The problem was already solved in the comments of the question. I just want to extend the answer with pointers.
The library ruta-core.jar and its dependencies (antlr-runtime, uima, uimafit, ...) need to be in the classpath of the application. The eclipse plugin ruta-ep-engine.jar contains the dependencies beside uima. For projects built with maven:
<dependency>
<groupId>org.apache.uima</groupId>
<artifactId>ruta-core</artifactId>
<version>2.2.0</version>
</dependency>
The documentation contains examples on how to call UIMA Ruta scripts from within Java: https://uima.apache.org/d/ruta-current/tools.ruta.book.html#ugr.tools.ruta.ae.basic https://uima.apache.org/d/ruta-current/tools.ruta.book.html#ugr.tools.ruta.integration
For developers that want to create a command line interface, this class might be interesting: https://svn.apache.org/repos/asf/uima/ruta/trunk/ruta-ep-ide-ui/src/main/java/org/apache/uima/ruta/ide/launching/RutaLauncher.java
If you are in a UIMA environment (CAS instance is already available), then the method Ruta.apply(CAS cas, String script) can be used for applying some rules on a CAS.
For developers that prefer to use uimaFIT: https://svn.apache.org/repos/asf/uima/ruta/trunk/ruta-core/src/test/java/org/apache/uima/ruta/engine/UimafitTest.java