«

On Fri, 13 Mar 2009 17:42:34 -0500, Tad J McClellan <> wrote:

> <> wrote:
>> On Wed, 11 Mar 2009 16:24:17 -0700 (PDT), msciwoj <> wrote:
>
>>>The mystery for me is this 'second' evaluation actually, like you
>>>said, "interpolates the values for $2 and $1 into the replacement
>>>string" and it seems it's the only way that does the job.
>>>A bit of mystery for me but at the end it works and only this counts,
>>>I guess.
>
>
>> In the nut of it, evaling a variable with a value '$str'
>> will work,
>
>
>Because the value is a valid Perl expression.
>
>
>> but with a value of '$str\n$str' will not.
>
>
>Because the value is not a valid Perl expression.
>
>
>> But if you notice, the vaule of '$str\n$str' and '$str\\n$str'
>> are equal.
>
>
>That is how single quoted strings work.
>
>The "Scalar value constructors" section in perldata points out that
>there are only 2 special characters in a single quoted string.
>
>Single quote is special (it marks the end of the string) unless
>it is preceded by a backslash.
>
>Backslash is special if it preceeds a single quote or a backslash.
>
>A backslash that does not preceed 1 of those 2 characters is literal.
>
>So the 1st string above has a literal backslash and an "n" in it.
>
>The 2nd string above has a backslashed backslash (which becomes
>a backslash) and an "n" in it.
>
>
>> In escence, there is a conflict with the parser on some
>> level that is not quite understood
>
>
>Don't let that stop you from pretending to answer questions about it.
>
>
>> as pertaining to eval.
>
>
>It has nothing to do with eval.
>
>It has to do with single-quoted strings.
>
>
>> Whitness the below code. All works fine. Replace "replacement2" variable
>> with this value: '$2\n' and watch the error's.
>
>
>That is because in the code below, the value of $replacement2
>is a valid expression:
>
> perl -e '$2'
>
>But the value you suggest above is not a valid expression:
>
> perl -e '$2\n'
>

Oh, I get it. Its all about "valid expression" now and not eval in regex's.
And now since you have stated '$2' is a valid expression and '$2\n' isn't,
it raises the question of the ilogic in your nothing, so nothing argument.

Now, given the "valid expression" of '$2', this should be equivalent in terms of eval:

$replacement2 = '$2';
$data =~ /(.)\n(.)/;
$e = eval ($replacement2);
print "->",$e,"\n";

$data =~ s/(.)\n(.)/$replacement2/eg;
print $data."\n";

Unfortunately, you forgot your brain today Tad.

-sln

<> wrote:
> On Fri, 13 Mar 2009 17:42:34 -0500, Tad J McClellan <> wrote:
>

>>> In the nut of it, evaling a variable with a value '$str'
>>> will work,
>>
>>
>>Because the value is a valid Perl expression.
>>
>>
>>> but with a value of '$str\n$str' will not.
>>
>>
>>Because the value is not a valid Perl expression.


> Oh, I get it. Its all about "valid expression"


You're welcome.


--
Tad McClellan
email: perl -le "print scalar reverse qq/moc.noitatibaher\100cmdat/"

On Fri, 13 Mar 2009 21:48:55 -0500, Tad J McClellan <> wrote:

> <> wrote:
>> On Fri, 13 Mar 2009 17:42:34 -0500, Tad J McClellan <> wrote:
>>
>
>>>> In the nut of it, evaling a variable with a value '$str'
>>>> will work,
>>>
>>>
>>>Because the value is a valid Perl expression.
>>>
>>>
>>>> but with a value of '$str\n$str' will not.
>>>
>>>
>>>Because the value is not a valid Perl expression.
>
>
>> Oh, I get it. Its all about "valid expression"
>
>
>You're welcome.

No thanks! But you are welcome.

Here's some valid expression for ya Tad:

$replacement2 = '$2';
$data =~ /(.)\n(.)/;
$e = eval ($replacement2);
print "->",$e,"\n";

$data =~ s/(.)\n(.)/$replacement2/eg;
print $data."\n";

<> wrote:
> On Fri, 13 Mar 2009 21:48:55 -0500, Tad J McClellan <> wrote:
>
>> <> wrote:
>>> On Fri, 13 Mar 2009 17:42:34 -0500, Tad J McClellan <> wrote:
>>>
>>
>>>>> In the nut of it, evaling a variable with a value '$str'
>>>>> will work,
>>>>
>>>>
>>>>Because the value is a valid Perl expression.
>>>>
>>>>
>>>>> but with a value of '$str\n$str' will not.
>>>>
>>>>
>>>>Because the value is not a valid Perl expression.
>>
>>
>>> Oh, I get it. Its all about "valid expression"
>>
>>
>>You're welcome.
>
> No thanks! But you are welcome.
>
> Here's some valid expression for ya Tad:
>
> $replacement2 = '$2';
> $data =~ /(.)\n(.)/;
> $e = eval ($replacement2);
> print "->",$e,"\n";
>
> $data =~ s/(.)\n(.)/$replacement2/eg;
> print $data."\n";


That works as expected. Since there is only one eval with s///e,
the result after evaluating $replacement2 is the string '$2'.

What were you expecting it to do instead?

If you were expecting the last print to print the value of the $2
variable, then you don't have enough levels of eval to get to where
'$2' is evaluated.

The 2nd level of eval can be gotten with either

$data =~ s/(.)\n(.)/$replacement2/eeg;

or

$data =~ s/(.)\n(.)/$e/eg;


--
Tad McClellan
email: perl -le "print scalar reverse qq/moc.noitatibaher\100cmdat/"