If the syntax for a recursive subpattern call (either by number
or by name) is used outside the parentheses to which it refers,
it operates like a subroutine in a programming language. The
called subpattern may be defined before or after the reference. A
numbered reference can be absolute or relative, as in these
examples:
(...(absolute)...)...(?2)...
(...(relative)...)...(?-1)...
(...(?+1)...(relative)...
An earlier example pointed out that the pattern
(sens|respons)e and \1ibility
matches "sense and sensibility" and "response and
responsibility", but not "sense and responsibility". If instead
the pattern
(sens|respons)e and (?1)ibility
is used, it does match "sense and responsibility" as well as the
other two strings. Another example is given in the discussion of
DEFINE above.
All subroutine calls, whether recursive or not, are always
treated as atomic groups. That is, once a subroutine has matched
some of the subject string, it is never re-entered, even if it
contains untried alternatives and there is a subsequent matching
failure. Any capturing parentheses that are set during the
subroutine call revert to their previous values afterwards.
Processing options such as case-independence are fixed when a
subpattern is defined, so if it is used as a subroutine, such
options cannot be changed for different calls. For example,
consider this pattern:
(abc)(?i:(?-1))
It matches "abcabc". It does not match "abcABC" because the
change of processing option does not affect the called
subpattern.
