(originally from https://web.archive.org/web/20150422001421/http://ajcsystems.com/blog/blog/2012/10/10/regular-expression-non-capturing-groups-or-whats-mean/)

When I was working recently on changing poodledo‘s parser from using a lexer to using regular expressions, I found myself implementing some crazy logic to try and ignore the results from matched groups whose matches I didn’t actually want (since I was using parentheses for their “pick one of these two alternatives” function instead of their “pull out this specific chunk as a match” function).

For example, take the string “task title @context name #due date”. Starting with the at-sign, I want to match all words until some other symbol is reached; or, to avoid confusing the parser, match everything following the at-sign and between two square brackets. Here’s the regex I came up with:

(^|\W)@([\b ]?(([\w\"\'.@]+ )*[\w\"\'.@]+)[\b]??|[\b(.+?)\b])

Using this regex, though, gave me a lot of extraneous matching groups:

>>> q = r'(^|\W)@([\b ]?(([\w\"\'.@]+ )*[\w\"\'.@]+)[\b]??|[\b(.+?)\b])'
>>> task = 'task title @context name #due date'
>>> re.findall(q, task)
[(' ', 'context name', 'context name', 'context ', '')]

How do I determine programmatically which of these groups is the one I want?

Digging around in the Python regex documentation, I found a section about non-capturing groups, which seemed like it would save me a significant amount of sanity. Apparently, Perl-5-compatible regular expression parsers often implement a regex extension syntax by following an open parenthesis with a question mark, and then some other symbols that indicate which extension is in use. In this specific case, “(?:” indicates a group whose matches shouldn’t be captured in the output list. Eureka!

>>> q = r'(?:^|\W)@(?:[\b ]?((?:[\w\"\'.@]+ )*[\w\"\'.@]+)[\b ]??|[\b(.+?)\b])'
>>> re.findall(q, task)
[('context name', '')]

Now the only logic I need is “Ignore matching groups which are empty”, my sanity is preserved, and all is well with the world.

(originally from https://web.archive.org/web/20150422203246/http://ajcsystems.com/blog/page/2/)

I recently had to fix a server which had been knocked off the network due to some bad configuration (very long story). When I logged into the remote graphical console, I was greeted with constant, unremitting spam from the kernel log! I wasn’t able to log in because the screen would clear instantly when I started typing.

This calls for the Magic SysRq key! A simple press of Alt+SysRq+0 to set the console log level to 0 silenced all of the log spam and let me log in and get the server back on its feet.

All hail Magic SysRq!

(originally from https://web.archive.org/web/20150423071705/http://ajcsystems.com/blog/blog/2012/10/02/the-unbearable-lightness-of-user-data/)

When you launch an instance in OpenStack, it has a big empty box called “User Data”:

I never knew what that box was for until today.

How it’s provided

If an image is configured to use cloud-init, when it starts up it will curl a special path:

$ curl http://169.254.169.254/2009-04-04/user-data/

Since this is a link-local “I don’t have a DHCP address yet” IP, it will work even before networking is configured. The output from this URL is whatever you put in “User Data”.

How it’s used

cloud-init has a bunch of different tweaks it can apply as an instance launches; it can run apt-get upgrade, import ssh keys, disable sshd PasswordAuthentication, run arbitrary commands, and much more. This sample config will give you a pretty good idea about the full capabilities of cloud-init.

Basic example

When you launch an instance, put the following in “User Data”:

#cloud-config
ssh_pwauth: True

This will make cloud-init enable PasswordAuthentication in sshd (which it otherwise disables by default), so that you don’t need to use a key to log in to the instance.

(originally from https://web.archive.org/web/20150504012625/http://ajcsystems.com/blog/blog/2012/08/11/i-love-it-when-a-plan-comes-together/)

I recently purchased a HDHomeRun Prime to replace my first-gen HDHomeRun, with the intention of using a CableCard to replace my ugly kludge of grabbing video from a cable box via Firewire. TL;DR – it just worked.

I set up the new HDHR and pointed MythTV at it, and it was detected and set up easily. I chatted with Comcast’s online tech support to ask how to get a CableCard, and they said I could either have a tech deliver one or pick it up myself at a service center. I opted to drive to the service center, where I waited for just a couple of minutes in a line of three people. The desk representative knew what a CableCard was and dug one out for me in a flash; I don’t think I was in the building more than five minutes. When I got home, I connected it to the new HDHR and went through the online activation process. After about a half-hour (as they warned me it might take that long), I fired up mythtv-setup and tried to scan channels on the CableCard. It worked great the first time, all of the channels and schedules downloaded, and I was able to start watching every channel I cared about on my HTPC immediately.

It’s so very rare that everything just works the way it’s supposed to with all this high-tech computer stuff, I like to savor the moments when things do go well. I had read a lot of horror stories on the web before planning all of this, but I couldn’t be happier about how it’s worked out.

Now to resolve the issue with the OpenGL UI painter so the program guide can occupy the entire screen… it’s good to know that there’ll always be something else to fix, too.