第一章Perl 概述

如此，便可以直接執行這個程式，此程式的第一行便會告訴作業系統到哪裡去找到執行這個perl的程式。

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perlÀɮפ¤ªº²Ä¤@¦æ©ñ¸m¤U¦Cªº±Ô­z¡AÅýµ{¦¡¦Û°Ê¥h°õ¦æperlµ{¦¡¡G #!/usr/local/bin/perl ³o¤@¦æ§i¶DUNIX³o­Óµ{¦¡¬O¥Ñ³o­Ó/USR/LOCAL/BIN/PERL¸ô®|ªºperl µ{¦¡¥h½Ķ©³¤Uªº©R¥O¡A¦p¦¹©p´N¥i¥HÅýµ{¦¡¦Û¦æ¥h°õ¦æ¡A¦ý­n§@¥H¤Uªº­×§ï¡G chmod+xsample.pl ¦p¦¹¡A«K¥i¥Hª½±µ°õ¦æ³o­Óµ{¦¡¡A¦¹µ{¦¡ªº²Ä¤@¦æ«K·|§i¶D§@·~¨t²Î¨ì­þ¸Ì¥h§ä¨ì°õ¦æ³o­Ó perlªºµ{¦¡¡C ¦bWindowsNT¤¤°õ¦æPERLµ{¦¡    «e­±©Ò´£¨ìªºªF¦è¦bUNIX¤¤¥i¥H«Ü¥¿±`ªº°õ¦æ¡A¦ý¬O¦bWindowsNT¤¤´N¤£¨£±o¥i¥H°õ¦æ ¡C§A¥i¥H¨Ï¥ÎNTªºÀɮ׺޲z­û¨Ó«Ø¥ßªþÀɦW¬°.PLªºÀɮשMPerlªºÀÉ®×ÃöÁp¡A³o¼Ë Perl´N¥i¥H¦bNT¤¤°õ¦æ¡C¥u­n¬OªþÀɦW¬°.PlNT´Nª¾¹D³o¬OPerlµ{¦¡¡A´N·|¦Û°Ê¥h ©I¥sPerl¨Ó°õ¦æ³o­ÓªþÀɦW¬°.PLªºµ{¦¡¡C ³Æµù ³q±`§Ú­ÌÁٻݭn´X­Ó¨BÆJ¤~¯àÅýWebserver¦Û°Ê¥h°õ¦æPerlµ{¦¡¡C °Ñ¾\ªþ¿ýA"Perl¨ú±o¤Î¦w¸Ë"¡Ð¦b¯S©wªº¥­¥x¤W«Ø¥ßscripts¤Îª½Ä¶¾¹ªºÃö Áp©Ê¡C ©R¥O¦C°Ñ¼Æ Perl´£¨Ñ¤F¦UºØ°w¹ï¤£¦P¥Øªº¦Ó¨Ï¥Îªº°Ñ¼Æ¡]°Ñ¾\Table1.1¡A¤j³¡¤Àªº°Ñ¼Æ³£¦C¦b³o ¸Ì¡^¡C the-tswitchinparticularisderigueurinWeb-basedPerlscripts. Table1.1  Perl©R¥O¦C°Ñ¼Æ °Ñ¼Æ ¥\¯à ¨Ï¥Î¥Øªº ³Æµù -0 Octalcharactercode Specifyrecordseparator Defaultisnewline(\n) -a   Automaticallysplitrecords Usedwith-nor-p -c   Checksyntaxonlydonotexecute   -d   Runscript,usingPerldebugger IfPerldebuggerisinstalled \-D Flags Specifydebuggingbehavior RefertothePERLDEBUGmanpageontheCD-ROMthatcomeswiththisbook -e Command PassacommandtoPerlfromthecommandline Usefulforquickoperations;seetipafterthistableforanexample -F Regularexpression Expressiontosplitbyif-aisused Defaultiswhitespace -i Extension Replaceoriginalfilewithresult Usefulformodifyingcontentsoffiles;seetipafterthistableforan example -I Directory Specifylocationofincludefiles   -l Octalcharactercode Dropnewlineswhenusedwith-nand-p,anduse designatedcharacterasline-terminationcharacter ¡@ -n   Processthescript,usingeachspecifiedfileasanargument Usedforperformingthesamesetofactionsonasetoffiles -p   Sameas-n,buteachlineisprinted ¡@ -P   RunthescriptthroughtheCpreprocessorbeforePerlcompilesit ¡@ -s   EnablepassingofarbitraryswitchestoPerl Use-s-what-evertohavethePerlvariables$whatand$ever definedwithinyourscript -S   TellPerltolookalongthepathforthescript ¡@ -T   Usetaintchecking;don'tevaluateexpressionssuppliedinthecommand line VeryimportantforWebuse -u   MakesPerldumpcoreaftercompilingyourscript;intendedtoallowfor generationofPerlexecutables Verymessy;waitforthePerlcompiler -U   ¤£¦w¥þ¼Ò¦¡;overridesPerl'snaturalcaution. ºÉ¶q¤£­n¨Ï¥Î¡I -v   Åã¥ÜPerlª©¥»   -w   ¦L¥X»yªk¿ù»~ªºÄµ§i»y «D±`¦³¥Î¡A¯S§O¬O¦bµo®i¶¥¬q TIP The-eoptionishandyforquickPerloperationsfromthecommandline.Want tochangeallthefoosinWIFFLE.BATtobars?Trythis: perl-i.old-p-e"s/foo/bar/g"wiffle.bat Thiscodesays,"TakeeachlineofWIFFLE.BAT(-p),storetheoriginalin WIFFLE.OLD(-i),replaceallinstancesoffoowithbar(-e), andwritetheresult(-p)totheoriginalfile(-i)." §A¥i¥H¦bUNIX¤¤¨Ï¥ÎPerl©R¥O¦C°Ñ¼Æ¡A©³¤U§Y¬°½d¨Ò¡G #!/usr/local/bin/perl-w-T ª`·N¨Æ¶µ The-wswitchisbestomittedinversionsofPerlolderthan5.002,becauseit mayproducespuriouswarnings. Also,takecarewhenyouusethe-wswitchinscriptsthatsenddatatoWeb browsers.Warningmessagessentbeforethebrowserreceivesacontent-typelinemayresult inanerrormessage. µ{¦¡ªº§G§½¡]ProgramLayout¡^ ¤@­ÓPerlµ{¦¡¬O¥Ñ¥]§t¤F¤£¦Pªºperl©R¥Oªº ¤å¦rÀÉ©Ò²Õ¦¨¡C³o¨Ç©R¥O¬O¥Ñ¤@¨Ç¬Ý°_¨Ó«D±`¹³C,shellscript, ¥H¤Î­^¤å©Ò²Õ¦¨¡C Perlµ{¦¡¥i¥H¬O«D±`¦Û¥Ñªº§Î¦¡¡A¦]¬°¥Lªº»yªk³W«h«D±`¼eÃP¡G ¶}ÀYªºªÅ¶¡·|³Qµ{¦¡©¿²¤¡C§A¥i¥H¦b¥ô¦ó§A³ßÅwªº¦a¤è¶}©lPERL ªºµ{¦¡¡C¥u­n§A³ßÅw¡A§A¥i¥H¦b²Ä¤@¦æ¶}©l¡A©Î¬O¨Ï¥Î¶Ç²Îªº¶¥¼h¦¡»yªk¡]«Øij¨Ï¥Î¡^¡A©Î¬O¦Û³Ðªº§Î¦¡¡]¥u­n§A¬ÝªºÀ´¡^¡C µ{¦¡¬O¥Î¤À¸¹¡]¡F¡^§@¬°µ²§ô¡C   ¤@­ÓªÅ®æ©M¤@¦Ê­ÓªÅ®æ¬O¤@¼Ë¦nªº¡C³o¤]´N¬O»¡§A¥i¥H±N¤Óªøªº±Ô­z¦¡¤À³Î¦¨¼Æ¦æ¥H¤è«K¾\Ū¡C ¦b³o­Ó²Å¸¹(#)«á¤èªº¥ô¦óªF¦è³£·|³Q©¿²¤¤£¥h°õ¦æ¡C  ©³¤U´N¬O¤@­ÓPerlªí¹F¦¡¡G print"MynameisAndyLiu\n; ·íPerl°õ¦æ¤W­±³o­Óµ{¦¡®É¡A·|¦b¿Ã¹õ¤W¦L¥X¡GMynameisAndy Liu¡C·íperlµ{¦¡°õ¦æ¨ì \n ®É¡Aµ{¦¡·|¦Û°Ê´«¦æ¡C ¡]¥Î¨ä¥Lªº¸Ü¨Ó»¡¡A´N¬O¸õ¨ì¤U¤@¦æªº¶}ÀY¡^¡C ¦C¦L§ó¦h¦æªº¤èªk¡A´N¬O±N¤W­±ªº¨Ò¤l°õ¦æ¦h¦¸ªºµ²ªG¡C print"MynameisYonYonson,\n"; print"IliveinWisconsin,\n", "Iworkinalumbermillthere.\n"; §¹¾ãªºPerlµ{¦¡¬Ý°_¨Ó¹³¤°»ò©O¡H©³¤U¬O¤@­ÓUNIX ªº½d¨Ò¡A¤@¶}©l«ü¥X¤Fperlµ{¦¡ªº©Ò¦b¦a¥H¤Î´X¦æµù¸Ñ¡G #!/usr/local/bin/perl-w#Åã¥Üĵ§i°T®§ print"MynameisAndyLiu,\n";#Let'sintroduceourselves print"IliveinTaiwan,\n", "Iworkinalumbermillthere.