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1回顶部 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
/\*.*?\*/
2回顶部
小结：
?与/U有类似功能，但同时出现彼此抵消
如下：
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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正在阅读：关于php正则表达式的两点备注关于php正则表达式的两点备注
2004-02-14 09:33
出处：PConline
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