Regex to allow a comma seperated list of codes

Question

I have an input form which I need to validate, the list must follow these rules

• comma separated
• each code can either
  • begin with a single letter followed by a single underscore only, followed by any number of letters or
  • a group of numbers
• the list must not end with a trailing comma

Valid example data

• A_AAAAA,B_BBBBB,122334,D_DFDFDF
• 12345,123567,123456,A_BBBBB,C_DDDDD,1234567

Invalid example data

• RR_RRR,12345
• 1_111,AVSFFF,
• A_SDDF,,123342

I am using http://www.regexr.com and have got as far as this: [A-Z_]_[A-Z],|[0-9],

The problem with this is the last code in each valid data example is not selected so the line does not pass the regex pattern

Answer 1

Try this -

^(?:[A-Z]_[A-Z]*|[0-9]+)(?:,(?:[A-Z]_[A-Z]*|[0-9]+))*$

Demo

Answer 2

Try this:

^(?:(?:[A-Za-z]_[A-Za-z]*|\d+)(?:,|$))+(?<!,)$

regex101 demo.

---

Explanation:

^  start of string
(?:  this group matches a single element in the list:
    (?:
        [A-Za-z]  a character
        _         underscore
        [A-Za-z]* any number of characters (including 0)
    | or
        \d+  digits
    )
    (?: followed by either a comma
        ,
    |  or the end of the string
        $
    )
)+  match any number of list elements
(?<!  make sure there's no trailing comma
    ,
)
$  end of string