Count number of citations from each proceedings with natbib

Count number of citations from each proceedings with natbib

I want to count the number of citations from each proceedings. If I was
using biblatex, I could define a counter:
\usepackage{biblatex}
\usepackage{xstring}
\newcounter{c_foo}
\newcounter{c_bar}
\AtDataInput{
% Increment the counter if the booktitle contains 'foo'
\IfSubStr{\thefield{Booktitle}}{foo}{ \stepcounter{c_foo} }{}
\IfSubStr{\thefield{Booktitle}}{bar}{ \stepcounter{c_bar} }{}
< ... etc ... >
{}
}
I can't do this, because I'm using natbib rather than biblatex, and using
both gives me errors like
biblatex2.sty:402: LaTeX Error: Command \bibhang already defined.
[\newlength{\bibhang}]
How can I count the number of citations where foo is contained in one of
the fields, while using natbib?