It's almost a year later, but as the OP has not yet accepted an answer, I still see my chance :-)
In the following, I use the same basic principle as in my other answer, that is, employ listings moredelim=** option to define delimiters, which styles then apply on top of all other formattings, so that the syntax formatting is kept.
However, instead of being limited to a standard font command for the delimiter style, I now use an lrbox to grab the content of the current group. The resulting box holds the (syntax-formatted) output of listings, which can be used to implement more fancy highlighting styles.
In the example code, the implementation of the \btHL command employs TikZ to typeset the lrbox into a TikZ node. Additional TikZ/PGF options to both, the tikzpicture and the node can be given in the optional argument (such as \btHL[fill=red!20,draw=red]), which provides for pretty flexible highlighters.
(Note: when the optional argument is used inside some listings argument, the whole \btHL[<key>=<value>,...] command has to be put into curly braces in order to not confuse the listings key=value parser.)
The \btHL command can as well be used inside normal text. It does, however, not work across line breaks, which is a possible disadvantage compared to the approach that culminated from Bens answer. Insides of listings, however, this usually is not an issue.
\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{listings,xcolor,beramono}
\usepackage{tikz}
\makeatletter
\newenvironment{btHighlight}[1][]
{\begingroup\tikzset{bt@Highlight@par/.style={#1}}\begin{lrbox}{\@tempboxa}}
{\end{lrbox}\bt@HL@box[bt@Highlight@par]{\@tempboxa}\endgroup}
\newcommand\btHL[1][]{%
\begin{btHighlight}[#1]\bgroup\aftergroup\bt@HL@endenv%
}
\def\bt@HL@endenv{%
\end{btHighlight}%
\egroup
}
\newcommand{\bt@HL@box}[2][]{%
\tikz[#1]{%
\pgfpathrectangle{\pgfpoint{1pt}{0pt}}{\pgfpoint{\wd #2}{\ht #2}}%
\pgfusepath{use as bounding box}%
\node[anchor=base west, fill=orange!30,outer sep=0pt,inner xsep=1pt, inner ysep=0pt, rounded corners=3pt, minimum height=\ht\strutbox+1pt,#1]{\raisebox{1pt}{\strut}\strut\usebox{#2}};
}%
}
\makeatother
\lstdefinestyle{SQL}{
language={SQL},basicstyle=\ttfamily,
moredelim=**[is][\btHL]{`}{`},
moredelim=**[is][{\btHL[fill=green!30,draw=red,dashed,thin]}]{@}{@},
}
\begin{document}
A listing with {\btHL highlighting of all \textbf{important} elements} looks as follows:
\begin{lstlisting}[style=SQL]
SELECT name, password `FROM` users @WHERE@ name=@UNION SELECT@
\end{lstlisting}
The problem is that the following two lines in your \lstdefinelanguage{XML} are in conflict:
morestring=[s]{>}{<},
morecomment=[s][\color{orange}]{<!--}{-->},
In addition, doesn't \lstdefinelanguage{XML} redefine the XML language settings, thereby losing a lot of configs? One of the most noticeable drawbacks is that you can no longer access tagstyle, while the identifierstyle setting would highlight both tag names and contents (the embedded LaTeX code).
Would it be feasible for you to move all the settings into \lstdefinestyle instead, dropping morestring=[s]{>}{<}, and using tagstyle in place of identifierstyle?
\lstdefinestyle{listXML}{language=XML, extendedchars=true, belowcaptionskip=5pt, xleftmargin=1.8em, xrightmargin=0.5em, numbers=left, numberstyle=\small\ttfamily\bf, frame=single, breaklines=true, breakatwhitespace=true, breakindent=0pt, emph={}, emphstyle=\color{red}, basicstyle=\small\ttfamily, columns=fullflexible, showstringspaces=false, commentstyle=\color{gray}\upshape,
morestring=[b]",
morecomment=[s]{<?}{?>},
morecomment=[s][\color{orange}]{<!--}{-->},
keywordstyle=\color{cyan},
stringstyle=\color{black},
tagstyle=\color{darkblue},
morekeywords={xmlns,version,type}
}
This is the output I got:
Best Answer
Load the defined language SQL and define more keywords, if needed. Alternetivily write a package
SPARQL.stywith a a complete language definition. Choose the one fromSQLas template which is in the filelstlang1.sty