I want to italicize a part of a single line comment, for example:
string new = old; // new is a *copy* of old In the above, I want "copy" to be italicized, while keeping what ever formatting the comments already have (i.e. the font size, color, etc...).
I've tried using moredelim but it does not work inside comments. Any help would be greatly appreciated, thank you.
As you are using tag listings, I assume you are using package listings. With listings, you can use escapechar=* to escape from the listing to LaTeX and escapebegin=\itshape to make the escaped part italic:
\documentclass{article} \usepackage{listings} \begin{document} \begin{lstlisting}[escapechar=*,escapebegin=\itshape] string new = old; // new is a *copy* of old \end{lstlisting} \end{document} 
See the manual for more information about escaping.
If you redefine the commenting mechanism for this style of comment, then you can make its style accumulative (with a *) and apply another style afterwards:
\documentclass{article} \usepackage{listings} \begin{document} \lstset{language=C,morecomment=*[l][]{//},moredelim=*[is][\itshape]{*}{*}} \begin{lstlisting} string new = old; // new is a *copy* of old \end{lstlisting} \end{document} 