Perl 5.10 introduced a feature whereby each alternative in a
subpattern uses the same numbers for its capturing parentheses.
Such a subpattern starts with (?| and is itself a non-capturing
subpattern. For example, consider this pattern:
(?|(Sat)ur|(Sun))day
Because the two alternatives are inside a (?| group, both sets of
capturing parentheses are numbered one. Thus, when the pattern
matches, you can look at captured substring number one, whichever
alternative matched. This construct is useful when you want to
capture part, but not all, of one of a number of alternatives.
Inside a (?| group, parentheses are numbered as usual, but the
number is reset at the start of each branch. The numbers of any
capturing parentheses that follow the subpattern start after the
highest number used in any branch. The following example is taken
from the Perl documentation. The numbers underneath show in which
buffer the captured content will be stored.
# before ---------------branch-reset----------- after
/ ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
# 1 2 2 3 2 3 4
A back reference to a numbered subpattern uses the most recent
value that is set for that number by any subpattern. The
following pattern matches "abcabc" or "defdef":
/(?|(abc)|(def))\1/
In contrast, a subroutine call to a numbered subpattern always
refers to the first one in the pattern with the given number. The
following pattern matches "abcabc" or "defabc":
/(?|(abc)|(def))(?1)/
If a condition test for a subpattern's having matched refers to a
non-unique number, the test is true if any of the subpatterns of
that number have matched.
An alternative approach to using this "branch reset" feature is
to use duplicate named subpatterns, as described in the next
section.
