I just found a neat trick to avoid duplicating code in XML:
Entities (as already defined at the top of most grammar.xml files) can also contain XML code, like this:
<!ENTITY date_numbers_pattern '
<pattern>
<token regexp="yes">\d\d?</token>
<token>.</token>
<token regexp="yes">\d\d?</token>
<token>.</token>
<token regexp="yes">\d\d\d\d</token>
</pattern>
'>
You would then use this later like this:
&date_numbers_pattern