Annotating a forest tree with its corresponding bracketing

Question

In order to teach people how to use forest to draw trees, it's helpful to be able to show the relationship between the tree itself and the bracketing. I've written some code to do this, but I suspect that there is also a more elegant way to do this using forest itself. Is there a simpler/better way to do this?

\documentclass[12pt,border=.5in]{standalone}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[linguistics]{forest}
\forestset{nice nodes/.style={for tree={inner sep=0pt,s sep=.4in}},default preamble=nice nodes}
\usepackage{xstring}
\usepackage{amsmath}
\makeatletter
% This code based on a suggestion from Alenanno
\newcommand\nd[1]{%
\noexpandarg
\IfSubStr{#1}{\\}
{\def\nodetext{\StrBefore{#1}{\\}}}
{\def\nodetext{#1}}
\leavevmode\llap{\color{red}[$_{\text{\nodetext}}$}#1}
\makeatletter
% This code due to David Carlisle
\newcommand*\dnx{\textcolor{red}{]}}
\newcommand\dn{\hbox to \z@\bgroup\let\dn\dnxx\dnxx}
\newcommand\dnxx{\dnx\@ifnextchar\dn{}{\hss\egroup}}
\makeatother
\begin{document}
\begin{forest}
[\nd{TP}
    [\nd{DP} [{\nd{D\\the\dn}} ] [\nd{NP} [\nd{N\\man\dn\dn\dn} ]]]
    [\nd{T’}
       [\nd{T\\will\dn} ]
       [\nd{VP}
          [\nd{V\\eat\dn} ]
          [\nd{DP} [\nd{D\\a\dn\dn} ] [\nd{NP} [\nd{N\\burger\dn\dn\dn\dn\dn\dn} ]]]
       ]
    ]
]
\end{forest}
\end{document}

Answer 1

Here's an all-Forest version thanks to Sašo's explaining how to detokenize to test for a string in multi-line content. No xstring or amsmath required!

\documentclass[12pt,border=25pt]{standalone}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[linguistics]{forest}
\forestset{%
  declare toks={dntoks}{},
  declare toks={ndtoks}{},
  nd/.style={%
    delay={%
      % detokenize to test for string in multi-line content: Sašo Živanović: https://tex.stackexchange.com/questions/330228/annotating-a-forest-tree-with-its-corresponding-bracketing/330237?noredirect=1#comment809615_330237
      temptoksa/.expanded=\detokenize{\\},
      temptoksb/.option=content,
      temptoksb/.wrap value=\detokenize{##1},
      temptoksb/.expanded/.register=temptoksb,
      if={instr(temptoksa,temptoksb)}{%
        split register/.wrap pgfmath arg={{temptoksb}{##1}{ndtoks,temptoksc}}{(temptoksa)},
      }{%
        ndtoks/.option=content,
      },
    },
    before typesetting nodes={%
      content/.wrap value={\llap{\textcolor{red}{[\textsubscript{\foresteoption{ndtoks}}}}##1},
    },
  },
  dn/.style={%
    repeat=#1{%
      dntoks+={]},
    },
    before typesetting nodes={%
      content/.wrap value={##1\rlap{\textcolor{red}{\foresteoption{dntoks}}}},
    },
  },
}
\begin{document}
\begin{forest}
  [TP, nd
    [DP, nd
      [D, nd
      ]
      [NP
        [N\\man, dn=2
        ]
      ]
    ]
    [T', nd
       [T\\will, nd, dn=1
       ]
       [VP, nd
         [V\\eat, nd, dn=1
         ]
         [DP, nd
           [D\\a, nd, dn=1
           ]
           [NP, nd
             [N\\burger, nd, dn=6
             ]
           ]
         ]
       ]
    ]
  ]
\end{forest}
\end{document}

If all nodes should take the nd style (like the example in the question), then this can obviously be simplified further.

\documentclass[12pt,border=25pt]{standalone}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[linguistics]{forest}
\forestset{%
  declare toks={dntoks}{},
  declare toks={ndtoks}{},
  nd/.style={%
    delay={%
      % detokenize to test for string in multi-line content: Sašo Živanović: https://tex.stackexchange.com/questions/330228/annotating-a-forest-tree-with-its-corresponding-bracketing/330237?noredirect=1#comment809615_330237
      temptoksa/.expanded=\detokenize{\\},
      temptoksb/.option=content,
      temptoksb/.wrap value=\detokenize{##1},
      temptoksb/.expanded/.register=temptoksb,
      if={instr(temptoksa,temptoksb)}{%
        split register/.wrap pgfmath arg={{temptoksb}{##1}{ndtoks,temptoksc}}{(temptoksa)},
      }{%
        ndtoks/.option=content,
      },
    },
    before typesetting nodes={%
      content/.wrap value={\llap{\textcolor{red}{[\textsubscript{\foresteoption{ndtoks}}}}##1},
    },
  },
  dn/.style={%
    repeat=#1{%
      dntoks+={]},
    },
    before typesetting nodes={%
      content/.wrap value={##1\rlap{\textcolor{red}{\foresteoption{dntoks}}}},
    },
  },
  default preamble={%
    for tree={nd},
  },
}
\begin{document}
\begin{forest}
  [TP
    [DP
      [D
      ]
      [NP
        [N\\man, dn=2
        ]
      ]
    ]
    [T'
       [T\\will, dn=1
       ]
       [VP
         [V\\eat, dn=1
         ]
         [DP
           [D\\a, dn=1
           ]
           [NP
             [N\\burger, dn=6
             ]
           ]
         ]
       ]
    ]
  ]
\end{forest}
\end{document}