XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (665 page)

The
i
flag

This flag causes the regular expression to use case-insensitive mode. In this mode, a letter used in the regular expression matches characters in the input string regardless of their case; for example, the regex
Monday
matches the strings
Monday
or
monday
or
MONDAY
. Without this flag, characters must match exactly. Note that collations are not used for regex comparisons.

Case-insensitive mode affects constructs in the regular expression as follows:

• When a character is used as an atom it represents that character and all its case variants. For example, the regex
  abc
  matches
  abc
  or
  ABC
  or
  Abc
  , among others.
  
• When a character is used as an
  XmlCharIncDash
  within square brackets, it represents that character and all its case variants. For example, the regex
  [ab]
  matches
  a
  ,
  A
  ,
  b
  , or
  B
  . This applies even if the character appears as part of a negative character group or a subtraction; for example,
  [
  ∧
  ab]
  matches any character except
  a
  ,
  A