diff --git a/fluent-syntax/src/errors.js b/fluent-syntax/src/errors.js
index 43c358987..0d124ae01 100644
--- a/fluent-syntax/src/errors.js
+++ b/fluent-syntax/src/errors.js
@@ -45,6 +45,8 @@ function getErrorMessage(code, args) {
       return 'Expected variant key';
     case 'E0014':
       return 'Expected literal';
+    case 'E0015':
+      return 'Only one variant can be marked as default (*)';
     default:
       return code;
   }
diff --git a/fluent-syntax/src/ftlstream.js b/fluent-syntax/src/ftlstream.js
index 705d74a8c..82aef23ea 100644
--- a/fluent-syntax/src/ftlstream.js
+++ b/fluent-syntax/src/ftlstream.js
@@ -43,6 +43,11 @@ export class FTLParserStream extends ParserStream {
       return true;
     }
 
+    if (ch === '\n') {
+      // Unicode Character 'SYMBOL FOR NEWLINE' (U+2424)
+      throw new ParseError('E0003', '\u2424');
+    }
+
     throw new ParseError('E0003', ch);
   }
 
@@ -56,7 +61,7 @@ export class FTLParserStream extends ParserStream {
 
   takeChar(f) {
     const ch = this.ch;
-    if (f(ch)) {
+    if (ch !== undefined && f(ch)) {
       this.next();
       return ch;
     }
@@ -64,6 +69,10 @@ export class FTLParserStream extends ParserStream {
   }
 
   isIDStart() {
+    if (this.ch === undefined) {
+      return false;
+    }
+
     const cc = this.ch.charCodeAt(0);
     return ((cc >= 97 && cc <= 122) || // a-z
             (cc >= 65 && cc <= 90) ||  // A-Z
@@ -98,8 +107,15 @@ export class FTLParserStream extends ParserStream {
 
     this.peek();
 
+    const ptr = this.getPeekIndex();
+
     this.peekLineWS();
 
+    if (this.getPeekIndex() - ptr === 0) {
+      this.resetPeek();
+      return false;
+    }
+
     if (this.currentPeekIs('*')) {
       this.peek();
     }
@@ -119,8 +135,15 @@ export class FTLParserStream extends ParserStream {
 
     this.peek();
 
+    const ptr = this.getPeekIndex();
+
     this.peekLineWS();
 
+    if (this.getPeekIndex() - ptr === 0) {
+      this.resetPeek();
+      return false;
+    }
+
     if (this.currentPeekIs('.')) {
       this.resetPeek();
       return true;
@@ -130,6 +153,36 @@ export class FTLParserStream extends ParserStream {
     return false;
   }
 
+  isPeekNextLinePattern() {
+    if (!this.currentPeekIs('\n')) {
+      return false;
+    }
+
+    this.peek();
+
+    const ptr = this.getPeekIndex();
+
+    this.peekLineWS();
+
+    if (this.getPeekIndex() - ptr === 0) {
+      this.resetPeek();
+      return false;
+    }
+
+    if (this.currentPeekIs('}') ||
+        this.currentPeekIs('.') ||
+        this.currentPeekIs('#') ||
+        this.currentPeekIs('[') ||
+        this.currentPeekIs('*') ||
+        this.currentPeekIs('{')) {
+      this.resetPeek();
+      return false;
+    }
+
+    this.resetPeek();
+    return true;
+  }
+
   isPeekNextLineTagStart() {
     if (!this.currentPeekIs('\n')) {
       return false;
@@ -137,8 +190,15 @@ export class FTLParserStream extends ParserStream {
 
     this.peek();
 
+    const ptr = this.getPeekIndex();
+
     this.peekLineWS();
 
+    if (this.getPeekIndex() - ptr === 0) {
+      this.resetPeek();
+      return false;
+    }
+
     if (this.currentPeekIs('#')) {
       this.resetPeek();
       return true;
diff --git a/fluent-syntax/src/parser.js b/fluent-syntax/src/parser.js
index 3ca8d7996..57f5e7f89 100644
--- a/fluent-syntax/src/parser.js
+++ b/fluent-syntax/src/parser.js
@@ -156,7 +156,7 @@ function getMessage(ps, comment) {
   }
 
   if (pattern === undefined && attrs === undefined && tags === undefined) {
-    throw new ParseError('E0005', id);
+    throw new ParseError('E0005', id.name);
   }
 
   return new AST.Message(id, pattern, attrs, tags, comment);
@@ -254,6 +254,9 @@ function getVariants(ps) {
     ps.skipLineWS();
 
     if (ps.currentIs('*')) {
+      if (hasDefault) {
+        throw new ParseError('E0015');
+      }
       ps.next();
       defaultIndex = true;
       hasDefault = true;
@@ -350,15 +353,12 @@ function getPattern(ps) {
         break;
       }
 
