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http://bugs.exim.org/show_bug.cgi?id=1251
Philip Hazel <ph10@???> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |RESOLVED
Resolution| |INVALID
--- Comment #3 from Philip Hazel <ph10@???> 2012-05-26 18:06:06 ---
This is not a bug, but an example of how PCRE differs from Perl, which does
match. This is an extract from the pcrepattern man page:
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Recursion processing in PCRE differs from Perl in two important ways. In PCRE
(like Python, but unlike Perl), a recursive subpattern call is always treated
as an atomic group. That is, once it 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.
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In your pattern, after it has matched '::', the fragment (?>(?2):)? is called.
Group 2 is ((?1)(?>:(?1)){0,4}) and group 1 matches a sequence of hex digits.
Thus, group 2 matches '0', then ':255' (that is, it matches the inner
subpattern once for the {0,4} repeat). However, as the next character is not a
colon, the whole of (?>(?2):) then fails to match. In Perl, there is can be a
recurse back into group 2 which can match (?>:(?1)) 0 instead of 1 times, but
in PCRE this does not happen, and so the whole match fails.
Changing the final ':' to '!' alters the entire behaviour, because group 2 then
matches ?>:(?1)) zero times right away.
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