Regex matching comma-separated list of values, trailing comma not allowed

Question

I want to match:

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

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

(\w{3},?)+

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

(\w{3})(,\w{3})*

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?

Answer 1

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

(?<big>[a-zA-Z0-9]+)

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

So, repeats (?&name) become easy:

^(?<big>[a-zA-Z0-9]+)(,(?&big))*$

Test it Online

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

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

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}$'

Answer 2

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.

Answer 3

Please use - ^((([a-zA-Z0-9]){1,45},){2}([a-zA-Z0-9]){1,45})$

Here, I have set max word size to 45, as longest word in English is 45 characters, can be changed as per requirement

Answer 4

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

Thanks to @Hkoof for their comment!