-      ps.peek();
-
-      if (!ps.currentPeekIs(' ')) {
-        ps.resetPeek();
+      if (!ps.isPeekNextLinePattern()) {
         break;
       }
 
-      ps.peekLineWS();
-      ps.skipToPeek();
+      ps.next();
+      ps.skipLineWS();
 
       firstLine = false;
 
diff --git a/fluent-syntax/test/fixtures_behavior/attribute_expression_with_wrong_attr.ftl b/fluent-syntax/test/fixtures_behavior/attribute_expression_with_wrong_attr.ftl
new file mode 100644
index 000000000..5fbf7b777
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/attribute_expression_with_wrong_attr.ftl
@@ -0,0 +1,7 @@
+key = { foo.23 }
+
+//~ ERROR E0004, pos 12, args "a-zA-Z"
+
+key = { foo. }
+
+//~ ERROR E0004, pos 31, args "a-zA-Z"
diff --git a/fluent-syntax/test/fixtures_behavior/attribute_starts_from_nl.ftl b/fluent-syntax/test/fixtures_behavior/attribute_starts_from_nl.ftl
new file mode 100644
index 000000000..380b08cc5
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/attribute_starts_from_nl.ftl
@@ -0,0 +1,3 @@
+foo = Value
+.attr = Value 2
+//~ ERROR E0002, pos 12
diff --git a/fluent-syntax/test/fixtures_behavior/attribute_with_empty_pattern.ftl b/fluent-syntax/test/fixtures_behavior/attribute_with_empty_pattern.ftl
new file mode 100644
index 000000000..66e4df3e2
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/attribute_with_empty_pattern.ftl
@@ -0,0 +1,2 @@
+key = Value
+    .label =
diff --git a/fluent-syntax/test/fixtures_behavior/attribute_without_equal_sign.ftl b/fluent-syntax/test/fixtures_behavior/attribute_without_equal_sign.ftl
new file mode 100644
index 000000000..f412f83c7
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/attribute_without_equal_sign.ftl
@@ -0,0 +1,3 @@
+key = Value
+    .label
+//~ ERROR E0003, pos 22, args "="
diff --git a/fluent-syntax/test/fixtures_behavior/broken_number.ftl b/fluent-syntax/test/fixtures_behavior/broken_number.ftl
new file mode 100644
index 000000000..46cce00c0
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/broken_number.ftl
@@ -0,0 +1,14 @@
+key = { -2.4.5 }
+//~ ERROR E0003, pos 12, args "}"
+
+key = { -2.4. }
+//~ ERROR E0003, pos 30, args "}"
+
+key = { -.4 }
+//~ ERROR E0004, pos 44, args "0-9"
+
+key = { -2..4 }
+//~ ERROR E0004, pos 61, args "0-9"
+
+key = { 24d }
+//~ ERROR E0003, pos 77, args "}"
diff --git a/fluent-syntax/test/fixtures_behavior/call_expression_with_trailing_comma.ftl b/fluent-syntax/test/fixtures_behavior/call_expression_with_trailing_comma.ftl
new file mode 100644
index 000000000..974cf99c9
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/call_expression_with_trailing_comma.ftl
@@ -0,0 +1 @@
+key = { BUILTIN(23, ) }
diff --git a/fluent-syntax/test/fixtures_behavior/call_expression_with_wrong_kwarg_name.ftl b/fluent-syntax/test/fixtures_behavior/call_expression_with_wrong_kwarg_name.ftl
new file mode 100644
index 000000000..b0bd8500c
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/call_expression_with_wrong_kwarg_name.ftl
@@ -0,0 +1,2 @@
+key = { BUILTIN(2: "foo") }
+//~ ERROR E0009, pos 17
diff --git a/fluent-syntax/test/fixtures_behavior/call_expression_with_wrong_value_type.ftl b/fluent-syntax/test/fixtures_behavior/call_expression_with_wrong_value_type.ftl
new file mode 100644
index 000000000..26ac13981
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/call_expression_with_wrong_value_type.ftl
@@ -0,0 +1,2 @@
+key = { BUILTIN(key: foo) }
+//~ ERROR E0006, pos 21, args "value"
diff --git a/fluent-syntax/test/fixtures_behavior/comment_continues_with_one_slash.ftl b/fluent-syntax/test/fixtures_behavior/comment_continues_with_one_slash.ftl
new file mode 100644
index 000000000..5dd830cd2
