XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (124 page)

, and
1987
. The subexpression that starts with the
n
th left parenthesis in the regular expression delivers the
n
th captured group in the result.

Some parenthesized subexpressions might not match any part of the string. For example, if the regex
([0-9]+)|([A-Z]+)
is used to match the string
12
, the first captured subgroup will be
12
and the second will be empty.

A parenthesized subexpression might also match more than one substring. For example, if the regex
([0-9]+)(,[0-9]+)*
is used to match the string
12,13,14
, then the second part in parentheses matches both
,13
and
,14
. In this case only the last one is captured. The first captured group in this example will be
12
, and the second will be
,14
.