Now, I need a very specific validation of a field called 'NIF', wich is the identifier of a tax-paying company in my country. And I need some help to make a regular expression to use like I did in the above code.

So I found an old javascript code online that validates 'NIF':

function IsValidNIF(nif){ var c; var checkDigit = 0; //Check if is not null, is numeric and if has 9 numbers if(nif != null && IsNumeric(nif) && nif.length == 9) { //Get the first number of NIF c = nif.charAt(0); //Check firt number is (1, 2, 5, 6, 8 or 9) if(c == ‘1' || c == ‘2' || c == ‘5' || c == ‘6' || c == ‘8' || c == ‘9') { //Perform CheckDigit calculations checkDigit = c * 9; var i = 0; for(i = 2; i <= 8; i++) { checkDigit += nif.charAt(i-1) * (10-i); } checkDigit = 11 - (checkDigit % 11); //if checkDigit is higher than ten set it to zero if(checkDigit >= 10) checkDigit = 0; //Compare checkDigit with the last number of NIF //If equal the NIF is Valid. if(checkDigit == nif.charAt(8)) return true; } } return false;}function IsNumeric(ObjVal){return /^\d+$/.test(ObjVal);}

So, is it possible to make a regular expression from this?

The rules are:

-must be only numbers
-must be 9 characters(numbers) long
-the first number must be 1 or 2 or 5 or 6 or 8 or 9
-the last number performs some validation(%module) that you should understand viewing the above code

-must be only numbers
-must be 9 characters(numbers) long
-the first number must be 1 or 2 or 5 or 6 or 8 or 9
characterReg = /^[125689][0-9]{8}$/

-the last number performs some validation(%module) that you should understand viewing the above code
This part would produce a more complicated regular expression than is worth it, so I'd stick to using code.

If possible let me know "how" complicated would the regex become. I don't mind if the regex is not "readable", the aproach of regex is just to cut some lines of code. Unless the regex could have some performance/other issues,. is that the case?

I have to agree with ozo: You're going to be attempting to reproduce modulo arithmetic via a regex. This isn't really what regex is designed for. The code approach would be much simpler. Just use ozo's suggested pattern, then add logic for the modulo check.

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Then how would I add up the 2 things (regex + code) in my form?

Should be like:

...//some codevar myRegex = /^[125689][0-9]{8}$/;if ( myRegex.test(inputValName) ){ // then implement other if here with the modulo% code??...//more code

but i a working with jquery here, I am more familiar with with. Don't know how to manipulate the DOM with pure js. Can I incorporate that function in my jquery?

function IsValidNIF(nif){ var checkDigit = 0; var i = 0; var characterReg = /^[125689][0-9]{8}$/; if (characterReg.test(nif) { //Perform CheckDigit calculations checkDigit = nif.charAt(0) * 9; for(i = 2; i <= 8; i++) { checkDigit += nif.charAt(i-1) * (10-i); } checkDigit = 11 - (checkDigit % 11); //if checkDigit is higher than ten set it to zero if(checkDigit >= 10) checkDigit = 0; //Compare checkDigit with the last number of NIF //If equal the NIF is Valid. if(checkDigit == nif.charAt(8)) return true; } return false;}

I found this questions asking how to do this in many different forums, so I will describe here how to implement a solution using PHP and AJAX.
The logical flow for the problem should be:
Write an event handler for the first drop down box to get …

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