Why does LaTeX replace some spaces with brackets or underscores in the listings environment?

Question

I'm using LaTeX on Windows with MikTeX and I'm seeing some odd behaviour. In the output, some of the spaces are replaced with bracket-like underscores. This is the code:

\documentclass[10pt]{article}
\usepackage{listings}
\begin{document}
\begin{lstlisting}[language=SAS, title=SAS code to format data on government patients]
code code;                                 
        code code;

code;
code;
code;
c
      identity = "Subject's identity"
      variable = "Subject's name in government archive of files";
FORMAT gender genderft.
       code
       response responseft.;
RUN;
\end{lstlisting}
\end{document}

This generates the following output:

I've replaced all tabs with spaces, copied the code into a plain text editor and retyped all the spaces, and still the problem persists. Even copying the code into the editor from my browser after the question is posted doesn't work. The bracket/underscores are still there.

No other code snippets in my document present this problem, yet they also use identical listings syntax to this example and also use SAS code.

Answer 1

The text between the two apostrophes (the one in the identity = "Subject' line and the apostrophe in the following line) is being interpreted as a string. Passing the option showstringspaces=false to the environment renders the desired output, namely

\begin{lstlisting}[showstringspaces=false,language=SAS, title=SAS code to 
format data on government patients, frame=single]

A cleaner solution would be to mark the quotes as string delimiters.

This could be accomplished by adding the line \lstdefinelanguage{SAS}{morestring=[b]"} in the preamble. The [b] allows quotes inside strings by preceding them by a backslash. Note that the showstringspaces=false option is still required.