protected static boolean isSimpleExpression(String expressionCandidate) {
      final Matcher matcher = EXPRESSION_PATTERN.matcher(expressionCandidate);
      return matcher.matches()
          && matcher.group(2).isEmpty() // Simple Expressions do not support any special operators
          && VARIABLE_LIST_PATTERN.matcher(matcher.group(3)).matches();
    }
Expression that adheres to Simple String Expansion as outlined in <a
href="https://tools.ietf.org/html/rfc6570#section-3.2.2>Simple String Expansion (Level 1)</a>
core/src/main/java/feign/template/Expressions.java