--delimited (-d)
In normal mode any chain of 16 or 19 digits are considered no matter the position inside the line, so card numbers embedded inside numeric strings can be detected.
By using this flag, we will discard those numbers that are included inside other numeric strings, only considering numbers delimted by non digit characters.
For example, if we have this value in a file:
98765451123456789012345678
Withour the flag, we can consider the value of length 16 (passes the Luhn check) as valid 98765451123456789012345678 so it is returned as a valid candidate
But with this flag present, then we will discard it because the characters before (5) and after (4) the number are digits. We will only consider it as a valid number if, for example, the contents of the line would be something like this:
98765 4511234567890123.45678