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The grossly inefficient regular expression problem:

Originally in my code I was using alarm to time-out grossly inefficient regexes. This was effective, but too effective; while the alarm was timing out, I was unable to trap the exception it threw, because the exception was happening inside a pattern match atom; eval, and Try::Tiny would both miss it unless I installed a custom exception handler. And when I did that, the alarm wouldn't ever stop the match because Perl considers the match an unsafe time to jump into a custom handler.

In versions without a custom $SIG{ALRM} handler, the alarm would go off, the process would die (because eval and Try::Tiny failed to trap it), and Dotcloud's supervisor would re-spawn the process. The end user would get a 404. This was the better of two evils; if a $SIG{ALRM} handler were installed the process wouldn't time out, and could run-away with a cleverly crafted regex. So up until recently, I took the first approach of letting it just die and respawn, to buy some time while I looked for a better alternative.

The solution:

The solution turned out to be to integrate Sys::SigAction into the timeout process. After some fiddling that worked out quite well. I was able to set a fairly short timeout, and instead of seeing a 404, the user who submits a particularly nasty regular expression gets a more friendly (though not terribly useful) experience.

The repo:

So now that I feel a little more comfortable with how exceptional circumstances are being handled, I guess it's time to follow through on my earlier commitment to make the code available. I've pushed it to a public Github repo at


I got a pleasant email from a representative at Dotcloud today thanking me for putting the Regex Tester online, and using Dotcloud to do it. Remember that this is a free "sandbox" app, so they're not making anything on it. I really put up the RE tester as a way to experiment with DC since I've been contemplating using them to deploy a larger private project I'm working on for a $client. Through this and other correspondence with them I'm learning that the folks at Dotcloud are pretty serious about making developers happy with their services. Their current pricing structure really opens the doors for developers to play with pet projects for free, presumably with the hope that as developers gain familiarity with the platform, paying business will follow. My experience with them so far has been good. One application I've been working on will deploy with a couple of web server instances (four processes each), and two database instances (with automatic replication). DC automatically places each instance in a different EC2 availability zone, and handles load balancing and automatic failover without any extra work on the part of the developer. Overall it's a bit more expensive than bare EC2, but they seem to provide enough benefits to justify the cost.


In reply to Re: The Perl Regex Tester by davido
in thread The Perl Regex Tester by davido

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