Re^2: why such an error happened?

[lightoverhead]

Thank you Darren. But I am still confused. $1 is matched to "this" or "that", so they will not be upper cased. $2 matched to other words other than "this" or "that" and change them to upper cases, in this example, the "and" was changed to "AND". It seems to me that the "and" was assigned to $2, why you thought that $2 will always be undefined? Thanks.

[Lawliet]

Because of the | in the middle of the regex there will never be a time where both parenthesis are used. That is, there will only be one set of parenthesis total. Ergo, $2 will never be defined.

[lightoverhead]

Thank you for your posts. Now I begin to understand the warning messages. But I thought $1 or $2 will not be defined at the same time,i.e. for any moment, only either of them is defined, the other is undefined. We can say sometime $1 is undefined, sometime $2 is undefined; but I still don't think that $2 will never be defined, since in this example, "\U$2" indeed change "and" to "AND". Is my understanding right? Thanks.

[CountZero]

$1 and $2 are statically assigned when the regex is being compiled. At that time the regex-engine does not yet know which --if any-- of the parentheses will match. So $1 will refer to what is between the first set of parentheses (counting from the left); $2 refers to the second set (counting from the left); etc. ...

In an (nested) alternation pattern (|) only one of these will ever match and that causes the warning.

In your case you can safely ignore it.

[Lawliet]

Uhh, what?

I think you are confusing what $1 and $2 are. $1 represents what was matched in the first set of matched parenthesis. $2 represents what was matched in the second set of matched parenthesis. Seeing as there will never be two matched sets of parenthesis in your regex, $2 will never be defined.