Hashtag label with tikzpicture

Question

I'm trying to compile the tree below and its working. The problem is that whenever I change the label of DP or LP to \#P (i.e. \node(\#P) {\#P}; \#), I get the following error message:

! Missing \endcsname inserted.
<to be read again>
\#
l.17 ...LP}; L [.YP Y [.FP ] ] ] ] ] ] ]
]
The control sequence marked <to be read again> should
not appear between \csname and \endcsname.
Missing character: There is no # in font nullfont!
Missing character: There is no P in font nullfont!

\documentclass[12pt]{article}

\usepackage[utf8]{inputenc}

\usepackage{tikz-qtree-compat}

\tikzset{every tree node/.style={baseline=(top.base), level distance=2em, sibling distance=4em, align=center, parent anchor=south, child anchor=north, anchor=north}, sibling distance=15pt}

\usetikzlibrary{positioning} 

\begin{document}

\begin{tikzpicture}  [inv/.style={overlay, coordinate  }]  

\Tree [.XP X [.ZP Z [.\node(DP) {DP}; D    [.\node(LP) {LP}; L      [.YP Y [.FP     ] ] ] ] ]  ] ] ] 

\node [ right=1cm of DP,font=\itshape] (X) {Blah};

\draw[<-] (X.west) -- (DP);

\node [ right=1cm of LP,font=\itshape] (X) {Blah};

\draw[<-] (X.west) -- (LP);    

\end{tikzpicture}

\end{document}

Answer 1

# is a special character in TeX which you have to escape if you want it printed. So, you have to escape it in normal text. However, names of nodes do not allow macros, so you cannot use \# in the node name.

Hence, I suggest you to use another letter character like x to replace \# in the node name (which is only used internally in your code and won't be typeset anyways).

Note that even most “special characters [...] including commas, semicolons, hyphens, braces, dots, parentheses, slashes, dashes, and more” (cited the TikZ manual) cannot be used in node names.

On changing \catcode, see here.

---

EDIT Possible solutions: 1) just name the node differently from its text; 2) if node name and node text have to be identical, you could use a macro to replace xD to \#D:

\documentclass[12pt]{article}
\usepackage[utf8]{inputenc}
\usepackage{tikz}

\usepackage{xstring}
\newcommand{\xtohash}[1]{\StrSubstitute[0]{#1}{x}{\#}}

\begin{document}

\begin{tikzpicture}

\node at (0,0) (xD) {\#D};

\node at (0,1) (xD) {\xtohash{xD}};

\end{tikzpicture}

\end{document}