\n";#§O§Ñ¤F¥H¤À¸¹³Ì¬°µ²§ô ¸ê®Æµ²ºc¡]DataTypes¡^ Perlªº¸ê®ÆºØÃþ«D±`¦a¤Ö¡C¦pªG§A²ßºDC»y¨¥³oºØ³s¦r¤¸³£¤À¬°unsigned¤Îsigned ¨º§A¨Ï¥ÎPerl±N·|«D±`´r§Ö¡C°ò¥»¤WPerl¥u¦³¨âºØ¸ê®Æµ²ºc¡G ¼Æ­Èscalars¥H¤Î°}¦Carrays¡CPerl¦P¼Ë¤]¦³Áp¦X°}¦C associativearrays³o¬O¤@ºØ¸û¯S®íªº°}¦C¡A´X¥G¥i¦Û¦¨¤@Ãþ¡C ¼Æ­ÈScalars ©Ò¦³ªº¼Æ¦r¤Î¦r¦ê³£ºÙ¬°¼Æ­È¸ê®Æscalars¡C©Ò¦³ªºScalar-variable «e­±³£¦³¤@­Ó($). µù¸Ñ Perlªº©Ò¦³ÅܼƦWºÙ¡A¥]§t¼Æ­È¸ê®Æ³£¦³¤j¤p¼g¤§¤À¡C¤ñ¦p»¡¡G$Name©M$name´N³Qµø¬°§¹¥þ¤£¦PªºªF¦è¡C Perl·|¦Û¤v±NScalarsÂà´«¦¨¼Æ­È¸ê®Æ©Î¬O¼Æ¦r«¬ºA¡C $a=2; $b=6; $c=$a.$b;#"."¹Bºâ¤¸·|±N¨â­Ó¦r¦ê³s°_¨Ó¡C $d=$c/2; print$d; °õ¦æªºµ²ªG¬°¡G 13 ¤W­±³o­Ó¨Ò¤l·|±N¨â­Ó¾ã¼ÆÂন¦r¦ê¡AµM«á±N¨â­Ó¦r¦ê³sµ²¦¨¤@­Ó·sªº¦r¦êÅܼơAµM«á ±N·s¦r¦êÂà´«¦¨¾ã¼Æ«¬ºA¡A¦A¥Î2¥h°£¡A¦A±Nµ²ªGÂà´«¦¨¦r¦ê¨Ã¦L¥Xµ²ªG¨ì¿Ã¹õ¤W¡C ThissituationmightbeaproblemifPerlwereregularlyusedfortasksinwhich explicitmemoryoffsetswereused,forexample,anddatatypeswerecritical.Butforthe typeoftaskforwhichPerlisnormallyused-andcertainlyforthetypesoftasksthat we'llbeusingitforinthisbook-theseautomaticconversionsaresmooth,intuitive,and generallyaGoodThing. Wecandeveloptheearlierexamplescriptwithsomestringvariables,asfollows: #!/usr/local/bin/perl-w#Åã¥Üĵ§i°T®§ $who='YonYonson'; $where='Wisconsin'; $what='inalumbermill'; print"Mynameis$who,\n";#¤¶²Ð¦Û¤v print"Ilivein$where,\n", "Iwork$whatthere.\n"; print"\nSigned:\t$who,\n\t\t$where.\n"; °õ¦æ«á¡A·|²£¥Í¤U¦Cµ²ªG¡G MynameisYonYonson, IworkinWisconsin, Iworkinalumbermillthere. Signed: YonYonson, Wisconsin. Don'tworry-itgetsbetter. °}¦CArrays ¼Æ­È¸ê®Æªº¶°¦X§Ú­ÌºÙ¬°°}¦Carray.°}¦CÅܼƷ|¥Î(@) ²Å¸¹§@¬°²Ä¤@­Ó¦rªº¶}©l¡C°}¦C¤¤ªº¤¸¯À¤§¶¡·|¥Î³r¸¹¹j¶}¡A´N¹³¤U­±Åã¥Üªº¨Ò¤l¤@¼Ë¡G @trees=("Larch","Hazel","Oak"); °}¦Cªº¤¸¯À¥i¥H¥Î¤¤¬A¸¹¨Óªí¥Ü¡AÁ|¨Ò¨Ó»¡¡G$trees[0]ªí¥Ü³o¬O@trees °}¦Cªº²Ä¤@­Ó¤¸¯À¡Cª`·N¡G³o¸Ì¬O¨Ï¥Î$trees[0]¦Ó¤£¬O@trees[0] ¦]¬°­Ó§Oªº¤¸¯À¬O¼Æ­È¸ê®Æ¡A©Ò¥H«e­±­n¥Î$¨Ó¨ú±o¤¸¯Àªº­È¡C Mixingscalartypesinanarrayisnotaproblem.Thecode @items=(15,'45.67',"case"); print"Take$items[0]$items[2]sat\$$items[1]each.\n"; ©³¤U¬O°õ¦æªºµ²ªG¡G Take15casesat$45.67each. Perl¸Ì­±ªº°}¦C³£¬O°ÊºAªº¡A©p®Ú¥»¤£»Ý­n¥h¾á¤ß°O¾ÐÅ骺°t¸m¤ÎºÞ²z¡CPerl·|¬°§A °µ¦n©Ò¦³ªº¤u§@¡C°}¦C¤§¤¤¤S¥i¥H¥]§t°}¦C¡A©³¤UÁ|­Ó¨Ò¤l¨Ó»¡¡G @A=(1,2,3); @B=(4,5,6); @C=(7,8,9); @D=(@A,@B,@C); ³o­Óµ{¦¡°õ¦æªºµ²ªG¡G°}¦C@D¥]§t¤F¸ê®Æ1¨ì9¡C©³¤U¬O§Ú­Ì±`¥Îªº°}¦C¥Îªk¡G @Annual=(@Spring,@Summer,@Fall,@Winter); ³o­Ó½d¨Òµ{¦¡±N¤@¦~¤¤ªº©u¸`¥Î¤@ºØ²¼ä¤Sª½±µ©öÀ´ªº°}¦C¨Óªí¹F¡C¦Ó°}¦C¤¤ªº©u¸`¤S ¥i¥HÂà´«¬°¥]§t¤ë¥÷ªº°}¦C¡A¨C­Ó¤ëªº°}¦C¤S¥i¥HÂà´«¦¨¬°¥]§t¤Ñ¼Æªº¸ê®Æ¡C @Annual°}¦C¥i¥HÅܦ¨¥Ñ¤@¦~¤¤ªº¨C¤@¤Ñªº­È©Ò²Õ¦¨¡C arraythenwouldconsistofavalueforeachdayoftheyear.Bydefiningyourdatain chunkssuchasthis,yougiveyourselftheoptionofhandlingitonadaily,monthly,or annualbasis. NOTE AnaspectofPerlthatoftenconfusesnewcomers(andoccasionallyoldhands,too)is thecontext-sensitivenatureofevaluations.Perlkeepstrackofthecontextinwhichan expressionisbeingevaluatedandcanreturnadifferentvalueinanarraycontextthanin ascalarcontext.Inthisexample,thearray@Bcontains1-4,whereas$C contains4(thenumberofvaluesinthearray): @A=(1,2,3,4); @B=@A; $C=@A; Thiscontextsensitivitybecomesmoreofanissuewhenyouusefunctionsandoperators thatcantakeeitherasingleargumentormultiplearguments.Thefunctionorargument behavesonewaywhenitispassedasinglescalarargumentandanotherwhenitis passedmultiplearguments,whichitmayinterpretasasinglearrayargument. ManyofPerl'sbuilt-infunctionstakearraysasarguments.Oneexampleissort, whichtakesanarrayasanargumentandreturnsthesamearray,sortedalphabetically.The code printsort('Beta','Gamma','Alpha'); printsAlphaBetaGamma. Youcanmakethiscodeneaterbyusinganotherbuilt-infunction,calledjoin. Thisfunctiontakestwoarguments:astringtoconnectwith,andanarrayofstringsto connect.joinreturnsasinglestringthatconsistsofallelementsinthearray joinedwiththeconnectingstring.Thecode printjoin(':','Name','Address','Phone'); returnsthestringName:Address:Phone. Becausesortreturnsanarray,youcanfeeditsoutputstraightintojoin. Thecode printjoin(',',sort('Beta','Gamma','Alpha')); printsAlpha,Beta,Gamma. Noticethatthiscodedoesn'tseparatetheinitialscalarargumentofjoin fromthearraythatfollowsit.Thefirstargumentisthestringtojointhingswith.The restoftheargumentsaretreatedasasingleargument:thearraytobejoined.Thisis trueevenifyouuseparenthesestoseparategroupsofarguments.Thecode printjoin(':',('A','B','C'),('D','E'),('F','G','H','I')); returnsA:B:C:D:E:F:G:H:I. Youcanuseonearrayormultiplearraysinacontextsuchasthisbecauseoftheway thatPerltreatsarrays;addinganarraytoanarraygivesyouonelargerarray,nottwo arrays.Inthiscase,allthreearraysarebundledintoone. TIP Forevenmorepowerfulstring-manipulationcapabilities,refertothesplice functioninChapter15,"FunctionList." ²Õ¦X°}¦CAssociativeArrays