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comp.lang.ruby

error with regular expression inside eval block

Barun Singh

1/28/2009 11:54:00 PM

I'm seeing strange behavior with Ruby 1.8 when I try to evaluate a
regular expression inside an eval block.

Consider the following:

puts '1' =~ /^[\d]+$/
>> outputs "0" to the terminal

eval %Q{'1' =~ /^[\d]+$/}
>> outputs "nil" to the screen

Could someone explain why the eval statement returns an incorrect result
for the regexp match? Thanks...
--
Posted via http://www.ruby-....
4 Answers

Iñaki Baz Castillo

1/28/2009 11:58:00 PM

0

El Jueves, 29 de Enero de 2009, Barun Singh escribi=C3=B3:
> I'm seeing strange behavior with Ruby 1.8 when I try to evaluate a
> regular expression inside an eval block.
>
> Consider the following:
>
> puts '1' =3D~ /^[\d]+$/
>
> >> outputs "0" to the terminal
>
> eval %Q{'1' =3D~ /^[\d]+$/}
>
> >> 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' =3D~ /^[\\d]+$/}
=3D> 0

Not sure why the \ must be escaped into a block code.

=2D-=20
I=C3=B1aki Baz Castillo

Barun Singh

1/29/2009 12:19:00 AM

0

Iñaki Baz Castillo wrote:
> El Jueves, 29 de Enero de 2009, Barun Singh escribió:
>>
>> >> 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...
--
Posted via http://www.ruby-....

Brian Candler

1/29/2009 9:40:00 AM

0

Barun Singh wrote:
> Iñaki Baz Castillo wrote:
>> El Jueves, 29 de Enero de 2009, Barun Singh escribió:
>>>
>>> >> 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
--
Posted via http://www.ruby-....

Robert Klemme

1/29/2009 12:56:00 PM

0

2009/1/29 Barun Singh <barunio@gmail.com>:
> I=F1aki Baz Castillo wrote:
>> El Jueves, 29 de Enero de 2009, Barun Singh escribi=F3:
>>>
>>> >> outputs "nil" to the screen
>>>
>>> Could someone explain why the eval statement returns an incorrect resul=
t
>>> for the regexp match? Thanks...
>>
>> It works if you escape the \ with \\ :
>>
>> eval %Q{'1' =3D~ /^[\\d]+$/}
>> =3D> 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 a block - it's a doubly quoted string!

Ruby version 1.8.7
irb(main):001:0> %Q{foo}
=3D> "foo"
irb(main):002:0> %Q{foo}.class
=3D> String
irb(main):003:0> %Q{foo\\no}
=3D> "foo\\no"
irb(main):004:0> %Q{foo\no}
=3D> "foo\no"
irb(main):005:0> puts %Q{foo\no}
foo
o
=3D> nil
irb(main):006:0> puts %Q{foo\\no}
foo\no
=3D> nil

Kind regards

robert


--=20
remember.guy do |as, often| as.you_can - without end