When editing the Pattern match question type (in Moodle 2.2), the online Help "More Help" link leads to:
question/type/pmatch
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There is, however a http://docs.moodle.org/dev/The_OU_PMatch_algorithm page in the wiki, but it's very hard to find it, and it is not directly linked to the Pattern Match question type as it should.

Given the complexity of the Pattern match question type, a full documentation - with lots of real-world examples - would be most welcome.