AssociativearrayshaveacertainelegancethatmakesexperiencedPerlprogrammersa littlesnobbishabouttheirlanguageofchoice.Rightlyso!AssociativearraysgivePerla degreeofdatabasefunctionalityataverylow,yetuseful,level.Manytasksthatwould otherwiseinvolvecomplexprogrammingcanbereducedtoahandfulofPerlstatementsby meansofassociativearrays. Arraysofthetypethatyou'vealreadyseenarelistsofvaluesindexedby subscripts.Inotherwords,togetanindividualelementofanarray,yousupplya subscriptasareference,asfollows: @fruit=("Apple","Orange","Banana"); print$fruit[2]; ThisexampleyieldsBanana,becausesubscriptsstartatzero,so2is [email protected]$fruit[7] herereturnsthenullvalue,becausenoarrayelementwiththatsubscripthasbeen defined. Now,here'sthepointofallthis:Associativearraysarelistsofvaluesindexedby strings.Conceptually,that'sallthereistothem.Theimplementationofassociative arraysismorecomplex,becauseallthestrings(keys)needtobestoredin additiontothevaluestowhichtheyrefer. Whenyouwanttorefertoanelementofanassociativearray,yousupplyastring(the key)insteadofaninteger(thesubscript).Perlreturnsthecorrespondingvalue.Consider thefollowingexample: %fruit=("Green","Apple","Orange","Orange","Yellow","Banana"); print$fruit{"Yellow"}; ThiscodeprintsBanana,asbefore.Thefirstlinedefinestheassociative arrayinmuchthesamewaythatyouhavealreadydefinedordinaryarrays;thedifference isthatinsteadoflistingvalues,youlistkey/valuepairs.ThefirstvalueisApple, anditskeyisGreen.ThesecondvalueisOrange,whichhappenstohave thesamestringforbothvalueandkey.Finally,thevalueBananahasthekeyYellow. Onasuperficiallevel,youcanusestringsubscriptstoprovidemnemonicsforarray references,allowingyoutoreferto$Total{'June'}insteadof$Total[5]. Butyouwouldn'tevenbebeginningtousethepowerofassociativearrays.Thinkofthe keysofanassociativearraysasyoumightthinkofakeythatlinkstablesina relationaldatabase,andyou'reclosertotheidea.Considerthisexample: %Folk=('YY','YonYonson', 'TC','TerraCotta', 'RE','RonEverly'); %State=('YY','Wisconsin', 'TC','Minnesota', 'RE','Bliss'); %Job=('YY','workinalumbermill', 'TC','teachnuclearphysics', 'RE','watchfootball'); foreach$person('TC','YY','RE'){ print"Mynameis$Folk{$person},\n", "Ilivein$State{$person},\n", "I$Job{$person}there.\n\n"; } ³o­Ó¨Ò¤l¸Ì¡A§Ú­Ì°½°½ªº¨Ï¥Î¤Fforeach³o­Óµ²ºc¦¡¡C³o­Óµ²ºc¦¡¦b¥»³¹µy «áªº¬yµ{±±¨î³¡¤À·|¦A¸Ô²Óªº¸ÑÄÀ¡CFornow,you'lljusthavetotakeitontrustthatforeachmakesPerlexecutethethreeprint statementsforeachofthepeopleinthelistaftertheforeachkeyword. Otherwise,youcouldtryexecutingthecodeinthesampleandseewhathappens. Youalsocantreatthekeysandvaluesofanassociativearrayasseparate(ordinary) arraysbyusingthekeysandvalueskeywords,respectively.Thecode printkeys%Folk; printvalues%State; printsthestringYYRETCWisconsinBlissMinnesota. Looksasthoughweneedtodosomemoreworkonstringhandling.Thattaskisbestleft untilafterwecoversomeflow-controlmechanisms,however. NOTE Aspecialassociativearraycalled%ENVstoresthecontentsofallenvironment variables,indexedbyvariablename.$ENV{'PATH'},forexample,returnsthe currentsearchpath.Followingisawaytoprintthecurrentvaluesofallenvironment variables,sortedbyvariablenameforgoodmeasure: foreach$var(sortkeys%ENV){ print"$var:\"$ENV{$var}\".\n"; } Theforeachclausesets$vartoeachoftheenvironment-variable namesinturn(inalphabeticalorder),andtheprintstatementprintseachname andvalue.Thebackslash-quote(\")inthereproducesquotation marksaroundthevalues. FileHandles ThischapterfinishesdiscussingPerldatatypesbydiscussingfilehandles.Afile handleisnotreallyadatatypeatall,butaspecialkindofliteralstring.Afile handlebehaveslikeavariableinmanyways,however,sothissectionisagoodplaceto coverthem.(Besides,youwon'tgetveryfarinPerlwithoutthem.) YoucanregardafilehandleasbeingapointertoafilefromwhichPerlistoreador towhichitwillwrite.(Cprogrammersarefamiliarwiththeconcept.)Thebasicideais thatyouassociateahandlewithafileordevice,andthenrefertothehandleinthe codewheneveryouneedtoperformareadorwriteoperation. Filehandlesgenerallyarewritteninuppercase.Perlhassomeusefulpredefinedfile handles,asTable1.2shows. Table1.2  Perl'sPredefinedFileHandles FileHandle Pointsto¡K STDIN Standardinput(normally,thekeyboard) STDOUT Standardoutput(normally,theconsole;inmanyWebapplications,the browser) STDERR Devicewhereerrormessagesshouldbewritten(normally,theconsole;ina Webserverenvironment,normally,theserver-errorlogfile) Theprintstatementcantakeafilehandleasitsfirstargument,asfollows: printSTDERR"Oops,somethingbroke.