diff --git a/CHANGELOG.md b/CHANGELOG.md
index dc50e2b..63064ce 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -32,6 +32,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 ### Fixed
 
 - Improved matching of identifiers by adding a scope and properly recognizing hyphens.
+- Support identifiers starting with an underscore character (#43).
 
 ---
 
diff --git a/Terraform.sublime-syntax b/Terraform.sublime-syntax
index dc4c3ac..b3419e5 100644
--- a/Terraform.sublime-syntax
+++ b/Terraform.sublime-syntax
@@ -28,10 +28,17 @@ file_extensions:
 scope: source.terraform
 
 variables:
-  # Identifiers: (UTF-8) (ID_Continue | '-')*;
+  # Identifiers: ID_Start (ID_Continue | '-')*;
   #
-  # https://github.com/hashicorp/hcl2/blob/master/hcl/hclsyntax/spec.md#identifiers
-  identifier: (?:\b(?!null|false|true)[[:alpha:]][[:alnum:]_-]*\b)
+  # There is an undocumented exception
+  # that an underscore character is also accepted
+  # as an id start character.
+  #
+  # https://github.com/hashicorp/hcl/blob/main/hclsyntax/spec.md#identifiers
+  # http://unicode.org/reports/tr31/#Table_Lexical_Classes_for_Identifiers
+  ID_Start: '[\p{ID_Start}_]'
+  ID_Continue: '[\p{ID_Continue}-]'
+  identifier: (?:{{ID_Start}}{{ID_Continue}}*)
 
   # Exponent: "e" or "E" followed by an optional sign
   #
diff --git a/syntax_test_terraform.tf b/syntax_test_terraform.tf
index b30789e..6047d43 100644
--- a/syntax_test_terraform.tf
+++ b/syntax_test_terraform.tf
@@ -274,6 +274,9 @@
       identifier-with-hyphens
 #     ^^^^^^^^^^^^^^^^^^^^^^^ variable.other.readwrite.terraform
 
+      __EOF__
+#     ^^^^^^^ variable.other.readwrite.terraform
+
       identifier.
 // comment
 # <- comment
@@ -3128,6 +3131,12 @@
     END
 #   ^^^^ keyword.control.heredoc.terraform
 
+    <<__EOF__
+#   ^^ keyword.operator.heredoc.terraform
+#     ^^^^^^^ keyword.control.heredoc.terraform
+    aaa
+    __EOF__
+#   ^^^^^^^^ keyword.control.heredoc.terraform
 /////////////////////////////////////////////////////////////////////
 // IMPORTS
 /////////////////////////////////////////////////////////////////////