From: Matt Armstrong <matt@...>
Date: 2002-11-16T02:03:24+09:00
Subject: Re: Regexp: What does //o do?

Jim Freeze <jim@freeze.org> writes:

> On Friday, 15 November 2002 at 15:23:16 +0900,
> nobu.nokada@softhome.net wrote:
>> Hi,
>> 
>> At Fri, 15 Nov 2002 15:02:51 +0900,
>> Jim Freeze wrote:
>> > In a regex, what is the /o for (as opposed to n, i or g)?
>> > 
>> > EG
>> >  re = /\A\s+/o
>> 
>> This o has no meanings.  It's usefull with interpolation.
>> 
>> ['\A\s+', '\S+\z'].map {|s| /#{s}/}	# => [/\A\s+/, /\S+\z/]
>> ['\A\s+', '\S+\z'].map {|s| /#{s}/o}	# => [/\A\s+/, /\A\s+/]
>> 
>
> I'm confused. Why does it affect the map iterator?
>
> ['a','b'].map {|s| /#{s}/o}  # => [/a/,/a/]

Perhaps this is more clear:

irb(main):009:0> def foo(arg)
irb(main):010:1> p /#{arg}/o
irb(main):011:1> end
nil
irb(main):012:0> foo('a')
/a/
nil
irb(main):013:0> foo('b')
/a/
nil


In other words, /o means the regexp will be compiled the first time
the particular piece of code executes and the same regexp will be used
from then on.  Use it only when you know the regexp will always be the
same anyway.  These two examples show how it can actually introduce
bugs into your code.