Restore support for legacy unquoted string literals

lib/jmespath/lexer.rb

@@ -243,7 +243,7 @@ def tokenize(expression)
           token = inside(chars, '"', T_QUOTED_IDENTIFIER)
           if token.type == T_QUOTED_IDENTIFIER
             token.value = "\"#{token.value}\""
-            token = parse_json(token)
+            token = parse_json(token, true)
           end
           tokens << token
         when STATE_EQ
@@ -295,9 +295,13 @@ def inside(chars, delim, type)
       Token.new(type, buffer.join, position)
     end
 
-    def parse_json(token)
+    def parse_json(token, quoted=false)
       begin
-        token.value = JSON.load(token.value)
+        if quoted
+          token.value = JSON.load(token.value)
+        else
+          token.value = JSON.load(token.value) rescue JSON.load(sprintf('"%s"', token.value.lstrip))
+        end
       rescue JSON::ParserError
         token.type = T_UNKNOWN
       end

spec/compliance_spec.rb

@@ -5,7 +5,7 @@
 SimpleCov.command_name('test:compliance')
 
 describe 'Compliance' do
-  Dir.glob('spec/compliance/*.json').each do |path|
+  Dir.glob('spec/{compliance,legacy}/*.json').each do |path|
 
     test_file = File.basename(path).split('.').first
     next if test_file == 'benchmarks'

spec/legacy/literal.json

@@ -0,0 +1,56 @@
+[
+    {
+        "given": {
+            "foo": [{"name": "a"}, {"name": "b"}],
+            "bar": {"baz": "qux"}
+        },
+        "cases": [
+            {
+                "expression": "`foo`",
+                "result": "foo"
+            },
+            {
+                "comment": "Double quotes must be escaped.",
+                "expression": "`foo\\\"quote`",
+                "result": "foo\"quote"
+            },
+            {
+                "expression": "`✓`",
+                "result": "✓"
+            },
+            {
+                "comment": "Double quote in literal",
+                "expression": "`foo\\\"bar`",
+                "result": "foo\"bar"
+            },
+            {
+                "expression": "`1\\``",
+                "result": "1`"
+            },
+            {
+                "comment": "Multiple literal expressions with escapes",
+                "expression": "`\\\\`.{a:`b`}",
+                "result": {"a": "b"}
+            }
+        ]
+    },
+  {
+    "comment": "Literals",
+    "given": {"type": "object"},
+    "cases": [
+      {
+        "expression": "`foo`",
+        "result": "foo"
+      },
+      {
+        "expression": "`      foo`",
+        "result": "foo"
+      },
+      {
+        "comment": "Literal on RHS of subexpr not allowed",
+        "expression": "foo.`bar`",
+        "error": "syntax"
+      }
+    ]
+  }
+]