Ruby Best Practices

Regular Expressions

At the time of writing this chapter, I was spending some time watching the Dow Jones Industrial Average, as the world was in the middle of a major financial meltdown. If you’re wondering what this has to do with Ruby or regular expressions, take a quick look at the following code:

require "open-uri"
loop do
  puts( open("http://finance.google.com/finance?cid=983582").read[
  /<span class="\w+" id="ref_983582_c">([+-]?\d+\.\d+)/m, 1] )
  sleep(30)
end

In just a couple of lines, I was able to throw together a script that would poll Google Finance and pull down the current average price of the Dow. This sort of “find a needle in the haystack” extraction is what regular expressions are all about.

Of course, the art of constructing regular expressions is often veiled in mystery. Even simple patterns such as this one might make some folks feel a bit uneasy:

/<span class="\w+" id="ref_983582_c">([+-]?\d+\.\d+)/m

This expression is simple by comparison to some other examples we can show, but it still makes use of a number of regular expression concepts. All in one line, we can see the use of character classes (both general and special), escapes, quantifiers, groups, and a switch that enables multiline matching.

Patterns are dense because they are written in a special syntax, which acts as a sort of domain language for matching and extracting text. The reason that it may be considered daunting is that this language is made up of so few special characters:

\ [ ] . ^ $ ? * + { } | ( ...