It requires the following features: p:ixml.
The pipeline
<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" version="3.0">
<p:output port="result"/>
<p:invisible-xml>
<p:with-input port="grammar">
<p:inline content-type="text/plain">
date: s?, day, s, month, (s, year)? .
-s: -" "+ .
day: digit, digit? .
-digit: "0"; "1"; "2"; "3"; "4"; "5"; "6"; "7"; "8"; "9".
month: "January"; "February"; "March"; "April";
"May"; "June"; "July"; "August";
"September"; "October"; "November"; "December".
year: (digit, digit)?, digit, digit .
</p:inline>
</p:with-input>
<p:with-input port="source">
<p:inline content-type="text/plain">31 December 2021</p:inline>
</p:with-input>
</p:invisible-xml>
</p:declare-step>
Schematron validation
<s:schema xmlns="http://www.w3.org/1999/xhtml" xmlns:s="http://purl.oclc.org/dsdl/schematron" queryBinding="xslt2">
<s:pattern>
<s:rule context="/">
<s:assert test="date">The root element is not 'date'.</s:assert>
<s:assert test="date/day">The root element does not have a 'day' child.</s:assert>
<s:assert test="date/day/text()='31'">The value of 'day' is not '31'.</s:assert>
<s:assert test="date/month">The root element does not have a 'month' child.</s:assert>
<s:assert test="date/month/text()='December'">The value of 'month' is not 'December'.</s:assert>
<s:assert test="date/year">The root element does not have a 'year' child.</s:assert>
<s:assert test="date/year/text()='2021'">The value of 'year' is not '2021'.</s:assert>
</s:rule>
</s:pattern>
</s:schema>
