Fix character escaping in Perl lexer (#1549)

In single- and double-quoted strings, the Perl lexer currently
tokenises `\` as an error if it is not followed by a character in part
of a recognised escape sequence. This is a bug. Perl accepts the use of
`\` in single- and double-quoted strings even if it is not part of a
valid escape sequence.

This commit permits the use of `\` in single-quoted and double-quoted
strings as well as increasing the range of escape sequences that are
recognised in double-quoted strings.

lib/rouge/lexers/perl.rb

@@ -178,16 +178,24 @@ def self.detect?(text)
       end
 
       state :sq do
-        rule %r/\\[']/, Str::Escape
+        rule %r/\\[\\']/, Str::Escape
         rule %r/[^\\']+/, Str::Single
         rule %r/'/, Punctuation, :pop!
+        rule %r/\\/, Str::Single
       end
 
       state :dq do
         mixin :string_intp
-        rule %r/\\[\\tnr"]/, Str::Escape
+        rule %r/\\[\\tnrabefluLUE"$@]/, Str::Escape
+        rule %r/\\0\d{2}/, Str::Escape
+        rule %r/\\o\{\d+\}/, Str::Escape
+        rule %r/\\x\h{2}/, Str::Escape
+        rule %r/\\x\{\h+\}/, Str::Escape
+        rule %r/\\c./, Str::Escape
+        rule %r/\\N\{[^\}]+\}/, Str::Escape
         rule %r/[^\\"]+?/, Str::Double
         rule %r/"/, Punctuation, :pop!
+        rule %r/\\/, Str::Escape
       end
 
       state :bq do

spec/visual/samples/perl

@@ -60,7 +60,15 @@ $s =~ y/[]|/()&/; # "backward" compatible
 
 
 # quoted strings
+my $a = 'foo';
+my $a = 'foo\'s bar';
+my $a = 'foo\bar';
+my $a = 'foo\\bar';
 my $a = "foo";
+my $a = "foo \" bar";
+my $a = "foo\bar";
+my $a = "foo \057 \x7f \x{263a} \cC \N{GREEK SMALL LETTER SIGMA} bar";
+my $a = "invalid escape \w";
 
 print "a: ";
 print $a, " - ";