An Application of Negative Lookahead

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Let’s say we want to validate a password that meets the following criteria:

  • At least 8 characters long
  • Contains a combination of letters (both uppercase and lowercase), numbers, and special characters
  • Does not contain the word “password” anywhere

We can use negative lookahead to implement this validation. Here’s an example using JavaScript:

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const password = "SecurePass123!";
const regex = /^(?!.*password).*$/;

if (regex.test(password) && password.length >= 8) {
  console.log("Password is valid.");
} else {
  console.log("Password is invalid.");

In this example, the regular expression ^(?!.*password).*$ is used. Let’s break it down:

  • ^: Matches the start of the string.
  • (?!.*password): Negative lookahead assertion that checks if the string does not contain the word “password” anywhere. The .* matches any characters (except newline), so .*password matches any characters followed by “password”.
  • .*: Matches any characters (except newline).
  • $: Matches the end of the string.

By using the negative lookahead (?!.*password), we ensure that the string does not contain the word “password”. If the password meets this condition and is at least 8 characters long (checked separately), it is considered valid.

Note that this is a basic example, and password requirements can vary depending on specific needs. However, it demonstrates the use of negative lookahead to exclude certain patterns from matching.