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/comment_continues_with_one_slash.ftl
@@ -0,0 +1,3 @@
+// This is a normal comment
+/ but this is not
+//~ ERROR E0003, pos 29, args "/"
diff --git a/fluent-syntax/test/fixtures_behavior/comment_with_eof.ftl b/fluent-syntax/test/fixtures_behavior/comment_with_eof.ftl
new file mode 100644
index 000000000..0ae104701
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/comment_with_eof.ftl
@@ -0,0 +1 @@
+// This is a comment with no new line
diff --git a/fluent-syntax/test/fixtures_behavior/empty_resource.ftl b/fluent-syntax/test/fixtures_behavior/empty_resource.ftl
new file mode 100644
index 000000000..e69de29bb
diff --git a/fluent-syntax/test/fixtures_behavior/empty_resource_with_ws.ftl b/fluent-syntax/test/fixtures_behavior/empty_resource_with_ws.ftl
new file mode 100644
index 000000000..4e4354436
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/empty_resource_with_ws.ftl
@@ -0,0 +1,2 @@
+   
+     
diff --git a/fluent-syntax/test/fixtures_behavior/entry_start_with_one_slash.ftl b/fluent-syntax/test/fixtures_behavior/entry_start_with_one_slash.ftl
new file mode 100644
index 000000000..440b3ffcd
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/entry_start_with_one_slash.ftl
@@ -0,0 +1,3 @@
+
+/ Test
+//~ ERROR E0003, pos 2, args "/"
diff --git a/fluent-syntax/test/fixtures_behavior/leading_empty_lines.ftl b/fluent-syntax/test/fixtures_behavior/leading_empty_lines.ftl
new file mode 100644
index 000000000..60e34ee63
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/leading_empty_lines.ftl
@@ -0,0 +1,3 @@
+
+
+foo = Value
diff --git a/fluent-syntax/test/fixtures_behavior/leading_empty_lines_with_ws.ftl b/fluent-syntax/test/fixtures_behavior/leading_empty_lines_with_ws.ftl
new file mode 100644
index 000000000..cd626c8d3
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/leading_empty_lines_with_ws.ftl
@@ -0,0 +1,5 @@
+
+   
+
+   
+foo = Value
diff --git a/fluent-syntax/test/fixtures_behavior/multiline_string.ftl b/fluent-syntax/test/fixtures_behavior/multiline_string.ftl
new file mode 100644
index 000000000..9f3cdba06
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/multiline_string.ftl
@@ -0,0 +1,3 @@
+key = { BUILTIN(key: "
+ text
+    ") }
diff --git a/fluent-syntax/test/fixtures_behavior/multiline_with_non_empty_first_line.ftl b/fluent-syntax/test/fixtures_behavior/multiline_with_non_empty_first_line.ftl
new file mode 100644
index 000000000..70417cc57
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/multiline_with_non_empty_first_line.ftl
@@ -0,0 +1,3 @@
+key = Value
+    Value 2
+//~ ERROR E0002, pos 12
diff --git a/fluent-syntax/test/fixtures_behavior/non_id_attribute_name.ftl b/fluent-syntax/test/fixtures_behavior/non_id_attribute_name.ftl
new file mode 100644
index 000000000..1cf580b29
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/non_id_attribute_name.ftl
@@ -0,0 +1,3 @@
+key = Value
+    .2 = Foo
+//~ ERROR E0004, pos 17, args "a-zA-Z"
diff --git a/fluent-syntax/test/fixtures_behavior/placeable_without_close_bracket.ftl b/fluent-syntax/test/fixtures_behavior/placeable_without_close_bracket.ftl
new file mode 100644
index 000000000..282cf2b61
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/placeable_without_close_bracket.ftl
@@ -0,0 +1,3 @@
+key = { $num
+
+//~ ERROR E0003, pos 12, args "}"
diff --git a/fluent-syntax/test/fixtures_behavior/second_attribute_starts_from_nl.ftl b/fluent-syntax/test/fixtures_behavior/second_attribute_starts_from_nl.ftl
new file mode 100644
index 000000000..e87c2b98f
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/second_attribute_starts_from_nl.ftl
@@ -0,0 +1,4 @@
+key = Value
+    .label = Value
