regex matching duplicates in a comma separated list

Question

I'm trying to regex match any duplicate words (i.e. alphanumeric and can have dashes) in some yaml with a PCRE tool.

(?<=,|^)([^,]*)(,\1)+(?=,|$)

it will catch:

hello-world,hello-world,goodbye-world,goodbye-world

but not the hello-worlds in

hello-world,goodbye-world,goodbye-world,hello-world

Could someone help me try to build a regex pattern for the second case (or both cases)?

anubhava · Accepted Answer · 2022-07-28T10:27:12.760

4

You may use this regex:

(?<=,|^)([^,]+)(?=(?>,[^,]*)*,\1(?>,|$)),

RegEx Details:

(?<=^|,): Assert that we have , or start position before current position
([^,]+): Match 1+ of non-comma text and capture in group #1
(?=(?>,[^,]*)*,\1(?>,|$)): Lookahead to assert presence of same value we captured in group #1 ahead of us
,: Match ,

edited Jul 28 '22 at 10:27

answered Jul 27 '22 at 18:37

anubhava

1

oh interesting! This is also a valid regex. @anubhava thank you for your explanation! :-) – torrho Jul 27 '22 at 18:53
Just for my understanding are you looking to match only `hello-world` and not match `goodbye-world` using this regex? – anubhava Jul 27 '22 at 19:05
the goal was to catch any duplicate. the regex i originally had only captured consecutive duplicates which was not sufficient. – torrho Jul 27 '22 at 21:48
1

@torrho: Thanks for confirming and I was also thinking same. But then I don't understand how you've selected `(?<=,|^)([^,]*)(?:,.*)?(,\1)(?=,|$)` as working answer which gives incorrect results i.e. showing only 1 match instead of 2. – anubhava Jul 28 '22 at 10:46
1

@anubhava the tool I am working with, semgrep, seems to work with the answer provided for the time being. I am still testing but I am beginning to think your original answer - sans the comma, might be _just_ a little more effective in what I am trying to do here. Should my testing change to your answer, i'll revise my "correct answer" selection. – torrho Jul 28 '22 at 16:17
1

@anubhava :-) I used your original regex as it has tested just a little bit better than the prior answer. Thank you Barmar for your answer as well. They both work for the purpose that I am trying to do however i think anubhava answer has cleaner test results in my tool. thank you! – torrho Jul 28 '22 at 23:30

score 2 · Answer 2 · answered Jul 27 '22 at 18:34

2

Put an optional ,.* between the capture group and the back-reference.

(?<=,|^)([^,]*)(?:,.*)?(,\1)(?=,|$)

answered Jul 27 '22 at 18:34

Barmar

2 Answers2