Fix backtracking issues, add string interpolation in Perl lexer (#1161)

The previous rules for strings used a regular expression that combined
different elements of the string. This could, in a pathological case,
cause Rouge to get stuck trying to lex the code. This problem can be
fixed by lexing a quoted string in a separate state to `:root`.

In addition to adding states for quoted string, this commit also adds
support for basic string interpolation. The basic code is lifted from
the Ruby lexer. This fixes #974.

lib/rouge/lexers/perl.rb

@@ -121,9 +121,9 @@ def self.detect?(text)
         rule /\d+(_\d*)*e[+-]?\d+(_\d*)*/i, Num::Float
         rule /\d+(_\d+)*/, Num::Integer
 
-        rule /'(\\\\|\\'|[^'])*'/, Str
-        rule /"(\\\\|\\"|[^"])*"/, Str
-        rule /`(\\\\|\\`|[^`])*`/, Str::Backtick
+        rule /'/, Punctuation, :sq
+        rule /"/, Punctuation, :dq
+        rule /`/, Punctuation, :bq
         rule /<([^\s>]+)>/, re_tok
         rule /(q|qq|qw|qr|qx)\{/, Str::Other, :cb_string
         rule /(q|qq|qw|qr|qx)\(/, Str::Other, :rb_string
@@ -179,6 +179,26 @@ def self.detect?(text)
         rule /;/, Punctuation, :pop!
       end
 
+      state :sq do
+        rule /\\[']/, Str::Escape
+        rule /[^\\']+/, Str::Single
+        rule /'/, Punctuation, :pop!
+      end
+
+      state :dq do
+        mixin :string_intp
+        rule /\\[\\tnr"]/, Str::Escape
+        rule /[^\\"]+?/, Str::Double
+        rule /"/, Punctuation, :pop!
+      end
+
+      state :bq do
+        mixin :string_intp
+        rule /\\[\\tnr`]/, Str::Escape
+        rule /[^\\`]+?/, Str::Backtick
+        rule /`/, Punctuation, :pop!
+      end
+
       [[:cb, '\{', '\}'],
        [:rb, '\(', '\)'],
        [:sb, '\[', '\]'],
@@ -193,6 +213,17 @@ def self.detect?(text)
         end
       end
 
+      state :in_interp do
+        rule /}/, Str::Interpol, :pop!
+        rule /\s+/, Text
+        rule /[a-z_]\w*/i, Str::Interpol
+      end
+
+      state :string_intp do
+        rule /[$@][{]/, Str::Interpol, :in_interp
+        rule /[$@][a-z_]\w*/i, Str::Interpol
+      end
+
       state :end_part do
         # eat the rest of the stream
         rule /.+/m, Comment::Preproc, :pop!

spec/visual/samples/perl

@@ -163,6 +163,12 @@ $p =~ s%
   %bar%x;
 print $p, "\n";
 
+my $str1 = "This is a ${ string } with fancy interpolation."
+my $cmd1 = `So is @{ this } one.`
+my $str2 = "This is a $string with plain interpolation."
+my $cmd2 = `So is @this one.`
+my $str3 = 'A boring $string.'
+
 ###### perl_misc ########
 
 # from http://gist.github.com/485595