freeCodeCamp/positive-and-negative-lookahead.english.md at dce5bceb8586ea13e7d58a8206705a8a0cc4e39f

Greg Brewton 7a1256df79 Minor clarification (#34250 )

(?=\D*\d) is looking for a non-number to precede a number. If '12Three' was passed in to test, it would not match though it matches the stated criteria of 5 character minimum length and two consecutive numbers.

2018-11-10 10:33:01 +05:30

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id, title, challengeType

id	title	challengeType
587d7dba367417b2b2512ba9	Positive and Negative Lookahead	1

Description

Lookaheads are patterns that tell JavaScript to look-ahead in your string to check for patterns further along. This can be useful when you want to search for multiple patterns over the same string. There are two kinds of lookaheads: positive lookahead and negative lookahead. A positive lookahead will look to make sure the element in the search pattern is there, but won't actually match it. A positive lookahead is used as (?=...) where the ... is the required part that is not matched. On the other hand, a negative lookahead will look to make sure the element in the search pattern is not there. A negative lookahead is used as (?!...) where the ... is the pattern that you do not want to be there. The rest of the pattern is returned if the negative lookahead part is not present. Lookaheads are a bit confusing but some examples will help.

let quit = "qu";
let noquit = "qt";
let quRegex= /q(?=u)/;
let qRegex = /q(?!u)/;
quit.match(quRegex); // Returns ["q"]
noquit.match(qRegex); // Returns ["q"]

A more practical use of lookaheads is to check two or more patterns in one string. Here is a (naively) simple password checker that looks for between 3 and 6 characters and at least one number:

let password = "abc123";
let checkPass = /(?=\w{3,6})(?=\D*\d)/;
checkPass.test(password); // Returns true

Instructions

Use lookaheads in the pwRegex to match passwords that are greater than 5 characters long, do not begin with numbers, and have two consecutive digits.

Tests

tests:
  - text: Your regex should use two positive <code>lookaheads</code>.
    testString: assert(pwRegex.source.match(/\(\?=.*?\)\(\?=.*?\)/) !== null, 'Your regex should use two positive <code>lookaheads</code>.');
  - text: Your regex should not match <code>"astronaut"</code>
    testString: assert(!pwRegex.test("astronaut"), 'Your regex should not match <code>"astronaut"</code>');
  - text: Your regex should not match <code>"airplanes"</code>
    testString: assert(!pwRegex.test("airplanes"), 'Your regex should not match <code>"airplanes"</code>');
  - text: Your regex should not match <code>"banan1"</code>
    testString: assert(!pwRegex.test("banan1"), 'Your regex should not match <code>"banan1"</code>');
  - text: Your regex should match <code>"bana12"</code>
    testString: assert(pwRegex.test("bana12"), 'Your regex should match <code>"bana12"</code>');
  - text: Your regex should match <code>"abc123"</code>
    testString: assert(pwRegex.test("abc123"), 'Your regex should match <code>"abc123"</code>');
  - text: Your regex should not match <code>"123"</code>
    testString: assert(!pwRegex.test("123"), 'Your regex should not match <code>"123"</code>');
  - text: Your regex should not match <code>"1234"</code>
    testString: assert(!pwRegex.test("1234"), 'Your regex should not match <code>"1234"</code>');

Challenge Seed

let sampleWord = "astronaut";
let pwRegex = /change/; // Change this line
let result = pwRegex.test(sampleWord);

Solution

var pwRegex = /(?=\w{5})(?=\D*\d{2})/;

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