Your subject line says &_#xA; (without the underscore) but the content says #xA; (with no ampersand). Perhaps that's a mailer corruption, or perhaps it's the bug you are looking for.
I'm not sure without detailed checking whether &_#xA; should work. I think in a regular expression it's safer to write it as \n.
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference