escape troublesome characters in regexp expressions

Found this issue while parsing the ABNF JSON string representation.
These two rules caused parsing issues because of the characters
generated within the regular expressions that they result in:

escape    = %x5C ; \
unescaped = %x20-21 / %x23-5B / %x5D-10FFFF ; [ -!] / [#-[] / []-\u{10FFF}]

lib/ebnf/rule.rb

@@ -367,11 +367,11 @@ def to_peg
     def to_regexp
       case expr.first
       when :hex
-        Regexp.new(translate_codepoints(expr[1]))
+        Regexp.new(Regexp.escape(translate_codepoints(expr[1])))
       when :istr
         /#{expr.last}/ui
       when :range
-        Regexp.new("[#{translate_codepoints(expr[1])}]")
+        Regexp.new("[#{escape_regexp_character_range(translate_codepoints(expr[1]))}]")
       else
         raise "Can't turn #{expr.inspect} into a regexp"
       end
@@ -770,5 +770,11 @@ def make_sym_id(variation = nil)
       @id_seq += 1
       ["_#{@sym}_#{@id_seq}#{variation}".to_sym, ("#{@id}.#{@id_seq}#{variation}" if @id)]
     end
+
+    # Escape "[", "]", and "\" in ranges so they don't result in a warning or error
+    # about empty character classes.
+    def escape_regexp_character_range(character_range)
+      character_range.gsub(/([\[\]\\])/) {|char| "\\#{char}"}
+    end
   end
-end
+end

spec/rule_spec.rb

@@ -481,9 +481,13 @@
   describe "#to_regexp" do
     {
       hex: [:hex, "#x20", / /],
+      hex: [:hex, "#x5c", /\\/],
       range: [:range, "a-b", /[a-b]/],
       range2: [:range, "a-zA-Z", /[a-zA-Z]/],
       range3: [:range, "abc-", /[abc-]/],
+      range4: [:range, "#x23-#x5b", /[#-\[]/],
+      range5: [:range, "#x5d-#x5e", /[\]-^]/],
+      range6: [:range, "#x5c-#x5e", /[\\-^]/],
     }.each do |title, (op, exp, regexp)|
       it title do
         expect(EBNF::Rule.new(title, nil, [op, exp]).to_regexp).to eql regexp
@@ -1055,4 +1059,4 @@
       end
     end
   end
-end
+end