Lookaheads and LookbehindsLesson 3.4

Combining lookaheads and lookbehinds for complex conditions

chaining assertions, extract between delimiters, password strength pattern, assertion order, real-world extraction examples

Assertions Chain — Order Matters

Combine lookahead and lookbehind at the same position to match text sandwiched between two delimiters without including the delimiters in the match.

// Extract content between [[ and ]]
const text = 'Use [[variable]] or [[name]] here';
text.match(/(?<=\[\[)[^\]]+(?=\]\])/g)
// => ['variable', 'name']

Strong Password Validator

function isStrongPassword(pw) {
  return /^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[!@#$%]).{10,}$/.test(pw);
}

Each lookahead runs at position 0, independently checking a separate requirement.