From: "David A. Black" <dblack@...>
Date: 2008-05-09T17:57:22+09:00
Subject: Re: Doing an AND in regexp char class

Hi --

On Fri, 9 May 2008, 7stud -- wrote:

> Joel VanderWerf wrote:
>> Todd Benson wrote:
>>>>>
>>>>> Thanks,
>>>> p 'foobar'.all_chars?('')
>>>>
>>>>
>>>>
>>>> cfp:~ > ruby a.rb
>>>> true
>>>> false
>>>> false
>>>
>>> Cool :)  #tr is one of those useful methods I somehow consistently forget about.
>>
>> But it can be done with regex, right? It's just more elegant with tr.
>>
>> class String
>>    def all_chars? chars
>>      if chars.empty?
>>        empty?
>>      else
>>        /\A[#{chars}]*\z/ === self
>>      end
>>    end
>> end
>>
>> p 'foobar'.all_chars?('rabof')  # => true
>> p 'foobar'.all_chars?('abc')    # => false
>> p 'foobar'.all_chars?('')       # => false
>
> Your method doesn't work, which can clearly be seen in these examples:
>
> strs = ["aaa", "bbb", "ccc"]
> chars = "abc"
>
> strs.each do |str|
>
>  if /\A[#{chars}]*/ =~ str
>    print str, " - yes"
>    puts
>  else
>    print str, " - no"
>    puts
>  end
>
> end
>
> --output:--
> aaa - yes
> bbb - yes
> ccc - yes
>
> It should be clear from the output that even though the string "aaa"
> passes your test, it is not true that all the characters in the string
> "abc"  appear in in the string "aaa".

Do it the other way around (and don't forget the \z):

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

It's really the characters in str that you're testing, to make sure
that none of them fail to match the characters in chars. If the
variable names seem backwards, you can change them. It's the logic
that's important, and it works fine.


David

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