Add note regarding regular expression unicode flag requirement

Author: Relequestual

jsonschema-core.xml

@@ -547,6 +547,11 @@
                     These regular expressions SHOULD be valid according to the regular expression
                     dialect described in <xref target="ecma262">ECMA 262, section 21.2.1</xref>.
                 </t>
+                <t>
+                    Regular expressions SHOULD be built with the "u" flag (or equivilent) to provide
+                    Unicode support, or processed in such a way which provides which provides Unicode
+                    as defined by ECMA 262.
+                </t>
                 <t>
                     Furthermore, given the high disparity in regular expression constructs support,
                     schema authors SHOULD limit themselves to the following regular expression