+.accesskey = K
+//~ ERROR E0002, pos 31
diff --git a/fluent-syntax/test/fixtures_behavior/second_tag_starts_from_nl.ftl b/fluent-syntax/test/fixtures_behavior/second_tag_starts_from_nl.ftl
new file mode 100644
index 000000000..00c9dcd96
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/second_tag_starts_from_nl.ftl
@@ -0,0 +1,4 @@
+key = Value
+    #tag
+#tag2
+//~ ERROR E0002, pos 21
diff --git a/fluent-syntax/test/fixtures_behavior/section_starts_with_one_bracket.ftl b/fluent-syntax/test/fixtures_behavior/section_starts_with_one_bracket.ftl
new file mode 100644
index 000000000..eb4235618
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/section_starts_with_one_bracket.ftl
@@ -0,0 +1,2 @@
+[ This is a broken section ]]
+//~ ERROR E0003, pos 1, args "["
diff --git a/fluent-syntax/test/fixtures_behavior/section_with_nl_in_the_middle.ftl b/fluent-syntax/test/fixtures_behavior/section_with_nl_in_the_middle.ftl
new file mode 100644
index 000000000..d4731b4a9
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/section_with_nl_in_the_middle.ftl
@@ -0,0 +1,3 @@
+[[ This is
+a broken section]]
+//~ ERROR E0003, pos 10, args "]"
diff --git a/fluent-syntax/test/fixtures_behavior/section_with_no_nl_after_it.ftl b/fluent-syntax/test/fixtures_behavior/section_with_no_nl_after_it.ftl
new file mode 100644
index 000000000..f11f4c062
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/section_with_no_nl_after_it.ftl
@@ -0,0 +1 @@
+[[ This is a correct section ]]
diff --git a/fluent-syntax/test/fixtures_behavior/section_with_one_bracket_at_the_end.ftl b/fluent-syntax/test/fixtures_behavior/section_with_one_bracket_at_the_end.ftl
new file mode 100644
index 000000000..0464afea0
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/section_with_one_bracket_at_the_end.ftl
@@ -0,0 +1,2 @@
+[[ This is a broken section ]
+//~ ERROR E0003, pos 29, args "]"
diff --git a/fluent-syntax/test/fixtures_behavior/select_expression_with_two_selectors.ftl b/fluent-syntax/test/fixtures_behavior/select_expression_with_two_selectors.ftl
new file mode 100644
index 000000000..0a05e57de
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/select_expression_with_two_selectors.ftl
@@ -0,0 +1,2 @@
+key = { $foo $faa }
+//~ ERROR E0003, pos 13, args "}"
diff --git a/fluent-syntax/test/fixtures_behavior/select_expression_without_arrow.ftl b/fluent-syntax/test/fixtures_behavior/select_expression_without_arrow.ftl
new file mode 100644
index 000000000..b89618dd8
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/select_expression_without_arrow.ftl
@@ -0,0 +1,2 @@
+key = { $foo - }
+//~ ERROR E0003, pos 13, args "}"
diff --git a/fluent-syntax/test/fixtures_behavior/select_expression_without_variants.ftl b/fluent-syntax/test/fixtures_behavior/select_expression_without_variants.ftl
new file mode 100644
index 000000000..198654bbc
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/select_expression_without_variants.ftl
@@ -0,0 +1,6 @@
+key = { $foo -> }
+//~ ERROR E0003, pos 16, args "␤"
+
+key = { $foo ->
+    }
+//~ ERROR E0003, pos 39, args "["
diff --git a/fluent-syntax/test/fixtures_behavior/selector_expression_ends_abruptly.ftl b/fluent-syntax/test/fixtures_behavior/selector_expression_ends_abruptly.ftl
new file mode 100644
index 000000000..a2ad26bec
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/selector_expression_ends_abruptly.ftl
@@ -0,0 +1,2 @@
+key = { $foo ->
+//~ ERROR E0003, pos 16, args "["
diff --git a/fluent-syntax/test/fixtures_behavior/foo.ftl b/fluent-syntax/test/fixtures_behavior/simple_message.ftl
similarity index 100%
rename from fluent-syntax/test/fixtures_behavior/foo.ftl
