下面图老师小编要向大家介绍下关于php正则表达式的两点备注，看起来复杂实则是简单的，掌握好技巧就OK，喜欢就赶紧收藏起来吧！
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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*/
（本文来源于图老师网站，更多请访问https://www.tulaoshi.com/webkaifa/）fails,becauseitmatchestheentirestringduetothe
greedinessofthe.*item.
However,ifaquantifierisfollowedbyaquestionmark,
（本文来源于图老师网站，更多请访问https://www.tulaoshi.com/webkaifa/）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
(？
来源:https://www.tulaoshi.com/n/20160219/1613185.html
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