I want to search a string and mask the a credit card for security.

"Hi here is my card number its visa 4242 4242 4242 4242"
"Hi here is my card number its visa 4242424242424242"
"Hi here is my card number its visa 4242-4242-4242-4242"

Should be converted to:

"Hi here is my card number its visa **** **** **** 4242"

I need to do this on the client in Javascript. I know there are quite a few resources and questions on the web and SO.

I have found two regex, but both throw errors: "Uncaught SyntaxError: Invalid or unexpected token"

"Hi here is my card number its visa 4242 4242 4242 4242".match("(\d{4}-?){4}")

and

"Hi here is my card number its visa 4242 4242 4242 4242".match(\b4\d{3}[ -]?\d{4}[ -]?\d{4}[ -]?\d{4}[ -]\b)

I think the expressions are not patible with JS?

I also understand they will return the string, I would then convert the string (mask it) and then use a simple replace on the original string.

Can anybody help me with the regex part of this problem?

I want to search a string and mask the a credit card for security.

"Hi here is my card number its visa 4242 4242 4242 4242"
"Hi here is my card number its visa 4242424242424242"
"Hi here is my card number its visa 4242-4242-4242-4242"

Should be converted to:

"Hi here is my card number its visa **** **** **** 4242"

I need to do this on the client in Javascript. I know there are quite a few resources and questions on the web and SO.

I have found two regex, but both throw errors: "Uncaught SyntaxError: Invalid or unexpected token"

"Hi here is my card number its visa 4242 4242 4242 4242".match("(\d{4}-?){4}")

and

"Hi here is my card number its visa 4242 4242 4242 4242".match(\b4\d{3}[ -]?\d{4}[ -]?\d{4}[ -]?\d{4}[ -]\b)

I think the expressions are not patible with JS?

I also understand they will return the string, I would then convert the string (mask it) and then use a simple replace on the original string.

Can anybody help me with the regex part of this problem?

• javascript
• regex

Share Improve this question Follow edited Feb 2, 2018 at 16:44 MartinWebb asked Feb 2, 2018 at 16:17 MartinWebbMartinWebb 2,00811 gold badge1414 silver badges1616 bronze badges 10

• 1 You should have a look at the syntax of regular expressions in javascript: developer.mozilla/en-US/docs/Web/JavaScript/Guide/… – Denys Séguret Commented Feb 2, 2018 at 16:18
• regex101. – Neoares Commented Feb 2, 2018 at 16:24
• Can someone explain why the Questions is down-voted. It seems to be harder and harder to ask anything on SO, can someone please explain where the question fails to meet approval? – MartinWebb Commented Feb 2, 2018 at 16:31
• @MartinWebb I'm not sure either. The original issue you're having though is that \d is not valid. You need to escape \. The second example would work other than you need to wrap your pattern in /. Also [ -]\b won't match anything in your case since \b matches positions between word characters and non-word characters (or start/end of line). I honestly think your question was downvoted for either 1. Not having used markdown (I edited your question to make it more legible) and/or 2. Because people assume everyone should know that you need to escape \ or delimit a regex pattern – ctwheels Commented Feb 2, 2018 at 16:40
• 1 @KyleFairns I'll agree with you on that one. Wanna see a neat one? stackoverflow./questions/30791292/… 68 downvotes. – ctwheels Commented Feb 2, 2018 at 16:42

 |  Show 5 more ments

2 Answers 2

Sorted by: Reset to default 12

See regex in use here

\b(?:\d{4}[ -]?){3}(?=\d{4}\b)

• \b Assert position as a word boundary. You can remove this if you want to catch cases such as My visa is1234567812345678
• (?:\d{4}[ -]?){3} Match the following exactly 3 times
  • \d{4} Match any digit 4 times
  • [ -]? Optionally match a space or hyphen
• (?=\d{4}\b) Positive lookahead ensuring what follows is 4 digits (and ensuring what follows it is not a word character). Similar to my first point, catching cards numbers that are followed by words such as My visa is 1234567812345678please use it carefully, use (?=\d{4}(?!\d)) instead.

const r = /\b(?:\d{4}[ -]?){3}(?=\d{4}\b)/gm
const a = [
  "Hi here is my card number its visa 4242 4242 4242 4242",
  "Hi here is my card number its visa 4242424242424242",
  "Hi here is my card number its visa 4242-4242-4242-4242"
]
const subst = `**** **** **** `

a.forEach(function(s) {
  console.log(s.replace(r, subst))
})

the regexp can be passed as a string ".." or regexp object /../. (when passed as string it is converted to RegExp object like with new RegExp("..")).

in first case the backslash must be doubled because of their special meaning between double quotes.

 "Hi here is my card number its visa 4242 4242 4242 4242".match("\\b(\\d{4}[- ]?){4}")

or

 "Hi here is my card number its visa 4242 4242 4242 4242".match(/\b(\d{4}[- ]?){4}/)