Allow defining complex expression like in interactive mode

src/main/java/org/apache/maven/plugins/help/EvaluateMojo.java

@@ -199,7 +199,10 @@ public void execute() throws MojoExecutionException, MojoFailureException {
                 }
             }
         } else {
-            handleResponse("${" + expression + "}", output);
+            if (!expression.contains("${")) {
+                expression = "${" + expression + "}";
+            }
+            handleResponse(expression, output);
         }
     }
 

src/test/java/org/apache/maven/plugins/help/EvaluateMojoTest.java

@@ -109,6 +109,29 @@ public void testEvaluateWithoutExpressionWithOutput() throws Exception {
         verify(inputHandler, times(2)).readLine();
     }
 
+    /**
+     * Tests evaluation of a complex expression.
+     * @throws Exception in case of errors.
+     */
+    public void testEvaluateForComplexExpression() throws Exception {
+        File testPom = new File(getBasedir(), "target/test-classes/unit/evaluate/plugin-config-complex.xml");
+
+        EvaluateMojo mojo = (EvaluateMojo) lookupMojo("evaluate", testPom);
+
+        ExpressionEvaluator expressionEvaluator = mock(PluginParameterExpressionEvaluator.class);
+        when(expressionEvaluator.evaluate("project_version=${project.version}"))
+                .thenReturn("project_version=1.0.0-SNAPSHOT");
+
+        setUpMojo(mojo, null, expressionEvaluator);
+
+        mojo.execute();
+
+        String ls = System.getProperty("line.separator");
+
+        assertTrue(interceptingLogger.infoLogs.contains(ls + "project_version=1.0.0-SNAPSHOT"));
+        verify(expressionEvaluator).evaluate("project_version=${project.version}");
+    }
+
     /**
      * This test will check that only the <code>project.groupId</code> is printed to
      * stdout nothing else.

src/test/resources/unit/evaluate/plugin-config-complex.xml

@@ -0,0 +1,38 @@
+<!--
+Licensed to the Apache Software Foundation (ASF) under one
+or more contributor license agreements.  See the NOTICE file
+distributed with this work for additional information
+regarding copyright ownership.  The ASF licenses this file
+to you under the Apache License, Version 2.0 (the
+"License"); you may not use this file except in compliance
+with the License.  You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing,
+software distributed under the License is distributed on an
+"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+KIND, either express or implied.  See the License for the
+specific language governing permissions and limitations
+under the License.
+-->
+
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
+  <modelVersion>4.0.0</modelVersion>
+  <groupId>org.apache.maven.its.help</groupId>
+  <artifactId>evaluate</artifactId>
+  <packaging>jar</packaging>
+  <version>1.0-SNAPSHOT</version>
+  <url>http://maven.apache.org</url>
+  <build>
+    <plugins>
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-help-plugin</artifactId>
+        <configuration>
+          <expression><![CDATA[project_version=${project.version}]]></expression>
+        </configuration>
+      </plugin>
+    </plugins>
+  </build>
+</project>