okay,  the limitation I was thinking of is that XML Schema's regex are
anchored (sort of sick so don't want to go find the spec and hunt up
the correct wording) and XSL-T not
necessarily

Unless the metacharacters ^ and $ are used as anchors, the string is considered to match the pattern if any substring matches the pattern. But if anchors are used, the anchors must match the start/end of the string (in string mode), or the start/end of a line (in multiline mode).

Note:
This is different from the behavior of patterns in [XML Schema Part 2: Datatypes Second Edition], where regular expressions are implicitly anchored.