freeCodeCamp/positive-and-negative-lookahead.english.md at 2c15bcbb438eb9af7e9296ee9abdd436d29fb03f

Divyanshu a987245e18 Added new test cases and corrected the solution for positive and negative lookahead for ensuring that strings beginning with numbers are not accepted (#37485 )

* Added new test case '8pass99' for positive and negative lookahead

* Changed the pwRegex to reflect the new solution

* Updated the pwRegex to reflect the latest solution as on forum

* Added another test case to catch "/^(?=\w{6,})(?=\D*\d{2})/" solution

2019-11-24 17:11:20 -05:00

3.2 KiB

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id, title, challengeType, forumTopicId

id	title	challengeType	forumTopicId
587d7dba367417b2b2512ba9	Positive and Negative Lookahead	1	301360

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);
  - text: Your regex should not match <code>"astronaut"</code>
    testString: assert(!pwRegex.test("astronaut"));
  - text: Your regex should not match <code>"airplanes"</code>
    testString: assert(!pwRegex.test("airplanes"));
  - text: Your regex should not match <code>"banan1"</code>
    testString: assert(!pwRegex.test("banan1"));
  - text: Your regex should match <code>"bana12"</code>
    testString: assert(pwRegex.test("bana12"));
  - text: Your regex should match <code>"abc123"</code>
    testString: assert(pwRegex.test("abc123"));
  - text: Your regex should not match <code>"123"</code>
    testString: assert(!pwRegex.test("123"));
  - text: Your regex should not match <code>"1234"</code>
    testString: assert(!pwRegex.test("1234"));
  - text: Your regex should not match <code>"8pass99"</code>
    testString: assert(!pwRegex.test("8pass99"));
  - text: Your regex should not match <code>"12abcde"</code>
    testString: assert(!pwRegex.test("12abcde"));

Challenge Seed

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

Solution

var pwRegex = /^(?=\w{6})(?=\D+\d{2})/;

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