Two notes on php regular expressions_PHP Tutorial

severaltipsaboutRegularExpressions
　　1.processfor"greedy"
　　Bydefault,thequantifiersare"greedy",thatis,they
　　matchasmuchaspossible(uptothemaximumnumberofper-
　　mittedtimes),withoutcausingtherestofthepatternto
　　fail.Theclassicexampleofwherethisgivesproblemsisin
　　tryingtomatchcommentsinCprograms.Theseappearbetween
　　thesequences/*and*/andwithinthesequence,individual
　　*and/charactersmayappear.AnattempttomatchCcom-
　　mentsbyapplyingthepattern
　　
　　/*.**/
　　
　　tothestring
　　
　　/*firstcommand*/notcomment/*secondcomment*/
　　
　　fails,becauseitmatchestheentirestringduetothe
　　greedinessofthe.*item.
　　
　　However,ifaquantifierisfollowedbyaquestionmark,
　　thenitceasestobegreedy,andinsteadmatchestheminimum
　　numberoftimespossible,sothepattern
　　
　　/*.*?*/
　　
　　小结：
　　?与/U有类似功能，但同时出现彼此抵消
　　

　　如下：
　　
　　$a="asdf/*asdfaldsfasdf*/asfdasldf;kfldsj*/asfddsaf";
　　$pattern="//*.*?*//";
　　//$pattern="//*.**//U";
　　//$pattern="//*.*?*//U";
　　preg_match($pattern,$a,$match);
　　print_r($match);
　　?>
　　
　　2.Assertions
　　w+(?=;)
　　
　　matchesawordfollowedbyasemicolon,butdoesnotinclude
　　thesemicoloninthematch,and
　　
　　foo(?!bar)
　　
　　matchesanyoccurrenceof"foo"thatisnotfollowedby
　　"bar".Notethattheapparentlysimilarpattern
　　
　　小结：
　　(?!)只前向判断匹配，如bar(?!foo)，而(?!foo)bar没有意义
　　(?

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