正则表达式之断言

[LiuL0703]

正则表达式--断言

Positive Lookbehind

• (?<=) #断言要匹配的文本前缀

  let str = 'qwert&code=1243&qerfvs=6577';
  str.replace(/(?<=code=)\d+/,'4321');     // "qwert&code=4321&qerfvs=6577"

Negative Lookbehind

• (?<!) #断言位置不能匹配的文本前缀

  "(?<!(T|t)he\s)(cat)"  => The cat sat on cat.

Positive Lookahead

• (?=) #断言要匹配的文本后缀

  "(T|t)he(?=\sfat)" => The fat cat sat on the mat.
  // 匹配 紧跟fat的 The或the 

Negative Lookahead

• (?!) #断言位置不能匹配的文本后缀

  "(T|t)he(?!\sfat)" => The fat cat sat on the mat.
  // 匹配文本后面不紧跟fat的The 或the

• https://regexper.com/

• https://regexr.com/

总结

• X(?=Y) Positive lookahead X if followed by Y
• X(?!Y) Negative lookahead X if not followed by Y
• (?<=Y)X Positive lookbehind X if after Y
• (?<!Y)X Negative lookbehind X if not after Y