rename to fluent-syntax/test/fixtures_behavior/simple_message.ftl
diff --git a/fluent-syntax/test/fixtures_behavior/single_char_id.ftl b/fluent-syntax/test/fixtures_behavior/single_char_id.ftl
new file mode 100644
index 000000000..b4d3b479c
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/single_char_id.ftl
@@ -0,0 +1,2 @@
+k = Value
+    .l = Foo
diff --git a/fluent-syntax/test/fixtures_behavior/standalone_identifier.ftl b/fluent-syntax/test/fixtures_behavior/standalone_identifier.ftl
new file mode 100644
index 000000000..d3ee59c70
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/standalone_identifier.ftl
@@ -0,0 +1,2 @@
+foo
+//~ ERROR E0005, pos 3, args "foo"
diff --git a/fluent-syntax/test/fixtures_behavior/tag_and_attribute_together.ftl b/fluent-syntax/test/fixtures_behavior/tag_and_attribute_together.ftl
new file mode 100644
index 000000000..2adfb21a5
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/tag_and_attribute_together.ftl
@@ -0,0 +1,4 @@
+key = Value
+    .label = Foo
+    #masculine
+//~ ERROR E0012, pos 28
diff --git a/fluent-syntax/test/fixtures_behavior/tag_starts_from_nl.ftl b/fluent-syntax/test/fixtures_behavior/tag_starts_from_nl.ftl
new file mode 100644
index 000000000..797ab57a9
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/tag_starts_from_nl.ftl
@@ -0,0 +1,3 @@
+key = Value
+#tag
+//~ ERROR E0002, pos 12
diff --git a/fluent-syntax/test/fixtures_behavior/bar.ftl b/fluent-syntax/test/fixtures_behavior/unclosed_empty_placeable_error.ftl
similarity index 100%
rename from fluent-syntax/test/fixtures_behavior/bar.ftl
rename to fluent-syntax/test/fixtures_behavior/unclosed_empty_placeable_error.ftl
diff --git a/fluent-syntax/test/fixtures_behavior/unknown_entry_start.ftl b/fluent-syntax/test/fixtures_behavior/unknown_entry_start.ftl
new file mode 100644
index 000000000..357d3b46d
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/unknown_entry_start.ftl
@@ -0,0 +1,3 @@
+
+8Foo = Foo
+//~ ERROR E0002, pos 1
diff --git a/fluent-syntax/test/fixtures_behavior/variant_ends_abruptly.ftl b/fluent-syntax/test/fixtures_behavior/variant_ends_abruptly.ftl
new file mode 100644
index 000000000..94dfcd4b5
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/variant_ends_abruptly.ftl
@@ -0,0 +1,3 @@
+key = { $foo ->
+    *[
+//~ ERROR E0004, pos 22, args "a-zA-Z"
diff --git a/fluent-syntax/test/fixtures_behavior/variant_expression_empty_key.ftl b/fluent-syntax/test/fixtures_behavior/variant_expression_empty_key.ftl
new file mode 100644
index 000000000..c6fd0b8d1
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/variant_expression_empty_key.ftl
@@ -0,0 +1,2 @@
+key = { foo[] }
+//~ ERROR E0004, pos 12, args "a-zA-Z"
diff --git a/fluent-syntax/test/fixtures_behavior/variant_starts_from_nl.ftl b/fluent-syntax/test/fixtures_behavior/variant_starts_from_nl.ftl
new file mode 100644
index 000000000..139f4f108
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/variant_starts_from_nl.ftl
@@ -0,0 +1,4 @@
+key = {
+*[one] Value
+    }
+//~ ERROR E0004, pos 7, args "a-zA-Z"
diff --git a/fluent-syntax/test/fixtures_behavior/variant_with_digit_key.ftl b/fluent-syntax/test/fixtures_behavior/variant_with_digit_key.ftl
new file mode 100644
index 000000000..3ab304d69
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/variant_with_digit_key.ftl
@@ -0,0 +1,3 @@
+key = {
+        *[-2] Foo
+    }
diff --git a/fluent-syntax/test/fixtures_behavior/variant_with_empty_pattern.ftl b/fluent-syntax/test/fixtures_behavior/variant_with_empty_pattern.ftl
new file mode 100644
index 000000000..8dfb5bc14
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/variant_with_empty_pattern.ftl
@@ -0,0 +1,3 @@
+key = {
+    *[one]
+    }
