diff --git a/fizz-core/src/main/java/we/fizz/function/FuncExecutor.java b/fizz-core/src/main/java/we/fizz/function/FuncExecutor.java
index 037544e..0fdbcba 100644
--- a/fizz-core/src/main/java/we/fizz/function/FuncExecutor.java
+++ b/fizz-core/src/main/java/we/fizz/function/FuncExecutor.java
@@ -212,19 +212,43 @@ public class FuncExecutor {
 		// int pos2 = funcExpression.lastIndexOf(")");
 		String argsStr = funcExpression.substring(pos1 + 1);
 		argsStr = StringUtils.trim(argsStr);
+		
+		Object[] args = new Object[paramTypes.length];
+		if (paramTypes.length == 0) {
+			// no argument method
+			if (hasCloseParenthesis(argsStr, 0)) {
+				ctx.funcExpression = argsStr.substring(1);
+			} else {
+				ctx.funcExpression = argsStr;
+			}
+			return args;
+		}
 		// check if there is any argument
 		if (StringUtils.isBlank(argsStr)) {
 			if (paramTypes == null || paramTypes.length == 0) {
-				return null;
+				ctx.funcExpression = argsStr;
+				return args;
 			} else if (paramTypes.length == 1 && isVarArgs) {
 				// check if variable arguments
-				return null;
+				ctx.funcExpression = argsStr;
+				return args;
+			} else {
+				throw new FizzRuntimeException(
+						String.format("missing argument, Function Expression: %s", funcExpression));
+			}
+		} else if (hasCloseParenthesis(argsStr, 0)) {
+			if (paramTypes == null || paramTypes.length == 0) {
+				ctx.funcExpression = argsStr.substring(1);
+				return args;
+			} else if (paramTypes.length == 1 && isVarArgs) {
+				ctx.funcExpression = argsStr.substring(1);
+				return args;
 			} else {
 				throw new FizzRuntimeException(
 						String.format("missing argument, Function Expression: %s", funcExpression));
 			}
 		}
-		Object[] args = new Object[paramTypes.length];
+		
 		List<Object> varArgs = new ArrayList<>();
 		for (int i = 0; i < paramTypes.length; i++) {
 			Class clazz = paramTypes[i];
@@ -439,9 +463,9 @@ public class FuncExecutor {
 			if (!Character.isWhitespace(argsStr.charAt(i))) {
 				if (")".equals(String.valueOf(argsStr.charAt(i)))) {
 					return true;
-				} /*else {
+				} else {
 					return false;
-				}*/
+				}
 			}
 		}
 		return false;
diff --git a/fizz-core/src/test/java/we/fizz/function/FuncExecutorTests.java b/fizz-core/src/test/java/we/fizz/function/FuncExecutorTests.java
new file mode 100644
index 0000000..d9ab62b
--- /dev/null
+++ b/fizz-core/src/test/java/we/fizz/function/FuncExecutorTests.java
@@ -0,0 +1,71 @@
+/*
+ *  Copyright (C) 2021 the original author or authors.
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package we.fizz.function;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+/**
+ * 
+ * @author Francis Dong
+ *
+ */
+class FuncExecutorTests {
+	
+	@Test
+	void contextLoads() {
+	}
+
+
+	@Test
+	void testNest() {
+		String funcExpression = "fn.codec.md5(fn.date.add(  fn.date.add(\"2021-07-09 22:44:55\", \"yyyy-MM-dd HH:mm:ss\", 1, fn.math.addExact(999,1 )   ), \"yyyy-MM-dd HH:mm:ss\", fn.math.addExact(0,1), 1000 ))";
+		Object result = FuncExecutor.getInstance().exec(null, funcExpression);
+		assertEquals(CodecFunc.getInstance().md5("2021-07-09 22:44:57"), result);
+	}
+	
+	@Test
+	void testNest2() {
+		String funcExpression = "fn.string.toUpperCase(fn.codec.sha256(fn.string.concat(\"a\",\"b\",fn.string.toString(fn.date.timestamp()))))";
+		Object result = FuncExecutor.getInstance().exec(null, funcExpression);
+		System.out.println(result);
+	}
+	
+	@Test
+	void testNest3() {
+		String funcExpression = "fn.string.toUpperCase(fn.string.concat(\"a\",\"b\", fn.string.concat())))";
+		Object result = FuncExecutor.getInstance().exec(null, funcExpression);
+		assertEquals("AB", result);
+	}
+	
+	@Test
+	void testNest4() {
+		String funcExpression = "fn.string.toUpperCase(fn.codec.sha256(fn.string.concat(\"a\",fn.string.concat(),\"b\")))";
+		Object result = FuncExecutor.getInstance().exec(null, funcExpression);
+		System.out.println(result);
+	}
+	
+	@Test
+	void testNest5() {
+		String funcExpression = "fn.string.toUpperCase(fn.codec.sha256(fn.string.concat(\"a\",fn.string.toString(fn.date.timestamp()),fn.string.concat(fn.string.concat(fn.string.concat(fn.string.concat())), \"c\"),\"b\")))";
+		Object result = FuncExecutor.getInstance().exec(null, funcExpression);
+		System.out.println(result);
+	}
+	
+
+}
\ No newline at end of file
diff --git a/fizz-core/src/test/java/we/fizz/function/StringFuncTests.java b/fizz-core/src/test/java/we/fizz/function/StringFuncTests.java
index a5a0193..83a97bc 100644
--- a/fizz-core/src/test/java/we/fizz/function/StringFuncTests.java
+++ b/fizz-core/src/test/java/we/fizz/function/StringFuncTests.java
@@ -97,7 +97,7 @@ class StringFuncTests {
 	
