Regex Practice 14/14

Fix the bug: the pattern that accepts junk

Every other lesson asked you to write a pattern from scratch. Real work is more often the opposite: a pattern is already there, it looks correct, and it is quietly wrong.

This one is the most common regex bug there is, and the nastiest kind: it fails by saying yes. isPostcode is supposed to accept exactly five digits and nothing else. Run it and it happily accepts abc12345xyz. Nothing throws, no error appears - the function just returns true for input it should reject. A validator that says yes too easily is how bad data gets into a database.

The reason is that .test() asks "does this pattern appear anywhere in the string?", not "is the string exactly this pattern?" You met the fix in phase 4.

Your task: run the code, look at what it returns for the junk input, and fix isPostcode so it accepts a five-digit postcode and rejects everything else.

You'll practice:

Reading a silent wrong answer instead of waiting for an error
Anchoring a pattern so it has to match the whole string

// This is broken: it accepts junk. Run it, look at the second line, and fix isPostcode.
function isPostcode(str) {
  return /\d{5}/.test(str);
}

console.log('12345        ->', isPostcode('12345'));
console.log('abc12345xyz  ->', isPostcode('abc12345xyz'));
console.log('123456       ->', isPostcode('123456'));