code: public static Selector expressionSelector(String expr, EvaluationContext evalCtx, SpelCompilerMode mode) {
          SpelParserConfiguration configuration = new SpelParserConfiguration(mode, null);
          SpelExpressionParser parser = new SpelExpressionParser(configuration);
          return new ExpressionSelector(parser.parseExpression(expr), evalCtx);
      }
docs: Helper method for creating an {@code ExpressionSelector}.
  
  @param expr
  The expression to parse.
  @param evalCtx
  The {@link org.springframework.expression.EvaluationContext} to use.
  @param mode
  The {@link org.springframework.expression.spel.SpelCompilerMode} to use.
  
  @return A new {@link reactor.bus.selector.Selector}