Monday, March 23, 2009

Features that fell off the radar

I was long pondering with the problem that it is quite tricky to enter regexps into syslog-ng configuration file, since if you enclose the string in double quotes (e.g. in ""), the backslash character needs to be escaped.

Since backslash is used in regexps quite often, it can become cumbersome to enter regexps like:

match("[a-z\\-]+");

Note that the backslash is doubled because otherwise the syslog-ng string parser would pass the sequence to the regexps compiler as: "[a-z-]+" which is certainly different in meaning what the above expression says.

I always remembered that syslog-ng also supports single quotes (aka apostrophes), but I remembered they behaved just as if you used normal quotation marks. Therefore I was thinking about a 3rd string format, one that would not require escaping.

However I was reading the related code the other day, and found that apostrophes work exactly the way I planned this 3rd string syntax to behave: not to get in the way when entering regexps. In fact it behaves just like apostrophes in the UNIX shells. It does not care about escaping, it only cares about the terminating apostrophe.

I was dealing with regexp related questions on the mailing list a lot, and the root cause of the problems was most times this escaping stuff, and I never knew the proper answer and behaviour is already in syslog-ng, I've just forgotten about it completely.

And now as I check the documentation for syslog-ng, it does not mention this syntax either, even though it had been present even in the 1.6.x times.

So if you had trouble writing lots of regexps in syslog-ng configuration, and I told you to properly escape your regexps, please forgive me. syslog-ng is better than I've thought :)

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