This question is a combination of regex practice and unit testing practice.
Regex part
I authored this problem separateThousands for personal practice:
Given a number as a string, introduce commas to separate thousands. The number may contain an optional minus sign, and an optional decimal part. There will not be any superfluous leading zeroes.
Here's my solution:
String separateThousands(String s) {
return s.replaceAll(
String.format("(?:%s)|(?:%s)",
"(?<=\\G\\d{3})(?=\\d)",
"(?<=^-?\\d{1,3})(?=(?:\\d{3})+(?!\\d))"
),
","
);
}
The way it works is that it classifies two types of commas, the first, and the rest. In the above regex, the rest subpattern actually appears before the first. A match will always be zero-length, which will be replaceAll with ",".
The rest basically looks behind to see if there was a match followed by 3 digits, and looks ahead to see if there's a digit. It's some sort of a chain reaction mechanism triggered by the previous match.
The first basically looks behind for ^ anchor, followed by an optional minus sign, and between 1 to 3 digits. The rest of the string from that point must match triplets of digits, followed by a nondigit (which could either be $ or \.).
My question for this part is:
Can this regex be simplified?
Can it be optimized further?
Ordering rest before first is deliberate, since first is only needed once
No capturing group
Unit testing part
As I've mentioned, I'm the author of this problem, so I'm also the one responsible for coming up with testcases for them. Here they are:
INPUT, OUTPUT
"1000", "1,000"
"-12345", "-12,345"
"-1234567890.1234567890", "-1,234,567,890.1234567890"
"123.456", "123.456"
".666666", ".666666"
"0", "0"
"123456789", "123,456,789"
"1234.5678", "1,234.5678"
"-55555.55555", "-55,555.55555"
"0.123456789", "0.123456789"
"123456.789", "123,456.789"
I haven't had much experience with industrial-strength unit testing, so I'm wondering if others can comment whether this is a good coverage, whether I've missed anything important, etc (I can always add more tests if there's a scenario I've missed).
