Please help with Regex script

[Factfinder]

I have a file with the following content:
 

<mokdjehspsksushshhhssssssss>
line
more lines
<subject>: "something"
line
more lines
<mnpmuksoelddldjdldjdlddjnyt>
line
more lines
<kopksuusssssssssssshhhhdsr>
line
more lines
<subject>: "something else"
line
more lines

I have a script that reads the random heading from another file then reads the above file to find a pattern that matches the random characters  then reads the string assigned to <subject>.
 
 This is my script:
 

$str = '<mokdjehspsksushshhhssssssss>'
$subject = StringRegExp(FileRead(@scriptdir & "\file"), $str & '.*\v*(?:.*\v*)+?\s*<subject>:\s*"(.*?)"', 1)
If IsArray($subject) Then
 MsgBox(0,"", $subject[0])
Else
 MsgBox(0,"", "No Subject")
EndIf

 
The issue with the script: When the sections with random characters have corresponding <subject> the script works correctly. But in case there is no corresponding <subject> (like <mnpmuksoelddldjdldjdlddjnyt>, the second section in above example), the script reads the <subject> listed in the following section instead of returning no match.
 
What I need is to match those random headings with the subject but stop matching at the next random heading. I have tried to use many combinations of (?![a-z]{20,}) before <subject> in the script with no success.
 
Thank in advance.

Edited January 23, 2014 by Factfinder

[jchd]

First, notice that StringRegExp doesn't read a file by itself: its first parameter is the subject string.

Now try this pattern which works for me:

(?s)<charactersequence>\R(?:.(?!<\w+>\R))*?<subject>:\s*"(.*?)"

[Factfinder]

Yes, I was creating an example and didn't include FileRead(), I edited my post.

Your script works great. I guess one of the challenges was using single line mode. Thanks a lot.