Single quoting weirdness

alien_life_form has asked for the wisdom of the Perl Monks concerning the following question:

Replies are listed 'Best First'.
(MeowChow) Re: Single quoting weirdness by MeowChow (Vicar) on Jun 07, 2002 at 09:01 UTC
Yes: "\\" is a backslash-escape, which produces a single backslash. Without it, you'd have no way to get a backslash into an interpolated string, or onto the end of an uninterpolated string. It escapes in both types of strings. MeowChow s aamecha.s a..a\u$&owag.print	[reply]
Re: Single quoting weirdness by grinder (Bishop) on Jun 07, 2002 at 09:55 UTC
It looks odd, but it is correct behaviour. \ is a meta-character for escaping, but in single quoted strings the only thing it escapes is ' (which allows you to embed a ' in a single quoted string without having to use the q{} syntax, or itself). Nothing else is quotable. So in the second line, \+, + is not magical, does not have to be quoted, so \ has no effect, is not consumed, and thus \+ is emitted. In the third line, \\+, \ escapes the following \, so \+ is emitted, and so on for the forth line. You might want to look at S_scan_str in toke.c for far more than you ever wanted to know about how perl scans strings. The most straightforward way of writing your code would be to double up each \ you want to print: `perl -e "print join(qq(\n),('+','\\+','\\\\+','\\\\\\+')),qq(\n)" # or, more concisely perl -le "print $_ for qw/+ \\+ \\\\+ \\\\\\+ \\\\\\\\+/"` [download] print@_{sort keys %_},$/if%_=split//,'= & *a?b:e\f/h^h!j+n,o@o;r$s-t%t#u'	[reply] [d/l]
Re: Single quoting weirdness by Abigail-II (Bishop) on Jun 07, 2002 at 10:58 UTC
Inside a single quoted string, you get everything as is, with the exceptions of two things: A backslash followed by a delimiter gives just that delimiter. A backslash followed by a backslash gives just a single backslash. The reason for the first exception is hopefully obvious. To see why the second exception is needed, imagine that the second exception didn't exists, and try to figure out how you would make a single quoted string that ends in a backslash. You can't, because then you'd escape the delimiter.... Abigail	[reply]
Re: Re: Single quoting weirdness by Anonymous Monk on Jun 07, 2002 at 11:52 UTC
I have never found the reason for the first exception to be obvious. Perl has enough quoting mechanisms that I think it would be more valuable to have one that makes it easy to include a row of backslashes in than it is to have every one able to put out any possible text you could want. Heck, just the fact that the rules for single-quoting are so widely understood (as can be seen from people wondering at what it does) strongly suggests that something is wrong with the rule as it stands.	[reply]
Re: Single quoting weirdness by Abigail-II (Bishop) on Jun 07, 2002 at 12:07 UTC
Escaping the escape character seems to be used in about any escaping mechanism that I can recall. That's hardly a Perl specific thing. And not being about to escape the delimiter is just very awkward. Do you like stuff like: `echo '$foo'"'"'$bar'` [download] just to be able to echo `$foo'$bar` [download] like you have to do in many shells? Abigail	[reply] [d/l] [select]
Re: Re: Single quoting weirdness by Anonymous Monk on Jun 08, 2002 at 00:58 UTC
Re: Single quoting weirdness by PodMaster (Abbot) on Jun 07, 2002 at 10:05 UTC
Have you read Gory details of parsing quoted constructs? You should.	[reply]
Re: Re: Single quoting weirdness by alien_life_form (Pilgrim) on Jun 07, 2002 at 10:45 UTC
Greetings, I have, now `:)` Thanks. Cheers, alf You can't have everything: where would you put it?	[reply]
Re: Single quoting weirdness by jmcnamara (Monsignor) on Jun 07, 2002 at 09:02 UTC
The `\\` is taken as an escaped backslash so in effect you only get one. -- John.	[reply]
Re: Single quoting weirdness by demerphq (Chancellor) on Jun 07, 2002 at 13:27 UTC
Well, there is one way that you can avoid the \\ rule in strings. Use the single quote variant of heredocs. as in the following `print join("\n",split(/\s+/,<<'END_OF_TEXT'); + \+ \\+ \\\+ END_OF_TEXT` [download] which outputs + \+ \\+ \\\+ Yes its annoying but single quoted here docs are the _only_ way (without using deep voodoo) to quote backslashes without escaping them. Updated in respect for Abigail-II Yves / DeMerphq --- Writing a good benchmark isnt as easy as it might look.	[reply] [d/l]
Re: Single quoting weirdness by Abigail-II (Bishop) on Jun 07, 2002 at 14:14 UTC
The _only_ way? Hah. You don't know Perl (yet) ;-). If you don't like the way Perl interpolates strings, Perl let's you change them. Abigail `#!/usr/bin/perl use strict; use warnings 'all'; use overload; BEGIN { overload::constant q => sub {$_ [$_ [2] && $_ [2] eq 'q' ? 0 : 1]} +; } print '+ \+ \\+ \\\+', "\n"; print "+ \+ \\+ \\\+", "\n"; __END__ + \+ \\+ \\\+ + + \+ \+` [download]	[reply] [d/l]