關於php正則表達式的兩點備註

severaltipsaboutRegularExpressions
  1.processfor”greedy”
  Bydefault,thequantifiersare”greedy”,thatis,they
  matchasmuchaspossible(uptothemaximumnumberofper-
  mittedtimes),withoutcausingtherestofthepatternto
  fail.Theclassicexampleofwherethisgivesproblemsisin
  tryingtomatchcommentsinCprograms.Theseappearbetween
  thesequences/*and*/andwithinthesequence,inpidual
  *and/charactersmayappear.AnattempttomatchCcom-
  mentsbyapplyingthepattern
  
  /*.**/
  
  tothestring
  
  /*firstcommand*/notcomment/*secondcomment*/
  
  fails,becauseitmatchestheentirestringduetothe
  greedinessofthe.*item.
  
  However,ifaquantifierisfollowedbyaquestionmark,
  thenitceasestobegreedy,andinsteadmatchestheminimum
  numberoftimespossible,sothepattern
  
  /*.*?*/
  
  小結:
  ?與/U有類似功能,但同時出現彼此抵消
  

  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
  
  小結:
  (?!)隻前向判斷匹配,如bar(?!foo),而(?!foo)bar沒有意義
  (?<!)隻後向判斷匹配,如(?<!foo)bar,而foo(?<!bar)沒有意義                    

You May Also Like