Reading Patrick McKenzie’s excellent practical example of metaprogramming, I came across a line of code I didn’t understand:
That line taught me three new things about Ruby:
- The syntax for the subscript operator
allows multiple arguments. (It turns out I already knew this in another context:
[1,1,2,3,5,7][2,3] => [2,3,5])
- You can subscript a String with a Regexp, returning the first match:
"goal"[/[aeiou]/] => "o"(
nilis returned if there is no match).
- If you throw in an index
n, then you get the
nth capturing group of the first match:
"xaabb"[/(.)\1/, 1] => "a"(or
nilagain if no match).
That last one is interesting, because it means there’s a concise way I didn’t previously know about to achieve a common regex task: checking if an input string matches a given format, and if so, extracting part of the format. Say we want to pull out the domain from an email address, but complain if we can’t find it:
Before learning this trick I would have either used a temporary match object a la Java, or gritted my teeth and used a global variable Perl-style:
Both of those seem rather verbose. They can be golfed into one-liners, but the readability starts to suffer:
So I’m left wondering what’s the most readable and/or idiomatic style for regexes in Ruby. TMTOWTDI indeed! Even now I know what it means,
"xaabb"[/(.)\1/,1] makes me double-take slightly - it’s an unusual way to use
 - but I guess it’s just another Ruby idiosyncracy I’ll come to know and love.