Neither the XSD nor the XPath regex syntax permits \x. If Xerces accepts it, then it's a non-conformant extension. You'll probably find it works in Saxon if you use the "j" flag, which is also a non-conformant extension - it switches from using the XSD regex syntax to the Java regex syntax.

The conformant way to write this in XSD is ` ` (but don't use this with the -x flag)