Typesetting "hollow slash"

You can use \holwd to set the gap.

\documentclass[12pt]{article}
\usepackage{stackengine}
\newcommand\hollowslash{\setbox0=\hbox{/}\def\holwd{3pt}%
  \stackengine{-.3pt}{/}{\rlap{\kern-1pt\rule{\holwd}{.4pt}}}{O}{r}{F}{F}{S}%
  \kern\dimexpr\holwd-\wd0-.2pt\relax%
  \stackengine{-.4pt}{/}{\llap{\rule{\holwd}{.4pt}\kern-1pt}}{U}{l}{F}{F}{S}%
}
\newcommand\hollowbackslash{\setbox0=\hbox{\textbackslash}\def\holwd{2.8pt}%
  \stackengine{-.4pt}{\textbackslash}{\rlap{\kern-1pt\rule{\holwd}{.4pt}}}{U}{r}{F}{F}{S}%
  \kern\dimexpr\holwd-\wd0-.2pt\relax%
  \stackengine{-.3pt}{\textbackslash}{\llap{\rule{\holwd}{.4pt}\kern-1pt}}{O}{l}{F}{F}{S}%
}
\usepackage{amsmath}
\begin{document}
$C\hollowslash(A\hollowbackslash B)$
\end{document}

If you don't mind loading the graphicx package, you can use \reflectbox to cut down on the code. Here it is with \holwd set to 2pt:

\documentclass[12pt]{article}
\usepackage{stackengine}
\newcommand\hollowslash{\setbox0=\hbox{/}\def\holwd{2pt}%
  \stackengine{-.3pt}{/}{\rlap{\kern-1pt\rule{\holwd}{.4pt}}}{O}{r}{F}{F}{S}%
  \kern\dimexpr\holwd-\wd0-.2pt\relax%
  \stackengine{-.4pt}{/}{\llap{\rule{\holwd}{.4pt}\kern-1pt}}{U}{l}{F}{F}{S}%
}
\newcommand\hollowbackslash{\reflectbox{\hollowslash}}
\usepackage{amsmath,graphicx}
\begin{document}
$C\hollowslash(A\hollowbackslash B)$
\end{document}

These symbols are defined by the stmaryrd package under the names \fat(b)slash (the spacing for \fatslash is not so good):

\documentclass{article}

\usepackage{stmaryrd}

\begin{document}

\[ A\fatbslash B\qquad C \fatslash D \]

\end{document} 

Edit: as suggested by @AndrewSwann, we can have a much better spacing for \fatslash using this code:

\documentclass{article}

\usepackage{stmaryrd}
\newcommand{\myfatslash}{\mathbin{\mkern-6mu\fatslash}}

\begin{document}

\[ A\fatbslash B\qquad C\myfatslash D \]

\end{document}