I have a String containing the following:
\"
Assuming you are passing the string to the XSLT stylesheet as a parameter, and processing a dummy XML document, you have two options:
Output the string with output escaping disabled, save the result as a new document, then process the new document with another XSLT stylesheet;
Use XSLT 3.0 (or a processor that supports parsing XML as an extension) to parse the string as XML before processing it.
Of course, you could save yourself all this trouble by writing the string to a file using your parent process, then pointing your XSLT processor to the resulting file.