正则表达式-信用卡验证

Question

I'm looking for a way to validate the beginning of a credit card pattern. So for example, let's take MasterCard.

It says that (ref: https://www.regular-expressions.info/creditcard.html ):

MasterCard numbers either start with the numbers 51 through 55...

I'm looking for a regex that returns true when the user enters:

 const regex = /^5|5[1-5]/; // this is not working :( regex.test("5"); // true regex.test("51"); // true regex.test("55"); // true regex.test("50"); // should be false, but return true because it matches the `5` at the beginning :( 

Answer 1

It should be:

const regex = /^5[1-5]/;

Your regex matches either a string beginning with 5 or a string that has 51 through 55 anywhere in it, because the ^ is only on the left side of the | .

If you want to allow a partial entry, you can use:

const regex = /^5(?:$|[1-5])/;

See Validating credit card format using regular expressions? for a regular expression that matches most popular cards.

Answer 2

Are you validating as the user types in? If so, you could add an end of line ($) to the first option, so that it returns true only if:

• 5 is the only character typed so far
• The string begins with 50-55

---

const regex = /^(5$|5[1-5])/;

regex.test("5"); // true
regex.test("51"); // true
regex.test("55"); // true
regex.test("50"); // false