diff --git a/core/src/ast.rs b/core/src/ast.rs
index 33ab59fc..7a986ddf 100644
--- a/core/src/ast.rs
+++ b/core/src/ast.rs
@@ -678,6 +678,46 @@ pub(crate) fn resolve_identifier<I: Interrupt>(
 	attrs: Attrs,
 	context: &mut crate::Context,
 	int: &I,
+) -> FResult<Value> {
+	let cloned_scope = scope.clone();
+	if let Some(ref scope) = cloned_scope {
+		if let Some(val) = scope.get(ident, attrs, context, int)? {
+			return Ok(val);
+		}
+	}
+	if let Some(val) = context.variables.get(ident.as_str()) {
+		return Ok(val.clone());
+	}
+
+	let builtin_result = resolve_builtin_identifier(ident, cloned_scope, attrs, context, int);
+	if !matches!(builtin_result, Err(FendError::IdentifierNotFound(_))) {
+		return builtin_result;
+	}
+	let unit_result = crate::units::query_unit(ident.as_str(), attrs, context, int);
+	if !matches!(unit_result, Err(FendError::IdentifierNotFound(_))) {
+		return unit_result;
+	}
+	
+	if !ident.as_str().bytes().all(|b| b.is_ascii_digit() || b.is_ascii_uppercase()) {
+		return unit_result;
+	}
+	let lowercase_builtin_result = resolve_builtin_identifier(
+			&ident.as_str().to_ascii_lowercase().into(),
+			scope,
+			attrs,
+			context,
+			int,
+		);
+	// "Unknown identifier" errors should use the uppercase ident.
+	lowercase_builtin_result.or(unit_result)
+}
+
+fn resolve_builtin_identifier<I: Interrupt>(
+	ident: &Ident,
+	scope: Option<Arc<Scope>>,
+	attrs: Attrs,
+	context: &mut crate::Context,
+	int: &I,
 ) -> FResult<Value> {
 	macro_rules! eval_box {
 		($input:expr) => {
@@ -690,14 +730,6 @@ pub(crate) fn resolve_identifier<I: Interrupt>(
 			)?)
 		};
 	}
-	if let Some(scope) = scope.clone() {
-		if let Some(val) = scope.get(ident, attrs, context, int)? {
-			return Ok(val);
-		}
-	}
-	if let Some(val) = context.variables.get(ident.as_str()) {
-		return Ok(val.clone());
-	}
 	Ok(match ident.as_str() {
 		"pi" | "\u{3c0}" => Value::Num(Box::new(Number::pi())),
 		"tau" | "\u{3c4}" => Value::Num(Box::new(Number::pi().mul(2.into(), int)?)),
@@ -774,7 +806,7 @@ pub(crate) fn resolve_identifier<I: Interrupt>(
 		"tomorrow" => Value::Date(crate::date::Date::today(context)?.next()),
 		"yesterday" => Value::Date(crate::date::Date::today(context)?.prev()),
 		"trans" => Value::String(Cow::Borrowed("🏳️‍⚧️")),
-		_ => return crate::units::query_unit(ident.as_str(), attrs, context, int),
+		_ => return Err(FendError::IdentifierNotFound(ident.clone())),
 	})
 }
 
diff --git a/core/tests/integration_tests.rs b/core/tests/integration_tests.rs
index 86f4870b..cfe28701 100644
--- a/core/tests/integration_tests.rs
+++ b/core/tests/integration_tests.rs
@@ -5994,3 +5994,14 @@ fn fibonacci() {
 	test_eval("fib 10", "55");
 	test_eval("fib 11", "89");
 }
+
+#[test]
+fn uppercase_identifiers() {
+	test_eval("SIN PI", "0");
+	test_eval("COS TAU", "1");
+	test_eval("LOG 1", "approx. 0");
+	test_eval("LOG10 1", "approx. 0");
+	test_eval("EXP 0", "approx. 1");
+
+	expect_error("foo = 1; FOO", Some("unknown identifier 'FOO'"));
+}