如何在.net中屏蔽信用卡号的前6位和后4位

Question

I'm very new to regex And I'm trying to use a regular expression to turn a credit card number which will be part of a conversation into something like 492900******2222

As it can come from any conversation it might contain string next to it or might have an inconsistent format, so essentially all of the below should be formatted to the example above:

• hello my number is492900001111222
• number is 4929000011112222ok?
• 4929 0000 1111 2222
• 4929-0000-1111-2222

It needs to be a regular expression which extracts the capture group of which I will then be able to use a MatchEvaluator to turn all digits (excluding non digits) which are not the first 6 and last 4 into a *

I've seen many examples here on stack overflow for PHP and JS but none which helps me resolve this issue.

Any guidance will be appreciated

UPDATE

I need to expand upon an existing implementation which uses MatchEvaluator to mask each character that is not the first 6 or last 4 and ideally I dont want to change the MatchEvaluator and just make the masking flexible based on the regular expression, see this for an example https://dotnetfiddle.net/J2LCo0

UPDATE 2

@Matt.G and @CAustin answers do resolve what I asked for but I am hitting another barrier where I cant have it be so strict. The final captured group needs to only take into account the digits and as such maintain the format of the input text. So for example:

If some types in my card number is 99 9988 8877776666 the output from the evaluation should be 99 9988 ******666666

OR my card number is 9999-8888-7777-6666 it should output 9999-88**-****-6666.

Is this possible?

Changed the list to include items that are in my unit tests https://dotnetfiddle.net/tU6mxQ

Answer 1

Try Regex: (?<=\\d{4}\\d{2})\\d{2}\\d{4}(?=\\d{4})|(?<=\\d{4}( |-)\\d{2})\\d{2}\\1\\d{4}(?=\\1\\d{4})

Regex Demo

C# Demo

Explanation:

2 alternative regexes
(?<=\d{4}\d{2})\d{2}\d{4}(?=\d{4}) - to handle cardnumbers without any separators (- or <space>)
(?<=\d{4}( |-)\d{2})\d{2}\1\d{4}(?=\1\d{4}) - to handle cardnumbers with separator (- or <space>)

1st Alternative (?<=\d{4}\d{2})\d{2}\d{4}(?=\d{4})
    Positive Lookbehind (?<=\d{4}\d{2}) - matches text that has 6 digits immediately behind it
    \d{2} matches a digit (equal to [0-9])
        {2} Quantifier — Matches exactly 2 times
    \d{4} matches a digit (equal to [0-9])
        {4} Quantifier — Matches exactly 4 times
    Positive Lookahead (?=\d{4}) - matches text that is followed immediately by 4 digits
        Assert that the Regex below matches
            \d{4} matches a digit (equal to [0-9])
            {4} Quantifier — Matches exactly 4 times

2nd Alternative (?<=\d{4}( |-)\d{2})\d{2}\1\d{4}(?=\1\d{4})

    Positive Lookbehind (?<=\d{4}( |-)\d{2}) - matches text that has (4 digits followed by a separator followed by 2 digits) immediately behind it
    1st Capturing Group ( |-) - get the separator as a capturing group, this is to check the next occurence of the separator using \1
    \1 matches the same text as most recently matched by the 1st capturing group (separator, in this case)
    Positive Lookahead (?=\1\d{4}) - matches text that is followed by separator and 4 digits

Answer 2

If performance is a concern, here's a pattern that only goes through 94 steps, instead of the other answer's 473, by avoiding lookaround and alternation:

\\d{4}[ -]?\\d{2}\\K\\d{2}[ -]?\\d{4}

Demo: https://regex101.com/r/0XMluq/4

Edit: In C#'s regex flavor, the following pattern can be used instead, since C# allows variable length lookbehind.

(?<=\\d{4}[ -]?\\d{2})\\d{2}[ -]?\\d{4}

Demo