How to get first Reference from a Mail header?

Question

One of the fields that may appear in an email message header is "References." I need to get the first item (which is a Message ID) in that field. However, when I try the following script for a message that has one or more IDs in References:
<pre style="font-family: 'Monaco', 'Courier New', Courier, monospace; overflow:auto; color: #222; background: #DDD; padding: 0.2em; font-size: 10px; width:800px">using terms from application "Mail"
on perform mail action with messages theMessages
repeat with eachMessage in theMessages
set theFirstRef to item 1 of content of header "References" of eachMessage</pre>
I get: "Mail got an error: Can’t make item 1 of content of header \"References\" of message id 17946 of mailbox \"INBOX\" of account \"David\" into type reference."
If I omit "item 1 of", I get the complete References field. So I've tried:
<pre style="font-family: 'Monaco', 'Courier New', Courier, monospace; overflow:auto; color: #222; background: #DDD; padding: 0.2em; font-size: 10px; width:800px"> set theRefs to content of header "References" of eachMessage
set theFirstRef to item 1 of theRefs</pre>
but that only gets me the first character (or subsequent characters if I use a value > 1), which is an open-angle. (BTW, I don't understand why breaking the action into 2 steps like this doesn't produce the same error as the first example.) I've also tried:
<pre style="font-family: 'Monaco', 'Courier New', Courier, monospace; overflow:auto; color: #222; background: #DDD; padding: 0.2em; font-size: 10px; width:800px"> set theRefs to (content of header "References" of eachMessage)
set theFirstRef to word 2 of theRefs</pre>
This skips the first open-angle in the References field and gets me all of the string up to the first delimiter. Unfortunately, the number of delimiters that may appear in a Message ID varies, so I can't simply grab a predetermined set and concatenate them. Furthermore, that would remove the delimiters, so the resulting Message ID in theFirstRef would be incorrect.
I need a syntax that gets me everything between the first set of open- and close-angles but haven't been able to determine what that is.

Answer 1

Thanks to your pointer, I found "text item delimiters" discussed on pp. 39-40 of the AppleScript Language Guide. A typical References field on my system for the last message in a somewhat long thread looks like this:
<[email protected]> <[email protected]> <[email protected]> <[email protected]> <[email protected]> <[email protected]>
So the message IDs are delimited by spaces and by open- and close-angles. Thus, if I use:
<pre style="font-family: 'Monaco', 'Courier New', Courier, monospace; overflow:auto; color: #222; background: #DDD; padding: 0.2em; font-size: 10px; width:800px"> set {savedDelims, my text item delimiters} to {my text item delimiters, " "}
repeat with eachMessage in theMessages
set theRefs to content of header "References" of eachMessage
set theFirstRef to text item 1 of theRefs</pre>
as you suggested, I get "<[email protected]>". That's close to what I want but includes the open and close angles in the returned string. If, however, I specify <pre style="font-family: 'Monaco', 'Courier New', Courier, monospace; overflow:auto; color: #222; background: #DDD; padding: 0.2em; font-size: 10px; width:30px">"<> "</pre> as the delimiters, I get the entire field, i.e., no parsing occurs. I've tried variations and found, for example, that <pre style="font-family: 'Monaco', 'Courier New', Courier, monospace; overflow:auto; color: #222; background: #DDD; padding: 0.2em; font-size: 10px; width:20px">"> "</pre>returns the first string without the close angle, whereas <pre style="font-family: 'Monaco', 'Courier New', Courier, monospace; overflow:auto; color: #222; background: #DDD; padding: 0.2em; font-size: 10px; width:20px">" >"</pre>(i.e., using the opposite order) returns the entire field -- don't understand why the order matters -- but haven't found the magic.
I know < and > are AppleScript operators (what's more, I've had to take special steps to show the quoted strings properly here) and am wondering whether they're a special case. I've tried using backslash, but that didn't help, either.