[perl #127352] [BUG] Behavior of Regular Expressions Contained in a Constant with a Sigil

[via RT]

# New Ticket Created by  Joseph Polanik 
# Please include the string:  [perl #127352]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=127352 >


Constants are not required to have a sigil but apparently they may.

Unfortunately, when a constant with a sigil contains a regex pattern, 
unpredictable results may occur.

The attached file show that a regex pattern that should detect the file 
system separator ('/' or '\') yield incorrect results when contained in 
a constant whose names contains a sigil but correct results otherwise.

Identical results were obtained on Linux (Ubuntu 12.04) and Mac OS X 10.10.5

Build information

Linux

$ perl6 -version
This is Rakudo version 2015.12 built on MoarVM version 2015.12
implementing Perl 6.c.

Mac

$ perl6 -version
This is Rakudo version 2015.12-176-gaefe2c2 built on MoarVM version 
2015.12-29-g8079ca5 implementing Perl 6.c.

Thanks,

Joseph Polanik

#!/usr/bin/env perl6

# Constants are not required to have a sigil. When they have a sigil and
# contain a regex, the results are unpredictable.

constant $sepreg = /(<[\\\/]>)/;
my $sepreg2      = /(<[\\\/]>)/;
constant sepreg3 = /(<[\\\/]>)/;

my $filenameW = "c:\\g\\b.mp4";
my $filenameL = "/g/b.mp4";

say "============= Looking for a Windows separator: \\";
# The match is made but $/ doesn't get populated
say "Testing regex as a constant with a sigil: Windows separator";
if ($filenameW ~~ $sepreg) {
    say $/[0].Str;
}
else {
    say "No match for SEP";
}

# Gives correct result
say "Testing regex as a constant with no sigil: Windows separator";
if ($filenameW ~~ sepreg3) {
    say $/[0].Str;
}
else {
    say "No match for SEP";
}

# Gives correct result
say "Testing regex as a lexical variable: Windows separator";
if ($filenameW ~~ $sepreg2) {
    say $/[0].Str;
}
else {
    say "No match for SEP";
}

# Gives correct result
say "Testing regex as a string: Windows separator";
if ($filenameW ~~ /(<[\\\/]>)/) {
    say $/[0].Str;
}

#
# Now try the same tests with a *nix file system
#
say "============= Looking for a *nix separator: /";

# The match is made but $/ doesn't get populated
say "Testing regex as a constant with a sigil: *nix separator: Wrong Result";
if ($filenameL ~~ $sepreg) {
    say $/[0].Str;
}
else {
    say "No match for SEP";
}

# Gives correct result
say "Testing regex as a constant with no sigil: *nix separator";
if ($filenameL ~~ sepreg3) {
    say $/[0].Str;
}
else {
    say "No match for SEP";
}

# Gives correct result
say "Testing regex as a lexical variable: *nix separator";
if ($filenameL ~~ $sepreg2) {
    say $/[0].Str;
}
else {
    say "No match for SEP";
}

# Gives correct result
say "Testing regex as a string: *nix separator";
if ($filenameL ~~ /(<[\\\/]>)/) {
    say $/[0].Str;
}