XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (649 page)

Within the

instruction, the contents of matched groups of characters can be referenced using the
regex-group()
function, described on page 860 in Chapter 13.

Back-References

A back-reference can be used to refer to a captured group from within the regular expression itself. A back-reference is written as a backslash followed by a positive number. Back-references are often used to match opening and closing quotes; for example, the regex
([’”]).*\1
matches the strings
“Hello”
and
‘Hello’
but not
“Hello’
.

A single digit following a
\
is always recognized as part of the back-reference; subsequent digits are recognized as part of the back-reference only if there are sufficiently many parenthesized subexpressions earlier in the regex. For example,
\15
is recognized as a back-reference only if there are at least 15 parenthesized subexpressions preceding it in the regular expression; if this is not the case, then it is interpreted as a back-reference
\1
followed by the digit
5
.