in reply to Weird quoting with /x modifier

I did the following: 
use strict;
use warnings;
use YAPE::Regex::Explain;

my $regex1 = qr{\QHello# World\E}x;
my $parser1 = YAPE::Regex::Explain->new($regex1)->explain;
print "$parser1\n";

print "*" x 20;
print "\n";

my $regex2 = qr{Hello\#\ World}x;
my $parser2 = YAPE::Regex::Explain->new($regex2)->explain;
print "$parser2\n";

[download]
This returned: 
The regular expression:
(?x-ims:Hello\#\ World\\E)
matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?x-ims:                 group, but do not capture (disregarding
                         whitespace and comments) (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n):
----------------------------------------------------------------------
  Hello                    'Hello'
----------------------------------------------------------------------
  \#                       '#'
----------------------------------------------------------------------
  \                        ' '
----------------------------------------------------------------------
  World                    'World'
----------------------------------------------------------------------
  \\                       '\'
----------------------------------------------------------------------
  E                        'E'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
********************
The regular expression:
(?x-ims:Hello\#\ World)
matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?x-ims:                 group, but do not capture (disregarding
                         whitespace and comments) (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n):
----------------------------------------------------------------------
  Hello                    'Hello'
----------------------------------------------------------------------
  \#                       '#'
----------------------------------------------------------------------
  \                        ' '
----------------------------------------------------------------------
  World                    'World'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

[download]
Note the addition of the "\E" at the end of the first explanation, but not the second.

Note also that the "\Q" is dropped by YAPE::Regex::Explain.

The behaviour is constant across 5.8.9, 5.10.1 and 5.12.1 with YAPE::Regex::Explain 3.011

HTH

UPDATE:
From perldoc perlre:

(?#text)
A comment. The text is ignored. If the /x modifier enables whitespace formatting, a simple # will suffice. Note that Perl closes the comment as soon as it sees a ), so there is no way to put a literal ) in the comment.

Replies are listed 'Best First'.
Re^2: Weird quoting with /x modifier
by proceng (Scribe) on May 29, 2010 at 22:40 UTC
    Interesting addition:
    With the following code:
    use strict;
use warnings;

my $teststring = qq/Hello# World/;

my $regex1 = qr{\QHello# World\E}x;
my $regex2 = qr{Hello\#\ World}x;
my $regex3 = qr{$teststring}x;
my $regex4 = qr{\Q$teststring\E}x;

print "String is $teststring\n";
print "Regex1 is $regex1\n";
print "Regex2 is $regex2\n";
print "Regex3 is $regex3\n";
print "Regex4 is $regex4\n";

    [download]
    I get the following output (perl 5.12.1): 
    String is Hello# World
Regex1 is (?x-ism:Hello\#\ World\\E)
Regex2 is (?x-ism:Hello\#\ World)
Regex3 is (?x-ism:Hello# World
)
Regex4 is (?x-ism:Hello\#\ World)

    [download]
    For regex3, the newline is inserted by the qr directive, as verified by a hex dump of the file.

    UPDATE:
    Output from perl 5.8.9: 

    $ perl8 regex.pl
String is Hello# World
Regex1 is (?x-ism:Hello\#\ World\\E
)
Regex2 is (?x-ism:Hello\#\ World
)
Regex3 is (?x-ism:Hello# World
)
Regex4 is (?x-ism:Hello\#\ World
)

    [download]
    Output from perl 5.10.1: 
    $ perl10 regex.pl
String is Hello# World
Regex1 is (?x-ism:Hello\#\ World\\E)
Regex2 is (?x-ism:Hello\#\ World)
Regex3 is (?x-ism:Hello# World
)
Regex4 is (?x-ism:Hello\#\ World)

    [download]
    So, it looks like there may be an edge case that was partially fixed.

    UPDATE 2: clarified that initial tests were on 5.12.1
    This is perl 5, version 12, subversion 1 (v5.12.1) built for x86_64-linux-thread-multi-ld
    This is perl, v5.10.1 (*) built for x86_64-linux-thread-multi-ld
    This is perl, v5.8.9 built for x86_64-linux-thread-multi-ld

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Re^2: Weird quoting with /x modifier
by ykar (Acolyte) on May 29, 2010 at 22:25 UTC

    >> Note the addition of the "\E" at the end of the first explanation, but not the second.

    This is exactly what bothering me. As far as I understand \Q and \E is just syntactic sugar, to avoid typing dozens of escapes. So neither \Q nor \E should appear in compiled regular expression.

    Regarding your excerpt from perlre: it is not my case. I'm intentionally quoting #.

    The "#" character is also treated as a metacharacter introducing a comment, just as in ordinary Perl code. This also means that if you want real whitespace or "#" characters in the pattern (outside a character class, where they are unaffected by "/x"), then you'll either have to escape them (using backslashes or "\Q...\E") or encode them using octal or hex escapes.

    By the way hex escapes not working inside /\Q...\E/ for me. I worked around my problem replacing /\Q...#...\E/x with /\Q...\E\#\Q...\E/x.

    Thank you for your answer!

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