Some help understanding this filter please

This was taken from /usr/share/doc/bogofilter/examples/contrib
procmailrc.example.

## Silently drop all completely unreadable mail
:0:
* 1^0 ^\/Subject:.*=\?(.*big5|iso-2022-jp|ISO-2022-KR|euc-kr|gb2312|ks_c_5601-1987|windows-1251|windows-1256)\?
* 1^0 ^\/Content-Type:.*charset="(.*big5|iso-2022-jp|ISO-2022-KR|euc-kr|gb2312|ks_c_5601-1987|windows-1251|windows-1256)
spam-unreadable.in

It goes well beyond my meager grasp how procmail works.  But first I
don't imagine there is much chance that flawed recipe got this far so
before getting into the mechanics of what this does. In second
(Content-Type) line at:

 * 1^0 ^\/Content-Type:.*charset="(  <== That double quote seems to have

no matching mate?

The only reason I even noticed it is that when I pasted it into my
.procmailrc file it threw the syntax highlighting off from that point to
the end of the file.  And I cannot find another matching double quote
anywhere in the 4 working lines of the recipe.

I'm pretty sure the people that put these examples together didn't
just make a mistake but I wondered if someone can explain how those
lines work?
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