Navid M. wrote:
> Hi Rob,
>
> I'm not very familiar with the qq() operator. I
> always thought this is included when the programmer
> wants to include the quote character in a string
> without having to escape it with a backslash.
> Ex: $string = qq(this is a " without escaping it);
Exactly. It lets you write a double-quoted string using
whatever delimiter characters you want. In the same
way as / .. / is a regex match, which can be written
m/ .. / or m( .. ) or m# .. #, the string "abc" can be
written qq"abc" or qq(abc) or qq#abc#. You can also
write a single-quoted string with q(abc), which is the
same as 'abc'. See perldoc perlop under "Regexp
Quote-Like Operators" for others.
>
> Can you please explain me the logic behind
> eval qq("$var"); and why it works.
Right. As I said in my post, and as you've decided yourself,
it's the same as
eval "\"$var\"";
or
eval qq(qq($var));
(if you use a pair of brackets, (), [], {} or <> as
the delimiters then they nest properly like this).
As we had
$fileName = "File";
$var = '1>>$fileName';
a call like this
eval "$var";
is the same as trying to compile the Perl statement
1>>$fileName;
which, as you found, isn't Perl. But
"1>>$fileName";
is, and the double-quotes will allow $fileName to be
interpolated. So we need a string with these characters:
'"', '1', '>', '>', '$', 'f', 'i', 'l', 'e', 'N', 'a', 'm', 'e', '"'
which, I think you can see, is
"\"1>>\$fileName\""
or
"\"$var\""
It gets a little tricky getting your head around the difference
between quote characters in the string, and quote characters
delimiting the Perl string constant. That's why I used the qq()
construct instead.
eval qq(qq($var)) gets interpolated at execution to
eval 'qq(1>>$fileName)' so the eval executes Perl code
qq(1>>$fileName) which interpolates a second time to
'1>>File'
Whew! I hope that helped!
Cheers,
Rob
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