diff --git a/fluent-syntax/test/fixtures_behavior/variant_with_leading_space_in_name.ftl b/fluent-syntax/test/fixtures_behavior/variant_with_leading_space_in_name.ftl
new file mode 100644
index 000000000..0b8f9c956
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/variant_with_leading_space_in_name.ftl
@@ -0,0 +1,4 @@
+key = {
+        *[  one] Foo
+    }
+//~ ERROR E0004, pos 18, args "a-zA-Z"
diff --git a/fluent-syntax/test/fixtures_behavior/variant_with_symbol_with_space.ftl b/fluent-syntax/test/fixtures_behavior/variant_with_symbol_with_space.ftl
new file mode 100644
index 000000000..68a60c6e7
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/variant_with_symbol_with_space.ftl
@@ -0,0 +1,3 @@
+key = {
+        *[New York] Nowy Jork
+    }
diff --git a/fluent-syntax/test/fixtures_behavior/variants_with_two_defaults.ftl b/fluent-syntax/test/fixtures_behavior/variants_with_two_defaults.ftl
new file mode 100644
index 000000000..f49c2998c
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/variants_with_two_defaults.ftl
@@ -0,0 +1,5 @@
+key = {
+    *[one] Foo
+    *[two] Two
+    }
+//~ ERROR E0015, pos 27
diff --git a/fluent-syntax/test/fixtures_structure/message_with_empty_pattern.ftl b/fluent-syntax/test/fixtures_structure/message_with_empty_pattern.ftl
new file mode 100644
index 000000000..179dbacc6
--- /dev/null
+++ b/fluent-syntax/test/fixtures_structure/message_with_empty_pattern.ftl
@@ -0,0 +1 @@
+foo = 
diff --git a/fluent-syntax/test/fixtures_structure/message_with_empty_pattern.json b/fluent-syntax/test/fixtures_structure/message_with_empty_pattern.json
new file mode 100644
index 000000000..e4ee002ef
--- /dev/null
+++ b/fluent-syntax/test/fixtures_structure/message_with_empty_pattern.json
@@ -0,0 +1,26 @@
+{
+  "type": "Resource",
+  "body": [
+    {
+      "type": "Message",
+      "span": {
+        "type": "Span",
+        "start": 0,
+        "end": 6
+      },
+      "annotations": [],
+      "id": {
+        "type": "Identifier",
+        "name": "foo"
+      },
+      "value": {
+        "type": "Pattern",
+        "elements": []
+      },
+      "attributes": null,
+      "tags": null,
+      "comment": null
+    }
+  ],
+  "comment": null
+}
diff --git a/fluent-syntax/test/fixtures_structure/resource_comment.ftl b/fluent-syntax/test/fixtures_structure/resource_comment.ftl
new file mode 100644
index 000000000..4c7a33c9d
--- /dev/null
+++ b/fluent-syntax/test/fixtures_structure/resource_comment.ftl
@@ -0,0 +1,2 @@
+// This is a resource wide comment
+// It's multiline
diff --git a/fluent-syntax/test/fixtures_structure/resource_comment.json b/fluent-syntax/test/fixtures_structure/resource_comment.json
new file mode 100644
index 000000000..d8a529e09
--- /dev/null
+++ b/fluent-syntax/test/fixtures_structure/resource_comment.json
@@ -0,0 +1,14 @@
+{
+  "type": "Resource",
+  "body": [],
+  "comment": {
+    "type": "Comment",
+    "span": {
+      "type": "Span",
+      "start": 0,
+      "end": 53
+    },
+    "annotations": [],
+    "content": "This is a resource wide comment\nIt's multiline"
+  }
+}
diff --git a/fluent-syntax/test/fixtures_structure/resource_comment_trailing_line.ftl b/fluent-syntax/test/fixtures_structure/resource_comment_trailing_line.ftl
new file mode 100644
index 000000000..14b5cced0
--- /dev/null
+++ b/fluent-syntax/test/fixtures_structure/resource_comment_trailing_line.ftl
@@ -0,0 +1,3 @@
+// This is a comment
+// This comment is multiline
+//
diff --git a/fluent-syntax/test/fixtures_structure/resource_comment_trailing_line.json b/fluent-syntax/test/fixtures_structure/resource_comment_trailing_line.json
new file mode 100644
index 000000000..49c8c1677
