From: Jim Freeze <jim@...>
Date: 2002-11-16T05:18:22+09:00
Subject: Re: Regexp: What does //o do?

On Saturday, 16 November 2002 at  2:03:24 +0900, Matt Armstrong wrote:
> 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.
> 

Yes. It's not completely clear to me when it will be recompiled
and when it won't be. Above it is not. Below it is.

irb(main):001:0> s="a"
"a"
irb(main):002:0> /#{s}/o
/a/
irb(main):003:0> s="b"
"b"
irb(main):004:0> /#{s}/o
/b/

I thought that your example above with the function would give the
same results as the irb example above because the re would get
destroyed when the function returned. Then I tried this:

irb(main):016:0> def foo(s)
irb(main):017:1> re = /#{s}/o
irb(main):018:1> p re
irb(main):019:1> re = nil
irb(main):020:1> end
nil
irb(main):021:0> foo("a")
/a/
nil
irb(main):022:0> foo("b")
/a/
nil

and I'm confused even more.

-- 
Jim Freeze
----------
Tonight's the night: Sleep in a eucalyptus tree.