From: "David A. Black" <dblack@...>
Date: 2008-05-09T09:35:08+09:00
Subject: Re: Doing an AND in regexp char class

Hi --

On Fri, 9 May 2008, Joel VanderWerf wrote:

> Todd Benson wrote:
>> I'm drawing a blank here with this one.  Why doesn't this work then...
>> 
>> irb(main):006:0> r = /\A[oh]*\z/
>> => /\A[oh]*\z/
>> irb(main):007:0> s = "hello, there"
>> => "hello, there"
>> irb(main):008:0> r === s
>> => false
>
> Maybe I'm confused about was wanted originally. The above tests the following 
> condition:
>
>  (set of chars occurring in given string)
>     is_a_subset_of
>  (given set of chars).
>
> irb(main):007:0> /\A[oh]*\z/ === "hohoho"
> => true
> irb(main):008:0> /\A[oh]*\z/ === "ho ho"
> => false
>
> If you want superset instead of subset, this works:
>
> irb(main):013:0> /(?=.*h)(?=.*o)/ === "h o"
> => true

That depends on the order, though. To do the superset test, you could
just do the subset, but in the other direction: check that the
character class, as a string, doesn't contain anything that isn't in
the main string:

   str = "h o"
   chars = "ho"

   /\A[#{str}]*\z/ === chars    # true

(though probably best to uniquify the string first).


David

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