Regex returns False when validating card

Question

I’m trying to validate a car plate in the format : ABC1234 3 letters and 4 numbers.

let regex = new RegExp("/[a-zA-Z]{3}[0-9]{4}/");
let isvalid = regex.test('abc1234');

When I put it on other sites,.

Any idea what it might be? Thank you.

Answer 1

If using the class RegExp you do not need to use the bars; these only serve to define a direct regular expression without using the class. If by removing the bars your expression matches other values that should not marry, your expression is wrong. How did you not give details about what the others were strings that the result was true, but should be false, I assumed it would be for values with more than 3 letters followed by more than 4 numbers or variants of it.

let regex = new RegExp("[a-zA-Z]{3}[0-9]{4}");

const tests = [
    'abc1234',
    'ab1234', // falta uma letra
    'abc123', // falta um número
    '1234abc', // começa com número
    'abc12345', // tem um número a mais
    'abcd1234', // tem uma letra a mais
];

for (let test of tests) {
    console.log(test, regex.test(test));
}

If this is, in fact the problem, just fix the expression by adding the characters ^ and $ to define the beginning and end of each value, thus to marry only values that start with 3 letters followed by 4 numbers and nothing more.

let regex = new RegExp("^[a-zA-Z]{3}[0-9]{4}$");

const tests = [
    'abc1234',
    'ab1234', // falta uma letra
    'abc123', // falta um número
    '1234abc', // começa com número
    'abc12345', // tem um número a mais
    'abcd1234', // tem uma letra a mais
];

for (let test of tests) {
    console.log(test, regex.test(test));
}

Using bars instead of class would be:

let regex = /^[a-zA-Z]{3}[0-9]{4}$/;

const tests = [
    'abc1234',
    'ab1234', // falta uma letra
    'abc123', // falta um número
    '1234abc', // começa com número
    'abc12345', // tem um número a mais
    'abcd1234', // tem uma letra a mais
];

for (let test of tests) {
    console.log(test, regex.test(test));
}

It is worth noting that in this case the use of bar notation is preferable because the expression is constant. This makes the code more performatic and more semantic. Prefer to use the class RexExp only when the expression may vary.

Answer 2

Complementing, you can also delimit using the meta-character \b using literal expression, adding also the ^ that defines the start of the string:

let regex = /^[a-zA-Z]{3}[0-9]{4}\b/;
console.log(regex.test('abc1234')); // correto
console.log(regex.test('abc12345')); // 5 números
console.log(regex.test('abcd2345')); // 4 letras
console.log(regex.test('abc145')); // 3 números
console.log(regex.test('ab2145')); // 2 letras

Answer 3

To validate the current standard and standard (Br) of cards, use this simple script for validation of standard cards and current cards using Javascript (vanilla) Regex:

let plate = "ABC1234";
let plateMerc = "ABC1D23"

const regexPlate = /^[a-zA-Z]{3}[0-9]{4}$/;
const regexPlateMerc = /^[a-zA-Z]{3}[0-9]{1}[a-zA-Z]{1}[0-9]{2}$/;

function validatePlate(plate) {
  if(regexPlate.test(plate)){
    console.warn('Placa válida (padrão atual)');
    return true
  }
  else if(regexPlateMerc.test(plate)){
    console.warn('Placa válida (padrão mercosul)');
    return true
  }
  else {
    console.error('Placa inválida no padrão atual e mercosul');
    return false
  }