diff --git a/src/inja.hpp b/src/inja.hpp
index 49595fd..105f6a6 100644
--- a/src/inja.hpp
+++ b/src/inja.hpp
@@ -597,13 +597,14 @@ public:
 	*/
 	static Regex function_regex(std::string name, int number_arguments) {
 		std::string pattern = name;
-		if (number_arguments > 0) {
-			pattern.append("\\(");
-			for (int i = 0; i < number_arguments; i++) {
-				if (i != 0) pattern.append(",");
-				pattern.append("(.*)");
-			}
-			pattern.append("\\)");
+		pattern.append("\\(");
+		for (int i = 0; i < number_arguments; i++) {
+			if (i != 0) pattern.append(",");
+			pattern.append("(.*)");
+		}
+		pattern.append("\\)");
+		if (number_arguments == 0) { // Without arguments, allow to use the callback without parenthesis
+			pattern = "(?:" + name + "|" + pattern + ")";
 		}
 		return Regex{"\\s*" + pattern + "\\s*"};
 	}
diff --git a/test/src/unit-renderer.cpp b/test/src/unit-renderer.cpp
index 9c4f19f..ffc3331 100644
--- a/test/src/unit-renderer.cpp
+++ b/test/src/unit-renderer.cpp
@@ -229,6 +229,7 @@ TEST_CASE("callbacks") {
 	CHECK( env.render("{{ double(age) }}", data) == "56" );
 	CHECK( env.render("{{ half(age) }}", data) == "14" );
 	CHECK( env.render("{{ double-greetings }}", data) == "Hello Hello!" );
+	CHECK( env.render("{{ double-greetings() }}", data) == "Hello Hello!" );
 	CHECK( env.render("{{ multiply(4, 5) }}", data) == "20.0" );
 	CHECK( env.render("{{ multiply(3, 4, 5) }}", data) == "60.0" );
 	CHECK( env.render("{{ multiply }}", data) == "1.0" );
diff --git a/test/src/unit-string-helper.cpp b/test/src/unit-string-helper.cpp
index 2eadc45..4696a21 100644
--- a/test/src/unit-string-helper.cpp
+++ b/test/src/unit-string-helper.cpp
@@ -159,6 +159,6 @@ TEST_CASE("match-functions") {
 
 TEST_CASE("create-regex-functions") {
 	CHECK( inja::Parser::function_regex("upper", 1).pattern() == "\\s*upper\\((.*)\\)\\s*" );
-	CHECK( inja::Parser::function_regex("upper", 0).pattern() == "\\s*upper\\s*" );
+	CHECK( inja::Parser::function_regex("upper", 0).pattern() == "\\s*(?:upper|upper\\(\\))\\s*" );
 	CHECK( inja::Parser::function_regex("lower", 2).pattern() == "\\s*lower\\((.*),(.*)\\)\\s*" );
 }