Pulling numbers out of a file name

The code below uses regular expressions from LaTeX3 to extract all of the numbers in the filename and then makes them available as \misha{1}, \misha{2}, .... There is no error checking so, for example, if you have \misha{100} in your document then this command will fail silently, doing nothing.

If you save the code below as the file ch3lec7.tex then run it you will get the output:

Here is the code:

\documentclass{article}

\usepackage{expl3}
\ExplSyntaxOn
\cs_generate_variant:Nn \regex_extract_all:nnN {nVN}
\seq_new:N \l_misha_seq
\regex_extract_all:nVN {\d+} \c_sys_jobname_str \l_misha_seq
\newcommand\misha[1]{\seq_item:Nn \l_misha_seq {#1}}
\ExplSyntaxOff

\begin{document}

   Chapter \misha{1}, lecture \misha{2}.

\end{document}

The work is all done by the command \regex_extract_all:nVN, which puts all of the numbers in \jobname into an internal LaTeX3 sequence. (As egreg pointed out, LaTeX3 stores the filename in the string constant \c_sys_jobname_str.) The command \misha{k} prints the kth element of this sequence.