Restore "regex" to the list of formats.

Addresses json-schema-org#392.  It's come up several times that removing
this format did not make sense and was quite possibly an accident.

The "regex" format was actually still in use in the meta-schema
for "pattern".  I've added it to "patternProperties" as well
using "propertyNames".

Regular expressions are a fundamental programming tool, and
play a part in JSON Schema's specification.  It seems incorrect
to not offer semantic documentation / optional validation of them.

Author: handrews

jsonschema-validation.xml

@@ -896,6 +896,15 @@
                         <xref target="RFC6901">RFC 6901, section 5</xref>
                     </t>
                 </section>
+                <section title="regex">
+                    <t>
+                        This attribute applies to string instances.
+                    </t>
+                    <t>
+                        A regular expression, which SHOULD be valid according to the
+                        <xref target="ecma262">ECMA 262</xref> regular expression dialect.
+                    </t>
+                </section>
             </section>
         </section>
 

schema.json

@@ -118,6 +118,7 @@
         "patternProperties": {
             "type": "object",
             "additionalProperties": { "$ref": "#" },
+            "propertyNames": { "format": "regex" },
             "default": {}
         },
         "dependencies": {