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🔰lookaround

🚧 under construction

JS ⟩ value ⟩ object ⟩ regex ⟩ pattern ⟩ lookaround

(?=...)	    // Positive lookahead (match the position before the specified regex)
(?!...)	    // Negative lookahead (don’t match, as above)
(?<=...)    // Positive lookbehind (match the position after the specified regex)
(?<!...)    // Negative lookbehind (don't match, as above)

JS.info ⟩ Lookahead & lookbehind
Regular Expression Quick Reference

examples

Password Validation

// Look ahead
// -----------
// (?=.*\d)       : at least one digit.
// (?=.*[a-z])    : at least one lower case.
// (?=.*[A-Z])    : at least one upper case.
// [0-9a-zA-Z]{6,}: at least 6 alphanumeric characters.
// /^...$/        : whole line
function validate(password) {
  return /^(?=.*\d)(?=.*[a-z])(?=.*[A-Z])[0-9a-zA-Z]{6,}$/.test(password);
}

words with (-) or (')

// (?=[a-zA-Z'-]*[a-zA-Z]): at least one alphabet character
// 
/(?=[a-zA-Z'-]*[a-zA-Z])[a-zA-Z'-]+/g

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Last updated 2 years ago

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const {log} = console;

/*
  Symbols
  -------
  • ? : lookaround
  • = : positive
  • ! : negative (NOT)
  • < : lookbehind

  lookahead: X(?=Y)
  ------------------
  match X (followed "immediately" by Y): 
  match X ("lookahead" for Y)
  
  lookahead: X(?=Y)(?=Z)
  -----------------------
  match X (followed "immediately" by Y & Z)
  match X ("lookahead" for Y & Z)
  
  negative lookahead: X(?!Y)
  ---------------------------
  match X (NOT followed "immediately" by Y)
  match X ("negative lookahead" for Y)
  
  Lookbehind: (?<=Y)X
  --------------------
  match X ("immediately" following Y)

  Negative Lookbehind: (?<!Y)X
  -----------------------------
  match X (NOT "immediately" following Y)
*/

//          ↱ space        ╭╮
let str = "1 turkey costs $30€";
//          ╰──── .*30 ─────╯

[
  // lookahead
  // ----------
  //         ╭X╮   Y
  str.match(/\d+(?=€)/),            // ["30"]
  
  //         ╭X╮   ╭╮    ╭──╮
  str.match(/\d+(?=\s)(?=.*30)/),   // ["1"]
  str.match(/\d+(?=.*30)(?=\s)/),   // ["1"]
  
  str.match(/\d+(?=30)(?=\s)/),     // ⭐️ null 
  // (no digits are followed "immediately" by "30")
  
  // negative lookahead
  // ------------------
  str.match(/\d+\b(?!€)/g),         // ["1"]
  //         ╰─╯╰╯   ↑
  
  // lookbehind: 
  // ----------
  //  escaped $ ➞╭╮ ╭─╮
  str.match(/(?<=\$)\d+/),          // ["30"]
  
].forEach(x => log(x))