Commented braces

This is something obvious, but it may not be entirely obvious to all.
So I'm copying it here for the benefit of potential readers.

Originally from:

http://perlmonks.org/?node_id=638287


Original question (sanPerl)
------------------------------------

Dear Monks,
I am trying to execute the code given below.

use strict;
my $abcd = "Hello how r you?";
my $hello="Hello";
$abcd =~ s{\w}
{
#{
#}{
if ($hello)
{"1"}
else
{"0"}
}exgs;
print $abcd;

It is giving me error

Substitution replacement not terminated at test.pl line 5.

When I removed line 6 & 7 the code looked as below

use strict;
my $abcd = "Hello how r you?";
my $hello="Hello";
$abcd =~ s{\w}
{
if ($hello)
{"1"}
else
{"0"}
}exgs;
print $abcd;

It works proper and gives me output 11111 111 1 111? My question is,
why the commented braces are playing role in this error ? Have I done
something wrong here?


Reply (moritz)
------------------------------------

The problem is that the code is potentially ambigous.

There are two ways to parse it:

s{...}{
# much stuff here
}x

The other, but less obvious, ist this:

s{...}{
# some characters
{}{
# more characters...
}
# no terminator

When perl tries to parses the regex it doesn't know if there is going
to come an /x modifier, so the braces have to be balanced - even in
comments.

That's one of the reasons Perl 6 puts the modifiers at the start of
the regexes.


Reply (ikegami)
------------------------------------

If Perl were to guess that "#" indicates a comment, it'll introduce a
paradox. Consider

s{foo}{
bar
#}xe
}

If Perl stops at the second "}", then the replacement expression is
not code and "#" are not comments and Perl should have stopped at the
first "}".

If Perl stops at the first "}", then the replacement expression is
code ("e") and "#" are comments ("x") and Perl should have stopped at
the second "}".

Perl needs to find the end of the operator to find the "e" and "x"
flags. To find the end of the operator, Perl initially treats the
expression as a replacement string. When the "e" flag is found is the
replacement string is reparsed as code. Only then does "x" have any
meaning.

Perl 6 fixes this by placing the flags before the replacement
expression.

------------------------------------


Other replies at the link above.


Michele
--
{$_=pack'B8'x25,unpack'A8'x32,$a^=sub{pop^pop}->(map substr
(($a||=join'',map--$|x$_,(unpack'w',unpack'u','G^ ..'KYU;*EVH[.FHF2W+#"\Z*5TI/ER 256),7,249);s/[^\w,]/ /g;$ \=/^J/?$/:"\r";print,redo}#JAPH,

Re: Commented braces

On Sep 12, 5:03 am, Michele Dondi wrote:
> This is something obvious, but it may not be entirely obvious to all.
> So I'm copying it here for the benefit of potential readers.
>
> Originally from:
>
> http://perlmonks.org/?node_id=638287
>
> Original question (sanPerl)
> ------------------------------------
>
> Dear Monks,
> I am trying to execute the code given below.
>
> use strict;
> my $abcd = "Hello how r you?";
> my $hello="Hello";
> $abcd =~ s{\w}
> {
> #{
> #}{
> if ($hello)
> {"1"}
> else
> {"0"}
> }exgs;
> print $abcd;
>
> It is giving me error
>
> Substitution replacement not terminated at test.pl line 5.
>
> When I removed line 6 & 7 the code looked as below
>
> use strict;
> my $abcd = "Hello how r you?";
> my $hello="Hello";
> $abcd =~ s{\w}
> {
> if ($hello)
> {"1"}
> else
> {"0"}
> }exgs;
> print $abcd;
>
> It works proper and gives me output 11111 111 1 111? My question is,
> why the commented braces are playing role in this error ? Have I done
> something wrong here?

`perldoc perlre` explains this just fine, IMO:

The "/x" modifier itself needs a little more explanation.

Note that you have to be careful not to include the pattern
delimiter in the comment--perl has no way of knowing you did
not intend to close the pattern early. See the C-comment
deletion code in perlop.


Paul Lalli

Re: Commented braces

On Wed, 12 Sep 2007 07:13:29 -0700, Paul Lalli
wrote:

>`perldoc perlre` explains this just fine, IMO:

I wasn't suggesting that it doesn't. It's a little piece of info that
may just elude someone's attention and intuition.


Michele
--
{$_=pack'B8'x25,unpack'A8'x32,$a^=sub{pop^pop}->(map substr
(($a||=join'',map--$|x$_,(unpack'w',unpack'u','G^ ..'KYU;*EVH[.FHF2W+#"\Z*5TI/ER 256),7,249);s/[^\w,]/ /g;$ \=/^J/?$/:"\r";print,redo}#JAPH,

Re: Commented braces

"Paul Lalli" wrote in message
news:1189606409.299898.311000@y42g2000hsy.googlegroups.com.. .
> On Sep 12, 5:03 am, Michele Dondi wrote:
this dude ^^^^^^.
>> This is something obvious, but it may not be entirely obvious to all.
>> So I'm copying it here for the benefit of potential readers.
>>
>> Originally from:
>>
>> http://perlmonks.org/?node_id=638287

>
> Paul Lalli
>
Do you know how to reach Mr. Dondi through e-mail?
--
Wade Ward
wade@zaxfuuq.net
'If they took all the "And it came to pass's" out
of the Book of Mormon, it would be a pamphlet.'
--Mark Twain

Re: Commented braces

On Thu, 13 Sep 2007 23:52:09 -0700, "Wade Ward"
wrote:

>Do you know how to reach Mr. Dondi through e-mail?



Or else I'm on Jabber as .


Michele
--
{$_=pack'B8'x25,unpack'A8'x32,$a^=sub{pop^pop}->(map substr
(($a||=join'',map--$|x$_,(unpack'w',unpack'u','G^ ..'KYU;*EVH[.FHF2W+#"\Z*5TI/ER 256),7,249);s/[^\w,]/ /g;$ \=/^J/?$/:"\r";print,redo}#JAPH,