 	@Test
 	void testConcatws() {
-		String funcExpression = "fn.string.concatws(\",\" ,  \"2021-07-09 22:44:55\", \"yyyy-MM-dd HH:mm:ss\")";
+		String funcExpression = "fn.string.concatws(\",\" ,  \"2021-07-09 22:44:55\", \"yyyy-MM-dd HH:mm:ss\"  )";
 		Object result = FuncExecutor.getInstance().exec(null, funcExpression);
 		assertEquals("2021-07-09 22:44:55,yyyy-MM-dd HH:mm:ss", result.toString());
 	}
@@ -124,6 +124,7 @@ class StringFuncTests {
 		String funcExpression = "fn.string.substring({data.dateStr},  {data.startIndex})";
 //		String funcExpression = "fn.string.substring(\"2021-07-09 22:44:55\",  1)";
 		Object result = FuncExecutor.getInstance().exec(ctxNode, funcExpression);
+		System.out.println(result);
 		assertEquals("2021-07-09 22:44:55".substring(1), result.toString());
 	}
 	
@@ -183,6 +184,14 @@ class StringFuncTests {
 		assertEquals("234", result);
 	}
 	
+	
+	@Test
+	void testToString4() {
+		String funcExpression = "fn.string.toString(fn.date.timestamp())";
+		String result = (String)FuncExecutor.getInstance().exec(null, funcExpression);
+		System.out.println(result);
+	}
+	
 	@Test
 	void testReplace() {
 		String funcExpression = "fn.string.replace(\"2021-07-09 22:44:55\",  \"44:55\", \"00:00\")";
@@ -197,6 +206,13 @@ class StringFuncTests {
 		assertEquals("2021-07-09 22:44:55 44:55".replaceAll("44:55", "00:00"), result);
 	}
 	
+	@Test
+	void testReplaceAll2() {
+		String funcExpression = "fn.string.replaceAll(\"1.2.3\",  \"\\.\", \"\")";
+		String result = (String)FuncExecutor.getInstance().exec(null, funcExpression);
+		assertEquals("1.2.3".replaceAll("\\.", ""), result);
+	}
+	
 	@Test
 	void testReplaceFirst() {
 		String funcExpression = "fn.string.replaceFirst(\"2021-07-09 22:44:55 44:55\",  \"44:55\", \"00:00\")";