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Javascript – unable to understand logic difference between regex

What’s the difference between /^(?=.*[w])$/ and /^(?=.*[w]).

  • /^(?=.*[w])$/.test('a') return false
  • /^(?=.*[w])/.test('a')
    return true.

Why is that?

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2

Answers


  1. 0

    Basically
    /^(?=.*[w])$/ -> $/ -> takes the entire string for matching – if yes – return true

    /^(?=.*[w]). -> Here it checks for the first character matching – if yes – return true

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  2. 0

    This pattern asserts a word character, but the match is an empty string ^$ so that will always be false:

    ^(?=.*w)$

    This pattern asserts a word character, which will be true for a, but also for a string like ab %

    ^(?=.*w)

    If you want to combine them and get a match for only a you can assert a word character followed by the end of the string inside the assertion:

    ^(?=w$)

    But in that case you don’t need an assertion, you can just match:

    ^w$

    console.log(/^(?=.*w)$/.test('a'));
console.log(/^(?=.*w)/.test('a'));
console.log(/^(?=w$)/.test('a'));
console.log(/^w$/.test('a'));
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