RegExp断言

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先行断言

  • lookahead assertion：x在y之前才匹配，格式为/x(?=y)/
  • negative lookahead assertion：只有x不在y之前才匹配，格式为/x(?!y)/
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let str = 'now is 02:11:44'

/\d+(?=:)/.exec(str) // ['02']
/\d+(?!\.)/.exec(str) // ['0']

后行断言

  • lookbehind assertion：x只有在y后面才匹配，格式为/(?<=y)x/
  • negative lookbehind assertion：只有x不在y后面才匹配，格式为/(?<!y)x/
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let str = 'now is 02:11:44'

/(?<=:)\d+/.exec(str) // ['11']
/(?<!:)\d+/.exec(str) // ['02']