Awk with reg. exp. question

[Bob Palowoda]

  I'm have a problem understanding regular expression usage in 'awk'.
Maybe someone can help me out here. I might be useing the wrong awk,
I have tried nawk, xenix's awk, oawk, and gnu awk version 1.7.

Essentially the below script is a mail file field parser. 
What I don't understand is the expressions with '\t' in them.
I look in all kinds of references.    

Included is a scirpt of the error, and what I thing the line
is to be parsed. (For the FIELD) var.

---------------------------------------
This would be the line to parse, it is converted to upper case by
ucasep.
From: palowoda at fiver.UUCP (Bob Palowoda)
--------------------------------------------------------------
# This assumes that the field-names are all upper case
for f in $*
do
        FIELD=$FIELD"|"`echo $f | ucasep`
done

${awk} "
/^($FIELD)[ \t]*:/"' {
        if (current != "") print current;
        current = $0;
        next;
}

/^[^ \t:]+[ \t]*:/ {                        <-------- Here is where I think
        if (current != "") print current;             the problem is
        current = "";
}

/^$/ {
        if (current != "") print current;
        current = "";
        exit;
}

/^[ \t]/ {
        if (current != "")
                current = current " " $0;
}

END {
        if (current != "") print current;
}
--------------------------------------
And now for the script of the error.

awk: illegal primary in regular expression ^(|REPLY-TO|FROM|SENDER)[ \t]*: at REPLY-TO|FROM|SENDER)[ \t]*:
 source line number 2
 context is
        /^(|REPLY-TO|FROM|SENDER)[ >>>  \t]*:/ <<<  {
                                         ^
                                         |
                                         Is this the error?
-----------------------------------------------------------------

---Bob