--- /dev/null
+++ b/fluent-syntax/test/fixtures_structure/resource_comment_trailing_line.json
@@ -0,0 +1,14 @@
+{
+  "type": "Resource",
+  "body": [],
+  "comment": {
+    "type": "Comment",
+    "span": {
+      "type": "Span",
+      "start": 0,
+      "end": 53
+    },
+    "annotations": [],
+    "content": "This is a comment\nThis comment is multiline\n"
+  }
+}
diff --git a/fluent-syntax/test/fixtures_structure/section.ftl b/fluent-syntax/test/fixtures_structure/section.ftl
new file mode 100644
index 000000000..60a26d326
--- /dev/null
+++ b/fluent-syntax/test/fixtures_structure/section.ftl
@@ -0,0 +1,2 @@
+
+[[ This is a section ]]
diff --git a/fluent-syntax/test/fixtures_structure/section.json b/fluent-syntax/test/fixtures_structure/section.json
new file mode 100644
index 000000000..657f491bc
--- /dev/null
+++ b/fluent-syntax/test/fixtures_structure/section.json
@@ -0,0 +1,20 @@
+{
+  "type": "Resource",
+  "body": [
+    {
+      "type": "Section",
+      "span": {
+        "type": "Span",
+        "start": 1,
+        "end": 25
+      },
+      "annotations": [],
+      "name": {
+        "type": "Symbol",
+        "name": "This is a section"
+      },
+      "comment": null
+    }
+  ],
+  "comment": null
+}
diff --git a/fluent-syntax/test/fixtures_structure/foo.ftl b/fluent-syntax/test/fixtures_structure/simple_message.ftl
similarity index 100%
rename from fluent-syntax/test/fixtures_structure/foo.ftl
rename to fluent-syntax/test/fixtures_structure/simple_message.ftl
diff --git a/fluent-syntax/test/fixtures_structure/foo.json b/fluent-syntax/test/fixtures_structure/simple_message.json
similarity index 100%
rename from fluent-syntax/test/fixtures_structure/foo.json
rename to fluent-syntax/test/fixtures_structure/simple_message.json
diff --git a/fluent-syntax/test/fixtures_structure/standalone_comment.ftl b/fluent-syntax/test/fixtures_structure/standalone_comment.ftl
new file mode 100644
index 000000000..49fce24e8
--- /dev/null
+++ b/fluent-syntax/test/fixtures_structure/standalone_comment.ftl
@@ -0,0 +1,3 @@
+foo = Value
+
+// This is a standalone comment
diff --git a/fluent-syntax/test/fixtures_structure/standalone_comment.json b/fluent-syntax/test/fixtures_structure/standalone_comment.json
new file mode 100644
index 000000000..0ae862574
--- /dev/null
+++ b/fluent-syntax/test/fixtures_structure/standalone_comment.json
@@ -0,0 +1,41 @@
+{
+  "type": "Resource",
+  "body": [
+    {
+      "type": "Message",
+      "span": {
+        "type": "Span",
+        "start": 0,
+        "end": 11
+      },
+      "annotations": [],
+      "id": {
+        "type": "Identifier",
+        "name": "foo"
+      },
+      "value": {
+        "type": "Pattern",
+        "elements": [
+          {
+            "type": "TextElement",
+            "value": "Value"
+          }
+        ]
+      },
+      "attributes": null,
+      "tags": null,
+      "comment": null
+    },
+    {
+      "type": "Comment",
+      "span": {
+        "type": "Span",
+        "start": 13,
+        "end": 45
+      },
+      "annotations": [],
+      "content": "This is a standalone comment"
+    }
+  ],
+  "comment": null
+}
diff --git a/package.json b/package.json
index 3408e82a3..d873679c7 100644
--- a/package.json
+++ b/package.json
@@ -12,6 +12,7 @@
     "commander": "^2.9",
     "documentation": "^4.0.0-beta10",
     "eslint": "^3.9.1",
+    "fuzzer": "^0.2.1",
     "gh-pages": "^0.12.0",
     "jsdoc": "^3.4.3",
     "mocha": "^3.0.2",
diff --git a/tools/README.md b/tools/README.md
index 166eb0a98..08579eaee 100644
--- a/tools/README.md
+++ b/tools/README.md
@@ -49,6 +49,26 @@ Examples:
     ./format.js -e data.json file.ftl
 
