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.

I have found a consecutive, duplicate regex matcher:

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

Answer 1

You may use this regex:

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

RegEx Demo

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 ,