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copied!<p>If the question really is "What is wrong with this expression?", here's the answer:</p> <ul> <li>You're trying to match newlines with a <code>.*?</code>. You need <code>(?s)</code> for that to work.</li> <li>You have explicit spaces and newlines in the middle of the regex that don't have any corresponding characters in the source text. You need <code>(?x)</code> for that to work.</li> </ul> <p>That may not be <em>all</em> that's wrong with the expression. But just adding <code>(?sx)</code>, turning it into a raw string (because I don't trust myself to mix Python quoting and regex quoting properly), and removing the <code>\n</code> gives me this:</p> <pre><code>r'(?sx)(\\problem{(.*?)}? \\subproblem{(.*?)} (.*?)) (\\problem|\\subproblem|\\end)' </code></pre> <p>That returns 2 matches instead of 0, and it's probably the smallest change to your regex that works.</p> <p>However, if the question is "How can I parse this?", rather than "What's wrong with my existing attempt?", I think impl's solution makes more sense (and I also agree with the point about using regex to parse TeX being usually a bad idea)—-or, even better, doing it in two steps as Regexident does.</p> <hr> <blockquote> <p>if using regex to parse TeX is not good idea, then what method would you suggest to parse TeX?</p> </blockquote> <p>First of all, as a general rule of thumb, if I can't write the regex to solve a problem by myself, I don't want to solve it with a regex, because I'll have a hard time figuring it out a few months from now. Sometimes I break it down into subexpressions, or use <code>(?x)</code> and load it up with comments, but usually I look for another way.</p> <p>More importantly, if you have a real parser that can consume your language and give you a tree (or whatever's appropriate) that you can walk and search—as with, e.g. <code>etree</code> for XML—then you've got 90% of a solution for every problem you're going to come up with in dealing with that language. A quick&dirty regex (especially one you can't write on your own) only gets you 10% of the way to solving the next problem. And more often than not, if I've got a problem today, I'm going to have more of them in the next few months.</p> <p>So, what's a good parser for TeX in Python? Honestly, I don't know. I know <code>scipy</code>/<code>matplotlib</code> has something that does it, so I'd probably look there first. Beyond that, check Google, PyPI, and maybe tex.stackexchange.com. The first things that turn up in a search are <a href="http://www.profv.de/texcaller/" rel="nofollow"><code>Texcaller</code></a> and <a href="http://plastex.sourceforge.net/plastex/" rel="nofollow"><code>plasTeX</code></a>. I have no idea how good they are, or if they're appropriate for your use case, but it shouldn't take long to skim the tutorials and find out.</p> <p>If it turns out that there's nothing out there, and it comes down to writing something myself with, e.g., <a href="http://pyparsing.wikispaces.com" rel="nofollow"><code>pyparsing</code></a> vs. regexes, then it's a tougher choice. Some languages, it's very easy to define just the subset you care about and leave the rest as giant uninterpreted tokens, in which case a real parser will be just as easy as a regex, so you might as well go that way. Other languages, you have to handle half the syntax before you can do anything useful, so I wouldn't even try. I'd have to put a bit of time into thinking about it and experimenting both ways before deciding which way to go.</p>

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