”先行断言“指的是 x只有在y前面才匹配，必须写成/x(?=y)/
比如，只匹配百分号之前的数字
/\d+(?=%)/.exec('100% of US presidents have been male'); // ["100", index: 0, input: "100% of US presidents have been male", groups: undefined]”先行否定断言“指的是 x只有不在y前面才匹配，必须写成/x(?!y)/
比如，只匹配不在百分号之前的数字
/\d+(?!%)/.exec('that’s all 44 of them'); //["44", index: 11, input: "that’s all 44 of them", groups: undefined]“先行否定断言”指的是 x只有在y后面才匹配，必须写成/(?<=y)x/
比如，只匹配美元符号之后的数字
/(?<=\$)\d+/.exec('Benjamin Franklin is on the $100 bill'); //["100", index: 29, input: "Benjamin Franklin is on the $100 bill", groups: undefined]”后行否定断言“指的是 x只有不在y后面才匹配，必须写成/(?<!y)x/
比如，只匹配不在美元符号后面的数字
/(?<!\$)\d+/.exec('it’s is worth about €90'); // ["90", index: 21, input: "it’s is worth about €90", groups: undefined]