\n"; Noticethatnocommaappearsafterthefilehandleinthisexample.ThathelpsPerl figureoutthattheSTDERRisnotsomethingtobeprinted.Ifyou'reuneasywith thisimplicitlistsyntax,youcanputparenthesesaroundalltheprint arguments,asfollows: print(STDERR"Oops,somethingbroke.\n"); Youstillhavenocommaafterthefilehandle,however. TIP Usethestandardfilehandlesexplicitly,especiallyincomplexprograms.Redefining thestandardinputoroutputdeviceforawhileisconvenientsometimes;makesurethat youdon'taccidentallywindupwritingtoafilewhatshouldhavegonetothescreen. Youcanusetheopenfunctiontoassociateanewfilehandlewithafile,as follows: open(INDATA,"/etc/stuff/Friday.dat"); open(LOGFILE,">/etc/logs/reclaim.log"); printLOGFILE"Logofreclaimprocedure\n"; Bydefault,openopensfilesforreadingonly.Ifyouwanttooverridethis defaultbehavior,addtothefilenameoneofthespecialdirectionsymbolslistedin Table1.3.(The>atthestartofthefilenameinthesecondoutput statementoftheprecedingexample,forexample,tellsPerlthatyouintendtowriteto thenamedfile.) Table1.3  PerlFile-AccessSymbols Symbol Meaning < Openthefileforreading(thedefaultaction) > Openthefileforwriting >> Openthefileforappending +< Openthefileforbothreadingandwriting +> Openthefileforbothreadingandwriting |(beforefilename) TreatfileascommandintowhichPerlistopipetext |(afterfilename) TreatfileascommandfromwhichinputistobepipedtoPerl Totakeamorecomplexexample,here'sonewaytofeedoutputtothemypr printeronaUNIXsystem: open(MYLPR,"|lpr-Pmypr"); printMYLPR"Alineofoutput\n"; closeMYLPR; AspecialPerloperatorforreadingfromfilesconsistsoftwoangle brackets-<>-aroundthefilehandleofthefilefromwhichyouwanttoread.This operatorreturnsthenextlineorlinesofinputfromthefileordevice,dependingon whethertheoperatorisusedinascalaroranarraycontext.Whennomoreinputremains, theoperatorreturnsfalse. Aconstructsuchas while(){ print; } simplyechoeseachlineofinputbacktotheconsoleuntilCtrl+D(Ctrl+ZinWindows NT)ispressed,becausetheprintfunctiontakesthecurrentdefaultargument here:themostrecentlineofinput.Foranexplanation,see"SpecialVariables" laterinthischapter. Iftheusertypes A Bb Ccc ^D thescreenlookslikethis: A A Bb Bb Ccc Ccc ^D Noticethatinthiscase,isinascalarcontext,soonelineof standardinputisreturnedatatime.Comparethatexamplewiththefollowingexample: print; Inthiscase,becauseprintexpectsanarrayofarguments(itcanbea single-elementarray,butit'sanarrayasfarasprintisconcerned),the<> operatorobliginglyreturnsallthecontentsofSTDINasanarray,andthenprint printsit.Becausethearrayisfullybuiltbeforeitisprinted,nothingiswrittento theconsoleuntiltheuserpressesCtrl+D: A Bb Ccc ^D A Bb Ccc Thisscriptprintsoutthecontentsofthefile.SIGNATURE,double-spaced: open(SIGFILE,".signature"); while(){ print;print"\n"; } Thefirstprintherehasnoarguments,soittakesthecurrentdefault argumentandprintsit.Thesecondprinthasanargument,soitprintsthat instead.Perl'shabitofusingdefaultargumentsextendstothe<> operator;ifthatoperatorisusedwithnofilehandle,Perlassumesthat isintended.expandstoeachlineinturnofeachfilelistedinthe commandline. Ifnofilesarelistedinthecommandline,PerlinsteadassumesthatSTDINis intended.Thefollowingcode,therefore,keepsprintingmoreaslongassomething otherthanCtrl+Dappearsinstandardinput: while(<>){ print"more...."; } NOTE Perl5allowsarrayelementstobereferencestoanydatatype.Asaresult,youcan buildarbitrarydatastructuresofthekindusedinCandotherhigh-levellanguages,but withallthepowerofPerl.Youcanhaveanarrayofassociativearrays,forexample. SpecialVariables Likealllanguages,Perlhasitsspecialhieroglyphs,whichareladenwithmeaning. Thissectionbrieflyexaminessomeofthemostcommonandusefulvariables,andprovides someexamplesoftypicalPerlidiomsinwhichyoumightfindthem. EnvironmentVariables Youhavealreadyseenonespecialvariable:theenvironment-variableassociativearray%ENV. Thisspecialassociativearrayallowsyoutoeasilyusethevalueofanyenvironment variablewithinyourPerlscripts: print"Lookingforfilesalongthepath($ENV{'PATH'})\n"; The%ENVarrayisquiteusefulinCGIprogramming,inwhichparametersare passedfromthebrowsertoCGIprogramsasenvironmentsettings. ProgramArguments AnyargumentsspecifiedinthePerlcommandlinearepassedtothePerlscriptin anotherspecialarray:@ARGV. CAUTION Cprogrammers,beware:Thefirstelementofthisarrayisthefirstactualargument, notthenameoftheprogram.Thespecialvariable$0containsthenameofthe Perlscriptthatisbeingexecuted. Thefollowingcodeprintsthecommand-lineargumentsoneperline,sorted alphabetically: printjoin("\n",sort@ARGV); Thecommand-lineargumentsareoflimiteduseinCGIscripts,inwhichargumentsare