[chaley]

Originally Posted by Sweetpea

I've to ask, what does the "|$" do?

Before explaining, I note that the expression can be simplified to
{tags:re( *(.[^\,]?)[^\,]*?(\,|$),\1\2)}
The extra parentheses were left over from an experiment.

The expression (\,|$) means (comma OR end_of_string). It causes the previous [^\,]*? to stop matching, and consumes the comma if there is one. As a side benefit, because it is parenthesized, the matched value can be used in the replacement to separate the two-letter shortened tags by that value, which is either a comma or nothing.

As long as I am in explain mode , the explanation of the rest of the expression is:

re( *(.[^\,]?)[^\,]*?(\,|$),\1\2)
    | |    |     | |   |    |_ Replace with first paren group value, followed by second
    | |    |     | |   |_Match comma or end of tags string. In parens, so remember as \2
    | |    |     | |_ As many non-comma chars as needed
    | |    |     |_Any character not a comma
    | |    |_zero or one non-comma. Still in the parens, so remembered
    | |_ Any character. It is in parens, so remember it as \1
    |_eat any leading blanks. They would be there because commas are followed by blanks.