Document regular expression APIs in REST API Guide

The regular expression APIs are not documented. Not sure why that is, since any content search policy will require a regular expression (that may not be provided by Discover product).

The following do actually work despite not being documented:
/policyengine/v1/regex: POST

Registers a regular expression.

Synopsis of the request URL
curl -k -H "Authorization: Bearer ${TOKEN}" https://$SDHOST/policyengine/v1/regex -d @regexp.json -H "Content-type:application/json" -X POST
with, the following as an example of the JSON file (regexp.json):
{ "regex_id": "US-SSN-blank-delimited", "pattern": "\\b\\d{3}\\s\\d{2}\\s\\d{4}\\b", "description": "US SSN delimited by white space, not dashes." }
policyengine/v1/regex: GET

Returns list of all regular expressions registered.

Synopsis of the request URL

curl -k -H "Authorization: Bearer ${TOKEN}" https://$SDHOST/policyengine/v1/regex -X GET
policyengine/v1/regex/<regular expression name>: GET

Returns the registered definition of a specific regular expression.

Synopsis of the request URL

curl -k -H "Authorization: Bearer ${TOKEN}" https://$SDHOST/policyengine/v1/regex/$REGEXNAME -H "Content-type:application/json" -X GET

Example output for regular expression with name 'US-SSN'

{
"regex_id": "US-SSN",
"pattern": "\\b\\d{3}-\\d{2}-\\d{4}\\b",
"description": "Matching United States Social Security Numbers (SSN) like: 513-84-7329"
}