The Backslash Challenge

[choroba]

To return the character that's used as the delimiter in a quote-like operator, you can use a backslash.

'/' eq q/\// or die "Failed";

Your task is to return a backslash using a backslash as the delimiter. Use qq\\ for level easy, use q\\ for level guru.

There are definitely many ways how to do it for level easy.

I haven't found a way how to do it with the single q.

[haukex]

I began reading Gory details of parsing quoted constructs in regards to q\\, then I clicked on your spoiler...

"However, when backslashes are used as the delimiters (like qq\\ and tr\\\), nothing is skipped." ;-P

Update: Looks to be true...

[LanX]

> then I clicked on your spoiler...

I should have done this earlier ...

[choroba]

OK, here are some solutions I was able to find:

Are you sure you don't want to try yourself?

Double spoilers don't work correctly on PerlMonks, but they hide the contents twice. Give it a few more minutes.

I started with the most obvious one. In double quotes, you can use the "babycart" operator (see perlsecret); inside it, you can have any code you like (well, in our case, any code not containing a backslash, because, as haukex correctly found in perlop, you can't include a backslash literally in a qq\\). To most straightforward way is to use chr:

1.

qq\@{[chr 92]}\

There are infinitely many ways how to get a backslash. We can use pack

2.

qq\@{[pack "c", 92]}\

or sprintf

3.

qq\@{[sprintf '%c', 92]}\

Another way is to use a bitwise operation, e.g. xor. Here are just two examples I liked:

4.

qq\@{['{'^"'"]}\
qq\@{['~'^'"']}\

Then I skimmed perlfunc searching for functions that can return a backslash. I found a few that return a bit more so we need to combine them with substr or a match:

5.

qq\@{[substr quotemeta ";", 0, 1]}\
qq\@{[prototype("CORE::push") =~ /^(.)/]}\

We can also require a core module that outputs a backslash.

6.

qq\@{[require File::Spec::Win32 and File::Spec::Win32->rootdir]}\
qq\@{[do{require Data::Dumper;substr Data::Dumper::Dumper("'"),-5,1}]}
+\

OK. Is there any other way than using the array dereference? I've found a way! Let's use a string dereference instead. We need to create a string reference inside, which I can't do without a backslash, but fortunately there's eval that makes it possible to generate the backslash without using a backslash.

7.

qq\${eval chr(92).'"'.(chr(92)x2).'"'}\

Finally, here's a joke. What a pity it doesn't work. Update: In a way, it does! See Tux's reply below!

∞.

qq\@{[reverse "/"]}\

To sum it up:

print
qq\@{[chr 92]}\,
qq\@{[pack "c", 92]}\,
qq\@{[sprintf '%c', 92]}\,
qq\@{['{'^"'"]}\, qq\@{['~'^'"']}\,
qq\@{[substr quotemeta ";", 0, 1]}\,
qq\@{[prototype("CORE::push") =~ /^(.)/]}\,
qq\@{[require File::Spec::Win32 and File::Spec::Win32->rootdir]}\,
qq\@{[do{require Data::Dumper;substr Data::Dumper::Dumper("'"),-5,1}]}
+\,
qq\${eval chr(92).'"'.(chr(92)x2).'"'}\,
qq\@{[reverse "/"]}\;

[Tux]

Two more options

• Encode::encode("cp1047","*")
• charnames::string_vianame("REVERSE SOLIDUS")

[LanX]

The goal is maximum obfuscation, right? ;)

  DB<267> x qq\@{[ (sort grep { $_ gt '[' } %::)[0] ]}\; 
0  '\\'
  DB<268> x qq\@{[ grep { '[' lt $_ lt ']' } %:: ]}\; 
0  '\\'
  DB<269>

The second one requires a recent Perl but can be easily rewritten to work with older versions. :)

update

also tried this but see comment

print  "\\\n" eq
  qq\@{[<<'']}\;
\

# is there an easy way to chomp a here doc?

update

added trailing ; to debugger examples

[choroba]

Nice trick!

Interestingly, it doesn't work for me. In the debugger, I'm getting

  cont:
Can't find string terminator "\" anywhere before EOF at (eval 13)[/usr
+/lib/perl5/5.26.1/perl5db.pl:738] line 2.
 at (eval 13)[/usr/lib/perl5/5.26.1/perl5db.pl:738] line 2.
    eval '' called at /usr/lib/perl5/5.26.1/perl5db.pl line 738
    DB::eval called at /usr/lib/perl5/5.26.1/perl5db.pl line 3140
    DB::DB called at -e line 1

And if I run it in a script instead, the output is _, not \ :-o

[LanX]

> cont:

A backslash at EOL marks a multiline statement in DB. I had a trailing whitespaces to avoid this, but the C&P from cmd.exe erased them. :( Fixed now!

> And if I run it in a script instead, the output is _, not \ :-o

both lines? lemme check...

...

Argh! Seems that the context of the debugger is initializing a "crucial thing" which can't be taken for granted :(

$\ is not in the symbol table til it's first set

update

yes a use DB; at the start of the script is "fixing" it.

Actually that could be a source of a Heisenbug.

[LanX]

> There are definitely many ways how to do it for level easy.

Please show me.

The only idea I had were control sequences, but they all start with a backslash.

update

well I consider it cheating, but since you keep repeating that there are no rules ;-)

  DB<163> $a='\\'; say $a eq qq\$a\
1

  DB<164>

[haukex]

qq\@{[chr 92]}\

[LanX]

yeah I updated something similar.

Mine is simpler, yours is less cheating... more obfuscated! ;-)