passedviatheenvironmentratherthanthecommandline.Theseargumentsarequiteuseful innormalPerlwork,ofcourse. CurrentLine Thespecialvariable$_isoftenusedtostorethecurrentlineofinput.This situationistruewhenthe<>inputoperatorisused.Thefollowingcode, forexample,printsanumberedlistingofthefilepointedtobySOMEFILE: $line=0; while(){ ++$line; print"Line$line:",$_; } Youoccasionallyneedtostorethecontentsof$_somewhere,asinthe followingexample: $oldvalue=$_; Buttheoppositeoperation-settingthevalueof$_manually-israrely appropriate,asinthisexample: $_=$oldvalue; Patternmatchingandsubstitutiontakeplaceonthecontentsofthisvariableunless youspecifyotherwise.Thesetopicsarecoveredin"RegularExpressions"later inthischapter. SystemErrorMessages Thespecialvariable$!containsthecurrentsystem-errornumber(errno, onUNIXsystems)orsystem-errorstring,dependingonwhetheritisevaluatedinanumeric orstringcontext.Thisvariablemaynotcontainanythingmeaningful;itshouldbeused onlyifanerroroccurred. Thisexamplereportsfailureiftheopencallfailed: open(INFILE,"./missing.txt")||die"Couldn'topen\"./missing.txt\"($!).\n"; The||hereistheBooleanoroperator,whichiscoveredin "FlowControl"laterinthischapter.diecausesPerltoterminate afterprintingthestringgiventodieasanargument. Ifthefiledoesnotexist,Perlterminatesafterdisplayingsomethinglikethis: Couldn'topen"./missing.txt"(Nosuchfileordirectory). Theformandcontentoferrormessagesvaryfromonesystemtothenext. ¬yµ{±±¨î Theexamplesthatyouhaveseensofarhavebeenquitesimple,withlittleornologicalstructurebeyondalinearsequenceofsteps.Wemanagedtosneakintheoccasionalwhileandforeach;thinkofthoseasbeingsneakpreviews.Perlhasalltheflow-controlmechanismsthatyou'dexpecttofindinahigh-levellanguage,andthissectiontakesyouthroughthebasicsofeachmechanism. ÅÞ¿è¹Bºâ¤¸ Twooperators-||(or)and&&(and)-areusedlikegluetoholdPerlprogramstogether.Theytaketwooperandsandreturneithertrueorfalse,dependingontheoperands.Inthefollowingexample,ifeither$Saturdayor$Sundayistrue,$Weekendwillbetrue,too: $Weekend=$Saturday||$Sunday; Inthenextexample,$Solventistrueonlyif$incomeisgreaterthan 3and$debtsislessthan10: $Solvent=($income>3)&&($debts<10); Nowconsiderthelogicofevaluatingoneoftheseexpressions.Itisn'talways necessarytoevaluatebothoperandsofeitheran&&ora|| operator.Inthefirstexampleearlierinthissection,if$Saturdayistrue,you knowthat$Weekendwillbetrue,regardlessofwhether$Sundayisalso true(themidnightcondition,perhaps?). Thismeansthatwhentheleftsideofanorexpressionisevaluatedastrue, therightsideisnotevaluated.CombinethiswithPerl'seasywaywithdatatypes,and youcansaythingslikethefollowing: $value>10||print"Oops,lowvalue\n"; If$valueisgreaterthan10,therightsideoftheexpressionisnever evaluated,sonothingisprinted.If$valueisnotgreaterthan10,Perl needstoevaluatetherightside,too,soastodecidewhethertheexpressionasawhole istrueorfalse.ThatmeansthatPerlevaluatestheprintstatement,printing outthemessage. OK,it'satrick,butit'saveryusefulone. Somethinganalogousappliestothe&&operator.Inthiscase,ifthe leftsideofanexpressionisfalse,theexpressionasawholeisfalse,soPerldoesnot evaluatetherightside.The&&operatorcan,therefore,beusedto producethesamekindofeffectasthe||trick,butwiththeoppositesense,as inthefollowingexample: $value>10&&print"OK,valueishighenough\n"; AsistrueofmostPerlconstructs,therealpowerofthesetrickscomeswhenyouapply alittlecreativethinking.Rememberthattheleftandrightsidesoftheseexpressions canbeanyPerlexpressions;thinkofthemasbeingconjunctionsinasentenceratherthan logicaloperators,andyou'llgetabetterfeelforhowtousethem.Expressionssuchas thefollowinggiveyoualittleoftheflavorofcreativePerl: $length<=80||die"Linetoolong.\n"; $errorlevel>3&&warn"Hmmm,strangeerrorlevel($errorlevel)\n"; open(LOGFILE,">install.log")||&bust("Logfile"); The&bustinthisexampleisasubroutinecall,bytheway.Referto "Subroutines"laterinthischapterformoreinformation. ConditionalExpressions Themostbasickindofflowcontrolisasimplebranch.Astatementiseitherexecuted ornot,dependingonwhetheralogicalexpressionistrueorfalse.Youcandothisby followingthestatementwithamodifierandalogicalexpression,asfollows: open(INFILE,"./missing.txt")if$missing; Theexecutionofthestatementiscontingentuponboththeevaluationofthe expressionandthesenseoftheoperator. Theexpressionisevaluatedaseithertrueorfalseandcancontainanyofthe relationaloperatorslistedinTable1.4(althoughitneednot).Followingareafew examplesofvalidexpressions: $full $a==$b Table1.4  Perl'sRelationalOperators Operator NumericContext StringContext Equality == eq Inequality != ne Inequalitywithsignedresult <=> cmp Greaterthan > gt Greaterthanorequalto >= ge Lessthan < lt Lessthanorequalto <= le NOTE Whenwe'recomparingstrings,lessthanmeanslexicallylessthan.If$left comesbefore$rightwhenthetwoaresortedalphabetically,$leftis lessthan$right. Perlhasfourmodifiers,eachofwhichbehavesthewaythatyoumightexpectfromthe correspondingEnglishword: if.Thestatementisexecutedifthelogicalexpressionistrueandisnot executedotherwise.Examples: $max=100if$min<100; print"Empty!