as far as = so far as. 朗文词典有这样一个例句：

It is their duty to ensure, so far as is reasonably practicable, that the equipment is safe. （他们有责任在合理可行的范围内确保设备安全。）

far在此为程度副词，as/so far as is reasonably practicable 为范围状语，修饰不定式 to ensure, 由于不定式带有that从句作宾语，所以范围状语插入不定式和that从句之间。as is reasonable practicable为比较分句，is的主语为比较对应项，必须省略。这个句子不宜按照非限制性定语从句分析。