XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (667 page)

• Back-references are matched using case-blind comparison; for example,
  ([A-Z]).*\1
  matches
  Ohio
  and
  Atlanta
  as well as
  OHIO
  ,
  ohio
  ,
  ATLANTA
  , and
  atlanta
  .
  

In most cases it's fairly obvious what counts as a
case variant
of a character. Officially it's defined in terms of the
upper-case()
and
lower-case()
functions described in Chapter 13:
$C
is a case variant of
$D
if
upper-case($C)
equals
upper-case($D)
, or
lower-case($C)
equals
lower-case($D)
, or both. Occasionally, this gives slightly unexpected results; for example, the letter
I
has four case variants:
I
,
i
,
, and
ı
. This oddity arises because some languages (English) drop the dot over the
i
when translating from lower case to upper case, whereas other languages (Turkish) keep it.