Double Quotes Vs Single Quotes

[moroshko]

Hi !

Can anyone tell me please why the following program does not found the word CAT ?
If I put the pattern in single quotes, it works.
Why ??



#!/usr/bin/perl
use warnings;
use strict;

$_ = 'laKJJlala AHAJJ--- CAT =-=-123 sad(!@*# xsaa KUKU ff';

my $pattern = ".*(\bCAT\b).*";
my @arr;
my $k = 0;

if (@arr = /$pattern/) {
print "The text matches the pattern '$pattern'.\n";

while ($arr[$k]) {
print "\$$k is '$arr[$k]'\n";
$k++;
}
} else {
print "'$pattern' was not found.\n";
}

 

[stevexff]

Inside single quotes hardly anything gets interpolated, but double quotes allow variables to be resolved. Try this to see if it helps, then you can see why it doesn't work:

use strict;
use warnings;

print ".*(\bCAT\b).*", "\n", '.*(\bCAT\b).*';

Steve

[small]"Every program can be reduced by one instruction, and every program has at least one bug. Therefore, any program can be reduced to one instruction which doesn't work." (Object:erlDesignPatterns)[/small]

 

[moroshko]

Thanks Steve,

So I understand that
.*(\bCAT\b).*
interpolates to
.*CA).*
i.e. I guess that
(\b
and
T\b
interpolates to empty string.

Is there some rule that any character followed by \b is interpolated to the empty string ?

Where I can find the full description of what exactly is interpolated in a double quoted string ?

Thank you very much !!

 

[KevinADC]

Your code can't work, this line is totally wrong:

if (@arr = /$pattern/) {

You used the assingment operator "=" and anyway, you can't do a pattern match on an array, you have to match each element of the array or use something like grep{} to search each element of the array to find a match.

------------------------------------------
- Kevin, perl coder unexceptional!

 

[spookie]

Hi,
I am also curious about the point moroshko has made that why .*(\bCAT\b).* interpolates to .*CA).*.

Can someone throw light on this?


--------------------------------------------------------------------------
I never set a goal because u never know whats going to happen tommorow.

 

[stevexff]

The escape sequence \b is interpolated as a backspace, so it backs up over the previous character. This is why the '(' and the 'T' seem to disappear.

Steve

[small]"Every program can be reduced by one instruction, and every program has at least one bug. Therefore, any program can be reduced to one instruction which doesn't work." (Object:erlDesignPatterns)[/small]

 

[spookie]

Thanks steve. could not figure out that..I think i need some rest...

--------------------------------------------------------------------------
I never set a goal because u never know whats going to happen tommorow.

 

[moroshko]

Oh.. now it makes sence !
Thank you !
Where I can read about the interpolation of special characters like '\b', '\n' and so on.... ?

 

[KevinADC]

\b is one of those exceptions in perl, it's a backspace in a double-quoted string but a word boundary anchor in a regexp. Most are mentioned in this page:

http://perldoc.perl.org/perlreref.html

------------------------------------------
- Kevin, perl coder unexceptional!

 

[moroshko]

Thanks a lot !!