\n"if!$full; unless.Thestatementisnotexecutedifthelogicalexpressionistrueandis executedotherwise.Examples: open(ERRLOG,"test.log")unless$NoLog; print"Success"unless$error>2; while.Thestatementisexecutedrepeatedlyuntilthelogicalexpressionis false.Examples: $total-=$decrementwhile$total>$decrement; $n=1000;"print$n\n"while$n->0; until.Thestatementisexecutedrepeatedlyuntilthelogicalexpressionis true.Examples: $total+=$value[$count++]until$total>$limit; printRESULTS"Nextvalue:$value[$n++]"until$value[$n]=-1; Noticethatthelogicalexpressionisevaluatedonlyonetimeinthecaseofif andunless,butmultipletimesinthecaseofwhileanduntil. Inotherwords,thefirsttwoaresimpleconditionals,andthelasttwoareloop constructs. CompoundStatements Thesyntaxchangeswhenyouwanttomaketheexecutionofmultiplestatements contingentontheevaluationofalogicalexpression.Themodifiercomesatthestartofa line,followedbythelogicalexpressioninparentheses,followedbytheconditional statementsinbraces.Noticethattheparenthesesaroundthelogicalexpressionare required,althoughtheyarenotrequiredinthesinglestatementbranchingdescribedin theprecedingsection. ThefollowingexampleissomewhatsimilartoC'sifsyntax: if(($total+=$value)>$limit){ printLOGFILE"Maximumlimit$limitexceeded.Offendingvaluewas$value.\n"; close(LOGFILE); die"Toomany!Checkthelogfilefordetails.\n"; } Theifstatementiscapableofalittlemorecomplexity,withelse andelsifoperators,asinthefollowingexample: if(!open(LOGFILE,"install.log")){ close(INFILE); die"Unabletoopenlogfile!\n"; } elsif(!open(CFGFILE,">system.cfg")){ printLOGFILE"Errorduringinstall:Unabletoopenconfigfileforwriting.\n"; close(LOGFILE); die"Unabletoopenconfigfileforwriting!\n"; } else{ printCFGFILE"Yoursettingsgohere!\n"; } Loops Theloopmodifiers(while,until,for,andforeach)areusedwithcompoundstatementsinmuchthesameway,asthefollowingexampleshows: until($total>=50){ print"Enteravalue:"; $value=scalar(); $total+=$value; print"Currenttotalis$total\n"; } print"Enough!\n"; Thewhileanduntilstatementsaredescribedin"ConditionalExpressions"earlierinthischapter.TheforstatementresemblestheoneinC.forisfollowedbyaninitialvalue,aterminationcondition,andaniterationexpression,allenclosedinparenthesesandseparatedbysemicolons,asfollows: for($count=0;$count<100;$count++){ print"Something"; } Theforeachoperatorisspecial;ititeratesoverthecontentsofanarrayandexecutesthestatementsinastatementblockforeachelementofthearray.Followingisasimpleexample: @numbers=("one","two","three","four"); foreach$num(@numbers){ print"Number$num\n"; } Thevariable$numfirsttakesonthevalueone,thentwo,andsoon.Thatexamplelooksfairlytrivial,buttherealpowerofthisoperatorliesinthefactthatitcanoperateonanyarray,asfollows: foreach$arg(@ARGV){ print"Argument:\"$arg\".\n"; } foreach$namekey(sortkeys%surnames){ printREPORT"Surname:$value{$namekey}.\n", "Address:$address{$namekey}.\n"; } Labels Youcanuselabelswiththenext,last,andredostatementstoprovidemorecontrolofprogramflowthroughloops.Alabelconsistsofanyword,usuallyinuppercase,followedbyacolon.Thelabelappearsjustbeforetheloopoperator(while,for,orforeach)andcanbeusedasananchorforjumpingtofromwithintheblock.Thefollowingcodesnippetprintsalltheodd-numberedrecordsinINFILE: RECORD:while(){ $even=!$even; nextRECORDif$even; print; } Thethreelabel-controlstatementsare: next.Jumpstothenextiterationoftheloopmarkedbythelabelortothe innermostenclosingloop,ifnolabelisspecified. last.Immediatelybreaksoutoftheloopmarkedbythelabeloroutofthe innermostenclosingloop,ifnolabelisspecified. redo.Jumpsbacktotheloopmarkedbythespecifiedlabelortotheinnermost enclosingloop,ifnolabelisspecified.redocausesthelooptoexecuteagain withthesameiteratorvalue. Subroutines SubroutinesinPerlaredefinedwiththesubkeyword,asfollows: subUsage{ print"Usage:\n", "twiddle[-args]infileoutfile\n"; print"Copyleft1996,JonathanF.Squirmsby."; } Subroutinesarecalledwith&,asfollows: subbust{ print"Oops,somekindoferrorseemstohaveoccurred.