Delan Azabani

Writing an IRC sedbot

The ComSSA IRC has an official utility bot, KhlavKalash, which currently does some trivia like URL title fetching and server uptime. It uses Twisted for IRC and has a good plugin system with Yapsy. Feeling a bit bored, I decided to try my hand at implementing what I call a "sedbot", which interprets messages that are sed replacement expressions, executing the replacement on the last normal message.

As I'm writing this, KhlavKalash allows you to create plugins by simply dropping Python files into a subdirectory with your own classes, each of which represents essentially a simple bot. The current interface is limited to callbacks on normal messages matching supplied regular expressions, and allows you to return zero or one response message to be sent back. This could change to become more flexible in the future.

Initially I planned to make replacements operate on the requesting user's last message, as opposed to the last message from anyone in the channel. This would currently not be possible with the KhlavKalash API because requesting nicks are not passed to the plugins. Thankfully, I discovered that most sedbots operate on the last message in general anyway, as it allows users to "correct" messages from other people, often for snarky effect.

The most difficult, or rather, verbose, part of the implementation was actually parsing the sed expressions. Not the regular expressions themselves, but simply splitting s/a/b/f into a, b and f — you can't simply split the strong on / because patterns and replacements can have slashes in them via backslash escapes. I wrote a simple finite state machine to parse the components of sed expressions, and about 100 lines later, I was in business.

There were a couple of things I couldn't implement in an obvious way. The regex module is much better than re but uses the same API. You can use the count argument to implement the global replacement flag, and the regex.IGNORECASE bit in flags to implement /i, but there isn't an easy way to choose which occurrence to start replacing from, which is provided by sed via a numeric flag, e.g. s/a/b/42.

sed allows you to specify multiple expressions by separating them with semicolons. This is unambiguous, because the last component of a sed expression are the flags, and a semicolon is not a valid flag. Some simple additions to my parser's two flags states and support for this was implemented. Unfortunately, this comes at the expense of the ability to allow the omission of the final slash, something I hadn't yet done but was requested by a few users, because a semicolon in the replacement section would be ambiguous.

I tackled Unicode support by treating incoming byte strings as UTF-8, and returning unicode strings back to the core module. An interesting thing to note is that the IRC protocol has no concept of text encoding at all; it only sees bytes, leaving it up to clients to decide how to interpret them. Ideally all clients would use UTF-8, but this is not always the case. Regardless, the bot assumes UTF-8 everywhere, in the hope that anyone using non-ASCII characters is using UTF-8, but there really is little other choice.

Formal grammars are apparently covered in Theoretical Foundations of Computer Science 300, which I'm not due to study until some time next year. Having written a couple of simple finite state machine parsers in the past as well as today, I was keen to see how it fit into the grand scheme of things.

I started looking for more information after writing the bot, and I found out that the very fact that I used a state machine indicates that the language of sed expressions (if you are not parsing the regular expressions inside) is a regular language, which is a subset of the context-free languages that can be parsed by, you guessed it, regular expressions.

Accepting the implicit challenge that the realisation yielded, I managed to create a regular expression that can be used to split a sed replacement expression into its three parts. I decided against switching to it, as the old parser is probably more readable and extensible, but it's worth sharing. Imagine these two lines as one:


You can use the regular expression to parse sed replacement expressions, even a semicolon delimited sequence of expressions, like the FSM parser. The regex will yield three captured groups per sed expression, one each for the needle, replacement and flags components respectively. All you need to do is collapse backslash escapes, and split flags as necessary, both of which are easy tasks. With verbose mode I can document the behemoth:

^			# start of the message
(?:			# BEGIN first sed expression
  s/			#   sed replacement expression delimiter
  (			#   BEGIN needle component
    (?:			#     BEGIN single needle character
      [^\\/]		#       anything that isn't a slash or backslash...
      |\\.		#       ...or any backslash escape
    )*			#     END single needle character, zero or more
  )			#   END needle component
  /			#   slash between needle and replacement
  (			#   BEGIN replacement component
    (?:			#     BEGIN single replacement character
      [^\\/]|\\.	#       escape or non-slash-backslash, as above
    )*			#     END single replacement character, zero or more
  )			#   END replacement component
  /			#   slash between replacement and flags
  (			#   BEGIN flags component
    (?:			#     BEGIN single flag
      g|i|\d+		#       "g", "i" or a sequence of digits
    )*			#     END single flag, zero or more
  )			#   END flags component
)			# END first sed expression
(?:			# BEGIN optional subsequent sed expressions
  ;			#   semicolon between sed expressions
  s/			#   sed replacement expression delimiter, as above
  ((?:[^\\/]|\\.)*)	#   needle component, as above
  /			#   slash between needle and replacement, as above
  ((?:[^\\/]|\\.)*)	#   replacement component, as above
  /			#   slash between replacement and flags, as above
  ((?:g|i|\d+)*)	#   flags component, as above
)*			# END optional subsequent sed expressions, zero or more
$			# end of the message