Question

This is from the PHP manual regarding PCRE conditional subpatterns:

The two possible forms of conditional subpattern are:
(?(condition)yes-pattern)
(?(condition)yes-pattern|no-pattern)

That’s OK as long as the condition is a digit or an assertion. But I don’t quite understand the following

If the condition is the string (R), it is satisfied if a recursive
call to the pattern or subpattern has been made. At “top level”, the
condition is false. (…) If the condition is not a sequence of digits
or (R), it must be an assertion.

I would be grateful if someone could explain on an example what (R) is in conditional subpattern and how to use it. Thanks in advance.

Answer 1

As an additional and clearer answer…

2 days ago I was writing a pattern to match an IPv4 address and I found myself using the recursion in condition so I thought I should share (because it makes more sense than imaginative examples).

~
(?:(?:f|ht)tps?://)?            # possibly a protocol
(
    (?(R)\.)                    # if it\'s a recursion, require a dot
    (?:                         # this part basically looks for 0-255
        2(?:[0-4]\d|5[0-5])
        | 1\d\d
        | \d\d?
    )
)(?1){3}                        # go into recursion 3 times
                                # for clarity I\'m not including the remaining part
~xi