I want to match:

  1. aaa
  2. aaa,bbb
  3. aaa,bbb,ccc

But not a list with a trailing comma. My current regex:


also matches lists with trailing commas (aaa,bbb,). I was thinking I could also do:


but that is rather ugly. My real regex is not matching 3-letter-words, but something bigger, and repeating the regex is ugly. How can this be fixed?

  • 1
    Do you need tthe capturing group? – Panki Jun 13 '19 at 8:00
  • 1
    Is it full line matching? – dedowsdi Jun 13 '19 at 8:29
  • What you have is a decent solution to the problem. If you have a much bigger problem, state that – Inian Jun 13 '19 at 8:31
  • Capturing group is not needed. Yes full line matching. And repeating the BIG regex twice does not sound decent ;) – Karlo Jun 13 '19 at 8:47
  • 1
    I think a regex as simple as: [^,]$ answers the question as posted. If it doesn't what information would explain why it's not sufficient? – Hkoof Jun 13 '19 at 10:59

You can give a name to your big regex in PCRE like:


Everything after the ?<name> will be recorded with the name given.
Called Regular Expression Subroutines

So, repeats (?&name) become easy:


Test it Online

So, matching an IP, for example, becomes simpler:


Test it OnLine.

Use it with grep as:

grep -P '^(?<ip>25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)(.(?&ip)){3}$'

The regex as simple as: [^,]$ answers the question.

Thanks to @Hkoof for their comment!

  • It answers the question in that it matches on those 3 samples and not on anything that ends in , but it would also match anything that ends in a character other than , like aaaa or a,,,,b... What about your big regexp? – Stéphane Chazelas Dec 3 '19 at 21:15

If using perl or PCRE regexps, you can avoid repetition by using things like (?1) to refer to the regexp in the first capture group:

grep -Px '(?:(\w{3}),)*(?1)'

Which would match on any non-empty comma separated list of 3 character words. Change to:

grep -Px '(?:(?:(\w{3}),)*(?1))?'

To allow an empty list.

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