Regex to extract last elements from a list

Question

I can have as input the following list: V1, V2, V3, V4, V5, V6, V7, V8, V9, V10

I would need a regex that extracts the last 3 elements from the list above. So after applying the regex the list should look like: V8, V9, V10

I have tried this regex /^.*(?=((,.*){3})$)/, it seems I'm getting close but I don't understand why when replacing with $1 I get the output duplicated.

Can anyone help me with an explanation? If I'm able to understand why this happens I should be able to correct it.

honestly, yes, a regex can do that, but it's very awkward doing with a regex, is there a particular reason this *must* me done with a regex, or would something simpler not do? — Marcus Müller, Dec 08 '21 at 10:40
Can you also post the replacement code, please? Are you using JavaScript? — Sree Kumar, Dec 08 '21 at 10:45

score 1 · Answer 1 · answered Dec 08 '21 at 10:55

The following regex should work

/^.*, ([^,]+, [^,]+, [^,]+)$/

^ Match the beginning of the string
.* Match 0 or more characters, taking as many as posible
, Match a comma followed by a space
( Begin capture
[^,]+ Match one or more not commas
This gets repeated so we can match the last 3 elements
) End capture
$ Match end of string

score 1 · Answer 2 · answered Dec 08 '21 at 10:56

You can simply extract the last three comma-separated values using

[^\s,][^,]*(?=(?:,[^,]*){0,2}$)

See the regex demo. Details:

[^\s,] - a char other than whitespace and comma
[^,]* - any zero or more chars other than a comma
(?=(?:,[^,]*){0,2}$) - a positive lookahead that requires zero, one or two occurrences of a comma and then zero or more non-comma chars till the end of string immediately to the right of the current location.

You get you match replaced twice because of a known issue, see Why do some regex engines match .* twice in a single input string?.

score 1 · Answer 3 · answered Dec 08 '21 at 11:00

1

For your case, you should use ?: instead of ?=, then you will get , V8, V9, V10

([a-zA-Z0-9]+(?:,\s[a-zA-Z0-9]+){2})$ will return the same without preceding comma V8, V9, V10

answered Dec 08 '21 at 11:00

morz79

11
2

score 0 · Answer 4 · answered Dec 08 '21 at 14:41

The captured part in your pattern has 3 comma's because the lookahead assertion repeats 3 times a pattern (,.*){3} starting with a comma

There is not language specified, but depending on the supported regex flavor, you could also use a single capture group for the last 3 values between 2 comma's:

^.*(?<!\S)([^,\n]*(?:,[^,\n]*){2})$

^ Start of string
.* Match the whole line
(?<!\S) Assert a whitespace boundary to the left
( Capture group 1
- [^,\n]* Optionally match any char except , or a newline
- (?:,[^,\n]*){2} Repeat 2 times matching the , and optional chars other than , or a newline
) Close group 1
$ End of string

Regex demo

Or as an alternative, split on , take the last 3 elements from the collection and join them back again with the same delimiter.

An example using JavaScript:

const str = "V1, V2, V3, V4, V5, V6, V7, V8, V9, V10";
console.log(str.split(", ").slice(-3).join(", "));

Regex to extract last elements from a list

4 Answers4