\n"; die"Fatalerror,terminating.\n"; } open(LOGFILE,">install.log")||&bust; Inthisexample,thesubroutinewasdefinedbeforeitwascalled.Youcandefineand callsubroutinesinanyorderinPerl;theconventionistodefinethemafterthemain routine. PassingArguments  Youcanpassargumentstoasubroutineintheusual way,asfollows: open(LOGFILE,">install.log")||&bust("Failedtoopenlogfile\"install.log\"."); ButhereiswherePerl'ssubroutinesyntaxstartstogetalittlestrange;C programmersmaywanttotakeaseatbeforereadingon. AllPerlsubroutinesreceivetheirargumentsasanarbitrarilylongarrayofscalars withthespecialnameof@_.Thereisnomechanismfordeclaringthearguments whenthesubroutineisdeclared.Thereisnofixednumberofarguments.Also,thecalling functioncanpassanymixtureofscalarsandarrays;theyarealltreatedasonebig@_ arraywhentheygettothesubroutine. Intheexampleearlierinthissection,inwhichbustiscalledwithasingle argument,youcanpickitupinthesubroutineanduseittoprovideamoresensibleerror message,asinthefollowingexample: subbust{ ($errortext)=@_; print"Oops,anerroroccurred($errortext).\n"; die"Fatalerror,terminating.\n"; } Noticethatwewenttothetroubleofassigningthescalar$errortexttothe argumentarray@_.Thisassignmentmayseemtobeunnecessary;infact,wecould havesimplyused@_insteadof$errortextintheprint statement.Explicitlyassigningvariablestothecontentsofthe@_arrayismuch clearer,though,especiallywhenthesubroutinetakesmultiplearguments.Comparethe example print"Error$_[0]openingfile$_[1].\n"; withthisone: ($errfile,$errtext)=@_; print"Error$errtextopeningfile$errfile.\n"; Notice,too,thatwhenweassignedthesinglevalue$errortexttothe@_ arrayinthebustexample,weplaceditinparentheses.Wedidsotoforcean arraycontext,sothatwhatgetsassignedto$errortextisthefirst(andonly) valueofthe@_array,notthenumberofvaluesin@_.Ineffect,we're tellingPerltotreat$errortextasasingle-elementarray.Theearlierexample thatuses$errfileand$errtextisaclearerexampleofan array-to-arrayassignment. In"VariableScope"laterinthischapter,youlearnhowtoprotectlocal variablessuchas$errortextinsubroutinesbyusingthelocalandmy keywords. PassingArrays  Perl'sgroupingofallsubroutineargumentsmakesit impossibletopassmorethanonearraytoaPerlsubroutine.Supposethatyouhavea subroutinecallofthefollowingform: &PrintRes("alpha",(1,3,5,7),"beta",(2,4,6,8)); Trytounpacktheseargumentsintothefollowingvaluesastheycomeintothe subroutine: $p1="alpha"; @p2=(1,3,5,7); $p3="beta"; @p4=(2,4,6,8); Astatementlike ($p1,@p2,$p3,@p4)=@_; won'tgetbeyondthesecondparameter.Thefollowinglistexplainswhathappens: Thefirstvariableinthelist,$p1,isassignedthevalueofthefirstscalar inthe@_argumentarray,whichisalpha. Thenthenextvariableinthelist,@p2,isassignedthevalueofthenext argumentinthe@_argumentarray.Thisisanarrayassignmentbecause@p2 isanarray,sotheentire@_array,fromitssecondelementon,isassignedto@p2-(1, 3,5,7,"beta",2,4,6,8),inotherwords. Thenextvariabletobeassignedis$p3.Thisvariableisassignedthevalueof thenextelementinthe@_argumentarray-buttherearen'tanyleft,because they'veallbeenslurpedby@p2.$p3,therefore,isnull. Thefinalvariable,@p4,suffersthesamefateandisalsonull. There'snopointintryingtospecifysubarrays,asinthefollowingexample,because Perlexpandsthearrayonthelefttothesamethingasbefore: ($p1,(@p2),$p3,(@p4))=@_; Themoralofthestoryis:Don'tpassmorethanonearrayintoasubroutine.Andifyou dopassanarray,makesurethatit'sthelastargument. ReturningValues  Perlisjustascasualaboutreturningvaluesfrom subroutinesasitisaboutpassingargumentstothem.Asubroutinereturnsasinglevalue: thevalueofthelastassign-mentmadeinthesubroutine.Ifyoupass(4,3) tothissubroutine,thevalue7isreturned: subAddIt{ ($a,$b)=@_; $a+$b; } Thatmeansthatthevalue7issubstitutedforthesubroutinecallafter evaluation.Thecode print"Summing4and3yields",&AddIt(4,3),".\n"; printsthefollowing: Summing4and3yields7. NoticethatwehadtokeepthesubroutinecalloutsidethequotestoallowPerlto recognize&asasubroutineinvocation. Itisn'talwaysclearwhichstatementisthelasttobeexecutedinasubroutine, particularlyifitcontainsloopsorconditionalstatements.Onewaytoensurethatthe correctvalueisreturnedistoplaceareferencetothevariableonalinebyitselfat theendofthesubroutine,asfollows: subMaybe{ #Variousloopsandconditionalsherewhichsetthevalueof"$result" $result; } CAUTION Takecarenottoaddseeminglyinnocuousstatementsneartheendofasubroutine.Aprint statementreturnsavalueof1(ifsuccessful)forexample,soasubroutinethatprints somethingjustbeforeitreturnsalwaysreturns1. Thereturnvaluecanbeascalar,anarray,oranassociativearray.Listing1.1shows acompleteexampleinwhichasubroutinebuildsanassociativearrayofnameskeyedby initialsandthenreturnstheassociativearray.Thekeysofthisarray-theinitials-are thenprintedinsortedorder.Takeyourtimereadingthroughthisexample;alotisgoing oninthere,butit'scomprehensivelycommented. Listing1.1  INITIALS.PL:ReturninganAssociativeArrayfromaSubroutine #!