A helpful comparison chart is here: http://www.regular-expressions.info/refflavors.html
The lack of opening and closing anchors would probably be the most trouble-making difference.
I suggest the current language be changed to:
@matchPattern should use only common-denominator features widely available in regular expression processors.
Unfortunately, there is no spec for a common-denominator subset. If it's felt the TEI spec must cite some standard and preferably one in the XML family, cite XPath 2.0. Most of the unique features it has (such as its Unicode support) are unlikely to be used on datapointers.