The TEI Council has been considering this ticket:
in which it is proposed that the regular expression syntax defined for
the @matchPattern attribute in TEI, which is currently W3C XML Schema
syntax, be changed to the much preferable and more flexible XPath syntax
(which was not available when the attribute was originally created).
Before doing this, we would like to check whether anyone using this
attribute (probably not a large number of people) is using it in such a
way that this change would actually have an effect on them. We believe
this is most unlikely, because even the TEI Stylesheets are written in
such a way that they assume that the regex is XPath, but before we go
ahead and make the change we would like to hear from anyone who feels
they might be affected by it. If you use @matchPattern, could you let us
know how your regular expressions are written (e.g. do they assume
anchoring at both ends, as would be the case with XML Schema regexes),
and how do you process them (XPath in XSLT or XQuery, PERL, etc.)?
If none of the above makes any sense, then don't worry: it doesn't apply