 
+fuzz.js
+--------
+
+`fuzz.js` is a fuzzer for fluent.js's parsers.
+
+    Usage: fuzz [options] [file]
+
+    Options:
+
+      -h, --help                output usage information
+      -V, --version             output the version number
+      -r, --runtime             Use the runtime parser
+      -i, --repetitions <int>   Number of repetitions [100000]
+
+Examples:
+
+    echo "foo = Foo" | ./fuzz.js
+    ./fuzz.js -i 1000 file.ftl
+
+
 perf
 ----
 
diff --git a/tools/fuzz.js b/tools/fuzz.js
new file mode 100755
index 000000000..3c8f0433f
--- /dev/null
+++ b/tools/fuzz.js
@@ -0,0 +1,68 @@
+#!/usr/bin/env node
+
+'use strict';
+
+const fs = require('fs');
+const readline = require('readline');
+const program = require('commander');
+const fuzzer = require('fuzzer');
+
+require('babel-register')({
+  plugins: ['transform-es2015-modules-commonjs']
+});
+
+fuzzer.seed(Math.random() * 1000000000);
+
+program
+  .version('0.0.1')
+  .usage('[options] [file]')
+  .option('-r, --runtime', 'Use the runtime parser')
+  .option('-i, --repetitions <int>', 'Number of repetitions [100000]', 100000)
+  .parse(process.argv);
+
+if (program.args.length) {
+  fs.readFile(program.args[0], fuzz);
+} else {
+  process.stdin.resume();
+  process.stdin.on('data', data => fuzz(null, data));
+}
+
+function fuzz(err, data) {
+  if (err) {
+    return console.error('File not found: ' + err.path);
+  }
+
+  const parse = program.runtime
+    ? require('../fluent/src/parser').default
+    : require('../fluent-syntax/src').parse;
+
+  const source = data.toString();
+  const mutations = new Set();
+
+  let i = 1;
+
+  while (i <= program.repetitions) {
+    const mutated = fuzzer.mutate.string(source);
+
+    if (mutations.has(mutated)) {
+      continue;
+    }
+
+    mutations.add(mutated);
+
+    const progress = Math.round(i / program.repetitions * 100);
+    readline.cursorTo(process.stdout, 0);
+    process.stdout.write(`mutation ${i} ... ${progress}%`);
+
+    try  {
+      parse(mutated);
+    } catch (e) {
+      console.log(`! mutation ${i}`);
+      console.log(e);
+      console.log(mutated);
+      break;
+    }
+
+    i++;
+  }
+}
diff --git a/tools/parse.js b/tools/parse.js
index d76e79f61..edcb9fdb7 100755
--- a/tools/parse.js
+++ b/tools/parse.js
@@ -64,10 +64,10 @@ function printAnnotations(source, entry) {
 }
 
 function printAnnotation(source, span, annot) {
-  const { name, message, pos } = annot;
+  const { name, message, span: { start } } = annot;
   const slice = source.substring(span.start, span.end).trimRight();
-  const lineNumber = FluentSyntax.lineOffset(source, pos) + 1;
-  const columnOffset = FluentSyntax.columnOffset(source, pos);
+  const lineNumber = FluentSyntax.lineOffset(source, start) + 1;
+  const columnOffset = FluentSyntax.columnOffset(source, start);
   const showLines = lineNumber - FluentSyntax.lineOffset(source, span.start);
   const lines = slice.split('\n');
   const head = lines.slice(0, showLines);