同じページにある、こっちが21世紀の常識なんだよ。おじいちゃん。
| In 2008 book "Beautiful Code", a collection of articles, It is Brian
| Kernighan saying " recursion is a win. This fundamental programming
| technique almost always leads to smaller, cleaner, and more elegant code
| than the equivalent written with explicit loops, and that is the case here.
| The idea of peeling off one matching character from the front of the regular
| expression and from the text, then recursing for the rest, echoes the
| recursive structure of the traditional factorial or string length examples,
| but in a much more interesting and useful setting."