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a discussion about creating more advanced types follows later in this chapter.
참고: to reduce complexity in the following source examples, we're only working with simple types such as integers at first. a discussion about creating more advanced types follows later in this chapter.
a description of how this works is given later, following the discussion of parenthesized subpatterns.
although lookbehind assertions must match fixed length strings, each alternative branch of a lookbehind assertion can match a different length of string. perl 5.005 requires them all to have the same length.
jeffrey friedl 's book contains a lot of discussion about optimizing regular expressions for efficient performance.
with both maximizing and minimizing repetition, failure of what follows normally causes the repeated item to be re- evaluated to see if a different number of repeats allows the rest of the pattern to match. sometimes it is useful to prevent this, either to change the nature of the match, or to cause it fail earlier than it otherwise might, when the author of the pattern knows there is no point in carrying on. consider, for example, the pattern \d+foo when applied to the subject line 123456bar after matching all 6 digits and then failing to match "foo", the normal action of the matcher is to try again with only 5 digits matching the \d+ item, and then with 4, and so on, before ultimately failing. once-only subpatterns provide the means for specifying that once a portion of the pattern has matched, it is not to be re-evaluated in this way, so the matcher would give up immediately on failing to match "foo" the first time. the notation is another kind of special parenthesis, starting with (? as in this example: (? \d+)bar this kind of parenthesis "locks up" the part of the pattern it contains once it has matched, and a failure further into the pattern is prevented from backtracking into it. back- tracking past it to previous items, however, works as normal. an alternative description is that a subpattern of this type matches the string of characters that an identical standalone pattern would match, if anchored at the current point in the subject string. once-only subpatterns are not capturing subpatterns. simple cases such as the above example can be thought of as a maximizing repeat that must swallow everything it can. so, while both \d+ and \d+? are prepared to adjust the number of digits they match in order to make the rest of the pattern match, (? \d+) can only match an entire sequence of digits. this construction can of course contain arbitrarily complicated subpatterns, and it can be nested. once-only subpatterns can be used in conjunction with look- behind assertions to specify efficient matching at the end of the subject string. consider a simple pattern such as abcd$ when applied to a long string which does not match. because matching proceeds from left to right, pcre will look for each "a" in the subject and then see if what follows matches the rest of the pattern. if the pattern is specified as ^.*abcd$ then the initial .* matches the entire string at first, but when this fails (because there is no following "a"), it backtracks to match all but the last character, then all but the last two characters, and so on. once again the search for "a" covers the entire string, from right to left, so we are no better off. however, if the pattern is written as ^(? .*)(? =abcd) then there can be no backtracking for the .* item; it can match only the entire string. the subsequent lookbehind assertion does a single test on the last four characters. if it fails, the match fails immediately. for long strings, this approach makes a significant difference to the processing time. when a pattern contains an unlimited repeat inside a subpattern that can itself be repeated an unlimited number of times, the use of a once-only subpattern is the only way to avoid some failing matches taking a very long time indeed. the pattern (\d+_bar_ \d+ )*[!?] matches an unlimited number of substrings that either consist of non-digits, or digits enclosed in, followed by either! or?. when it matches, it runs quickly. however, if it is applied to aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa it takes a long time before reporting failure. this is because the string can be divided between the two repeats in a large number of ways, and all have to be tried. (the example used [!?] rather than a single character at the end, because both pcre and perl have an optimization that allows for fast failure when a single character is used. they remember the last single character that is required for a match, and fail early if it is not present in the string.) if the pattern is changed to ((? \d+)_bar_ \d+ )*[!?] sequences of non-digits cannot be broken, and failure happens quickly.
this document specifies an internet standards track protocol for the internet community, and requests discussion and suggestions for improvements.
이 문서는 인터넷 커뮤니티 및 요청 토론을 위한 인터넷 표준 트랙 프로토콜 및 개선 위한 제안 지정합니다.
最后更新： 2017-06-15
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choose a username and password for synchronising opera. this account can also be used to participate in forums and create a weblog or photo album in the my opera community.
opera 동기화에 사용할 사용자 이름과 비밀번호를 선택하십시오. 이 계정을 사용해 포럼에 참여하고 내 opera 커뮤니티에서 웹 블로그나 포토 앨범을 만들 수도 있습니다.
最后更新： 2016-12-17
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for a more complete reference and discussion see applied cryptography by schneier (isbn 0-471-11709-9).
cbc (cipher block chaining) is especially suitable for encrypting files where the security is increased over ecb significantly.
最后更新： 2011-10-24
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his companion replied, in the course of their discussion, "do you deny him who created you from dust, from a small drop of fluid, then formed you into a man?
이때 그의 형제가 대항하여 말하더라 흙에서 너를 창조한 후 정액으로 완전한 인간이 되도록 하여 주신 그분을 불신하느뇨
最后更新： 2014-07-03
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警告：包含不可见的HTML格式