From: "Radosław Bułat" <radek.bulat@...>
Date: 2008-01-17T20:17:59+09:00
Subject: Re: Greedy and non greedy quantifiers

There are 4 (I hope that didn't miss anything) operators to specify repetitions.

*, +, ? and {m, n}
* - matches 0 or more
+ - matches 1 or more
? - matches 0 or 1 (same as {0,1} )
{m,n} matches m..n

All these operators all greedy (I'll show you examples). To make
non-greedy operator you must add '?' after operator. So non-greedy
operators are:
*?, +?, ??, {m,n}?
*? - matches 0 or more
+? - matches 1 or more
?? - matches 0 or 1 (same as {0,1}?)
{m,n} matches m..n

Maybe '??' looks strange, but all of these are absolutely correct.

So far no differences.

Why we have greedy and non-greedy operators? Because they works in
different way. Greedy operators tries to match as many character
(precisely: left expression) as can and if during matching it must
move back it tries to match fewer characters.

For example:
puts "aaaa".match(/a*/) # matches whole string
puts "aaaa" =~ /a*?/ # matches no characters (but it success)

Another example:
puts "aaaabaaaab".match(/.*b/) # matches whole string
puts "aaaabaaaab".match(/.*?b/) # matches "aaab"

I always recommend Jeffrey Friedl's book "Mastering Regular
Expressions" (http://regex.info/). After reading this book you will be
master of regexp ;-).

-- 
Rados³aw Bu³at

http://radarek.jogger.pl - mój blog