From: Brian Candler <b.candler@...>
Date: 2009-01-29T18:39:49+09:00
Subject: Re: error with regular expression inside eval block

Barun Singh wrote:
> I単aki Baz Castillo wrote:
>> El Jueves, 29 de Enero de 2009, Barun Singh escribi坦:
>>>
>>>  >> outputs "nil" to the screen
>>>
>>> Could someone explain why the eval statement returns an incorrect result
>>> for the regexp match?  Thanks...
>> 
>> It works if you escape the \ with \\ :
>> 
>>   eval %Q{'1' =~ /^[\\d]+$/}
>>   => 0
>> 
>> Not sure why the \ must be escaped into a block code.
> 
> Ah, thanks that worked. I wonder why extra escaping is needed just 
> because it's in a block...

It's not in a block, it's just inside a string. There are two types of 
string quoting, normally given using double-quotes and single-quotes, 
and they behave differently with regards to escaping. The double-quote 
variety converts special sequences like \n to newline, and \d just 
becomes d.

irb(main):001:0> puts %Q{'1' =~ /^[\d]+$/}
'1' =~ /^[d]+$/
=> nil

irb(main):002:0> puts %q{'1' =~ /^[\d]+$/}
'1' =~ /^[\d]+$/
=> nil

irb(main):003:0> puts %Q{\n}

=> nil
irb(main):004:0> puts %q{\n}
\n
=> nil
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