perl regex not start and end wi - regex

Question

I am trying to write a perl script that get all strings that is does not start and end with a single quote. And a string cannot be a part of comment # and each line in DATA is not necessary at the beginning of a line.
use warnings;
use strict;
my $file;
{
local $/ = undef;
$file = <DATA>;
};
my #strings = $file =~ /(?:[^']).*(?:[^'])/g;
print join ("\n",#strings);
__DATA__
my $string = 'This is string1';
"This is string2"
# comment : "This is string3"
print "This is comment syntax #"."This is string4";
I am getting no where with this regex.
The expected output is
"This is a string2"
"This is comment syntax #"
"This is string 4"

Answer 1

Obviously this is only an exercise, as there are been many students asking about this problem lately. Regex's will only ever get you part of the way there, as there will pretty much always be edge cases.
The following code is probably good enough for your purposes, but it doesn't even successfully parse itself because of quotes inside a qr{}. You'll have to figure out how to get strings that span lines to work on your own:
use strict;
use warnings;
my $doublequote_re = qr{"(?: (?> [^\\"]+ ) | \\. )*"}x;
my $singlequote_re = qr{'(?: (?> [^\\']+ ) | \\. )*'}x;
my $data = do { local $/; <DATA> };
while ($data =~ m{(#.*|$singlequote_re|$doublequote_re)}g) {
my $match = $1;
if ($match =~ /^#/) {
print "Comment - $match\n";
} elsif ($match =~ /^"/) {
print "Double quote - $match\n";
} elsif ($match =~ /^'/) {
print "Single quote - $match\n";
} else {
die "Carp! something went wrong! <$match>";
}
}
__DATA__
my $string = 'This is string1';
"This is string2"
# comment : "This is string3"
print "This is comment syntax #"."This is string4";

Answer 2

Do not know how to achieve that by using regular expression, so here is a simple hand-written lexer:
#!/usr/bin/perl
use strict;
use warnings;
sub extract_string {
my #buf = split //, shift;
while (my $peer = shift #buf) {
if ($peer eq '"') {
my $str = "$peer";
while ($peer = shift #buf) {
$str .= "$peer";
last if $peer eq '"';
}
if ($peer) {
return ($str, join '', #buf);
}
else {
return ("", "");
}
}
elsif ($peer eq '#') {
return ("", "");
}
}
}
my ($str, $buf);
while ($buf = <DATA>) {
chomp $buf;
while (1) {
($str, $buf) = extract_string $buf;
print "$str\n" if $str;
last unless $buf;
}
}
__DATA__
my $string = 'This is string1';
"This is string2"
# comment : "This is string3"
print "This is comment syntax #"."This is string4";
Another option is using Perl module such as PPI.