208: Regular Expressions

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Revision as of 14:49, 7 March 2014

Regular Expressions
Wait, forgot to escape a space. Wheeeeee[taptaptap]eeeeee.
Title text: Wait, forgot to escape a space. Wheeeeee[taptaptap]eeeeee.


In computing, a regular expression provides a concise and flexible means to "match" (specify and recognize) strings of text, such as particular characters, words, or patterns of characters.

Looking for a specific pattern on 200MB of text is an equivalent to "looking for a needle in a haystack". But this task can be made easy by using "regexes", since this script can find a "match", a specific string pattern, much faster than humans can achieve.

Perl is a popular scripting language, and is especially well known for the flexible and simple regular expression features that it offers.

The title text explains how sensitive the response is to small missing characters.


Whenever I learn a new skill I concoct elaborate fantasy scenarios where it lets me save the day.
Megan: Oh no! The killer must have followed her on vacation!
[Megan points to computer.]
Megan: But to find them we'd need to search through 200MB of emails looking for something formatted like an address!
Cueball: It's hopeless!
Everybody stand back.
I know regular expressions.
[A man swings in on a rope, toward the computer.]
tap tap
[The man swings away, and the other characters cheer.]


  • This comic is featured on one of the T-shirts sold at the xkcd store.
  • Randall no longer likes Perl, having discovered Python.
  • Writing a good street address finder in regular expressions is a difficult task, especially internationally. Examples.

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Hi, sorry about the previous "poor explanation". This one is pretty straight-forward (I think), given enough information about "regex"es and Perl. On a side-note, how did you find out the date for the comic? -- NariOx

The explanation was mostly accurate, my gripe was with the fact that a number of fields were empty - the transcript, date and categories weren't done. You can find the date in the "all comics" page, accessible from the sidebar. Davidy22[talk] 23:26, 29 November 2012 (UTC)

*Grumble grumble*, email should be intrinsically treated as 7-bit ASCII. 200MB of email would be almost 210 million bytes, raw. Even assuming half of that is header overheads on top of a whole lot of short message bodies, that's a lot more characters still to processed than 5 million. Of course, if people send more complex data through common MIME extensions (or the old favourites of uuencoding, etc) so as to send extended characters and binary-attachments (including various compressed formats of data, but again needing significant overheads when not containing large documents within), then that reduces the amount of characters needing searching (but probably needs a few more "use <foo>::<bar>;" bits on your Perl code, in order to give you the tools to isolate such blocks and decode what's in them for further searching within). But things went downhill ever since people could start sending emails with fancy graphical signatures and backgrounds... Yeah, I'm an Old Fogey from the dark ages. 00:00, 22 June 2013 (UTC)

I don't think a regex can isolate an address, it has so many forms that it can not be considered regular language. This discussion comes to similar conclusions: http://stackoverflow.com/questions/9397485/regex-street-address-match 05:41, 1 November 2013 (UTC)

It's an EMAIL address they're trying to match. Not quite obvious if you're not already thinking emails. At the same time, you COULD most likely catch a lot of the most common street address types, like 1234 Name Rd/St/Ave/etc. 00:52, 30 January 2015 (UTC)
No. "But to find them we'd need to search through 200MB of emails looking for something formatted like an address!" To find someone you need a physical address, not an email address. -- 14:48, 30 January 2015 (UTC)

The explanation misses the main point of the comic: a regex expert is so efficient he can solve a hard problem in a split second. Assuming, for instance, he jumped from 2 m high and he can type while he is less than 50 cm from the keyboard, he would have about .15 second to type. There's no way he could enter a long program implementing a complex algorithm but, according to Randall, this is sufficient to enter a compact regex that solves the problem. In the title text, he is swinging back like a pendulum and using his new split-second typing opportunity to fix a bug in his initial regex. Zetfr 09:55, 20 May 2015 (UTC)

It's kinda funny that people are taking this so seriously when the comic itself (and the fact that it's in a comic) establishes this as a fantastical scenario. -Pennpenn 06:47, 30 June 2015 (UTC)

This is probably how they wrote Encyclopedia Brown. -- 20:18, 26 November 2016 (UTC)

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