, I provide \getmydefstring to do that:
\documentclass{article}
\usepackage{listofitems,tokcycle}
\stripgroupingtrue
\ignoreemptyitems
\newcommand\getmystring[1]{%
\tokcycle{\addcytoks{##1,}}{\addcytoks{##1,}}{}{}{#1}%
\expandafter\readlist\expandafter\mystring\expandafter{\the\cytoks}}
\newcommand\getmydefstring[1]{%
\def\tmp{\tokcycle{\addcytoks{####1,}}{\addcytoks{####1,}}{}{}}
\expandafter\tmp\expandafter{#1}%
\expandafter\readlist\expandafter\mystring\expandafter{\the\cytoks}}
\begin{document}
\def\mystring{abcdef{ghi}jkl{mn}opqrs}
\getmydefstring{\mystring}
The number of strings is \mystringlen.
\mystring[3], \mystring[6], \mystring[7], \mystring[11], \mystring[16]
\end{document}
<ghi>or could you use the syntax{ghi}(which would simplify things quite a bit) – David Carlisle May 6 '20 at 11:45<or>also? What about nesting<..>, e.g.,ab<<cde<fg>hi>jk>lm? – Ulrich Diez May 6 '20 at 17:49{ghi}– Girish May 10 '20 at 17:19