code: protected boolean hasValidVersionSpecified( EnforcerRuleHelper helper, Plugin source, List<PluginWrapper> pluginWrappers )
      {
          boolean found = false;
          boolean status = false;
          for ( PluginWrapper plugin : pluginWrappers )
          {
              // find the matching plugin entry
              if ( source.getArtifactId().equals( plugin.getArtifactId() )
                  && source.getGroupId().equals( plugin.getGroupId() ) )
              {
                  found = true;
                  // found the entry. now see if the version is specified
                  String version = plugin.getVersion();
                  try
                  {
                      version = (String) helper.evaluate( version );
                  }
                  catch ( ExpressionEvaluationException e )
                  {
                      return false;
                  }
  
                  if ( StringUtils.isNotEmpty( version ) && !StringUtils.isWhitespace( version ) )
                  {
  
                      if ( banRelease && version.equals( "RELEASE" ) )
                      {
                          return false;
                      }
  
                      if ( banLatest && version.equals( "LATEST" ) )
                      {
                          return false;
                      }
  
                      if ( banSnapshots && isSnapshot( version ) )
                      {
                          return false;
                      }
                      // the version was specified and not
                      // banned. It's ok. Keep looking through the list to make
                      // sure it's not using a banned version somewhere else.
  
                      status = true;
  
                      if ( !banRelease && !banLatest && !banSnapshots )
                      {
                          // no need to keep looking
                          break;
                      }
                  }
              }
          }
          if ( !found )
          {
              log.debug( "plugin " + source.getGroupId() + ":" + source.getArtifactId() + " not found" );
          }
          return status;
      }
docs: Checks for valid version specified.
  
  @param helper the helper
  @param source the source
  @param pluginWrappers the plugins
  @return true, if successful