"用立法规定" can be translated into English as "stipulate by legislation" or "regulate through legislation".
"Stipulate by legislation" emphasizes the act of clearly setting out requirements or provisions through legal means.
"Regulate through legislation" highlights the function of legislation in controlling or governing certain matters.
Both expressions are appropriate depending on the context and the specific meaning you want to convey. For example:
"We should stipulate clear penalties for environmental violations by legislation."（我们应当用立法规定对环境违法行为的明确处罚。）
"The government regulates the use of personal data through legislation."（政府用立法规定来监管个人数据的使用。）