From: Joel VanderWerf <vjoel@...>
Date: 2003-09-28T04:27:16+09:00
Subject: Re: regexp needed to split letters from numbers

Rodrigo B. de Oliveira wrote:
>>irb(main):001:0> s = "clonidine300 mg"
>>=> "clonidine300 mg"
>>irb(main):002:0> s[/(?=\d+ mg)/] = " "
>>=> " "
>>
> 
> 
> What's exactly happening here?

String#[]=, as in "s1[pat] = s2", is a destructive slice operator. It 
replaces the first match of pat with the r.h.s. (raising IndexError if 
no match).

In this case, pat is /(?=\d+ mg)/, which has a lookahead pattern 
(?=...). This lookahead expression matches the point in the string *just 
before* the match of /\d+ mg/, but it doesn't consume the "300 mg". So 
slicing out the match (which is empty) and substituting " " has the 
effect of inserting a space before the match of /\d+ mg/.