Regex patterns in Java vs Javascript

Question

I have java Regex pattern for UK Postcode:

^([a-zA-Z]){1}([0-9][0-9]|[0-9]|[a-zA-Z][0-9][a-zA-Z]|[a-zA-Z][0-9][0-9]|[a-zA-Z][0-9]){1}([ ])([0-9][a-zA-z][a-zA-z]){1}$

I want to use it in Javascript as well but it does not validate Postcodes correctly in Javascript. Is there any difference between Regex patterns in Java and Javascript?

What is the format of UK's postal code, give us the pattern that you would like to search? — Touchstone
– Touchstone, Commented Apr 21, 2016 at 16:31
It works in js for "DE56 2SR". What postal code do you have that it is not working with? — Jonathan M
– Jonathan M, Commented Apr 21, 2016 at 16:39

collapsar · Accepted Answer · 2016-04-21 16:36:56Z

7

There is a difference between Java and JavaScript regex flavors: JS does not support lookbehind.

A tabulation of differences between regex flavors can be found on Wikipedia.

However, this does not apply to your case. I surmise that your JS test string has spurious characters, eg. cr/lf.

Try to use the regex without the anchors and check the lengths of the test strings.

answered Apr 21, 2016 at 16:36

collapsar

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forgivenson · Accepted Answer · 2016-04-21 16:49:01Z

Your regex works just as well in JavaScript as Java. Below is a short example using some data I made up.

var regex = /^([a-zA-Z]){1}([0-9][0-9]|[0-9]|[a-zA-Z][0-9][a-zA-Z]|[a-zA-Z][0-9][0-9]|[a-zA-Z][0-9]){1}([ ])([0-9][a-zA-z][a-zA-z]){1}$/;

var testData = [

  // valid
  'A11 1AA',
  'a1 1aa',
  'Aa1a 1AA',
  'aA11 1aa',
  'Aa1 1AA',

  // invalid
  'aa1aa 1aa',
  '@a11 1aa',
  'a1aa 1aa'
];

for (var i = 0; i < testData.length; i++) {
  if (testData[i].match(regex)) {
    console.log(testData[i] + ' is valid.');
  } else {
    console.log(testData[i] + ' is invalid.');
  }
}

Bonus tips:

You could simplify your regex some. The {1} is unnecessary, and you don't need to wrap the space in [].

 ^[a-zA-Z]([0-9][0-9]|[0-9]|[a-zA-Z][0-9][a-zA-Z]|[a-zA-Z][0-9][0-9]|[a-zA-Z][0-9]) [0-9][a-zA-z][a-zA-z]$

Of course, you can leave in any () that you want for capture groups, but I don't know if you are using those.

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