private static boolean isPathVariable(final String key) {
        return Objects.nonNull(key) && key.startsWith("{") && key.endsWith("}");
    }
determines whether the string is path variable.

@param key path string
@return true or false
shenyu-plugin/shenyu-plugin-base/src/main/java/org/apache/shenyu/plugin/base/trie/ShenyuTrie.java