/usr/local/bin/perl-w #Passthenamesintothesubroutine. #Storetheresultsinanassociativearraycalled"keyedNames". %keyedNames=&GetInitials("JaneAusten","EmilyBronte","MaryShelley"); #Printouttheinitials,sorted: print"Initialsare",join(',',sortkeys%keyedNames),".\n"; #TheGetInitialssubroutine. subGetInitials{ #Let'sstoretheargumentsina"names"arrayforclarity. @names=@_; #Processeachnameinturn: foreach$name(@names){ #The"split"functionisexplainedinChapter15,"FunctionList". #Inthisstatement,we'regettingsplittolookforthe''inthename; #Itreturnsanarrayofchunksoftheoriginalstring(i.e.$name)whichwere #separatedbyspaces,i.e.theforenameandsurnamerespectivelyinourcase. #Thevariables"$forename"and"$surname"arethenassignedtothisarray #usingparenthesestoforceanarrayassignment. ($forename,$surname)=split('',$name); #OK,nowwehavetheforenameandsurname.Weusethe"substr"function, #alsoexplainedinchapter15,toextractthefirstcharacterfromeach ofthese. #The"."operatorconcatenatestwostrings(forexample,"aa"."bb"is"aabb") #sothevariable"$inits"takesonthevalueoftheinitialsofthename: $inits=substr($forename,0,1).substr($surname,0,1); #Nowwestorethenameinanassociativearrayusingtheinitialsasthekey: $NamesByInitials{$inits}=$name; } #Havingbuilttheassociativearray,wesimplyrefertoitattheendofthe #subroutinesothatit'svalueisthelastthingevaluatedhere.Itwillthen #bepassedbacktothecallingfunction. %NamesByInitials; } VariableScope Perlusesseparatenamespacestostorescalars,arrays,associativearrays,andsoon. Asaresult,youcanusethesamenameforvariablesofdifferenttypeswithoutfearof confusion(atleastonPerl'spart;foryourownsake,useuniquenames).This exampleusesthreedifferentkindsofvariables,eachcalledname: $name="Dana"; @name=("Donna","Dana","Diana"); %name=("Donna","Elephants","Dana","Finches","Diana","Parakeets"); print"Isaid$name{$name},not$name{$name[0]}!\n"; Thebadnewsisthatbydefault,Perlusesjustonenamespaceforeachdatatype,for allfunctions.Soifyouhaveavariablecalled$tempinthemainfunction,and youcallaroutinethatusesanothervariablecalled$temp,thevalueof$temp inthemainfunctiongetsclobbered.Thereferencestothetwovariablesareinfacttwo referencestothesamevariable,asfarasPerlisconcerned. That'swherethelocal(Perl4and5)andmy(Perl5only)functions comein.ThesefunctionsforcePerltotreatvariablesasthoughtheyarelocaltothe currentcodeblock,whetherthatblockisaloop,anif-block,orasubroutine. Thefollowingexampleusestwovariablescalled$temp(oneoutsideandone insideawhileloop): $temp="Stillhere!\n"; print"Enterafewwordsatatime,Ctrl+Dtoterminate:\n"; while(<>){ local($temp,@etc)=split('',$_); print"Yousaid$temp"; @etc&&print"andthenyousaid@etc"; print".Entersomemore,orpressCtrl+Dtoend:\n"; } print$temp; ThedifferencebetweenPerl4'slocal()andPerl5'smy()isthatlocal variablesarelocaltothecurrentpackage,whereasmyvariablesarereally local. Patterns We'llfinishthisoverviewofPerlbydiscussingitspattern-matchingcapabilities.The capabilitytomatchandreplacepatternsisvitaltoanyscriptinglanguagethatclaimsto becapableofusefultextmanipulation.Bythisstage,youprobablywon'tbesurprisedto readthatPerlmatchespatternsbetterthananyothergeneral-purposelanguagedoes.Perl 4'spatternmatchingisexcellent,butPerl5introducessomesignificantimprovements, includingthecapabilitytomatchonevenmorearbitrarystringsthanbefore. Thebasicpattern-matchingoperationsdiscussedinthissectionare: Matching,inwhichwewanttoknowwhetheraparticularstringmatchesapattern Substitution,inwhichwewanttoreplaceportionsofastringbasedonapattern Thepatternsreferredtoherearemoreproperlyknownasregularexpressions,andwe'll startbylookingatthem. RegularExpressions Aregularexpressionisasetofrulesthatdescribesageneralizedstring.If thecharactersthatmakeupaparticularstringconformtotherulesofaparticular regularexpression,theregularexpressionissaidtomatchthatstring. Afewconcreteexamplesusuallyhelpafteranoverblowndefinitionlikethatone.The regularexpressionb.matchesthestringsbovine,above,Bobby,andBobJones,butnotthe stringsBell,b,orBob.That'sbecausetheexpressioninsiststhattheletterb (lowercase)mustbeinthestringandmustbefollowedimmediatelybyanothercharacter. Theregularexpressionb+,ontheotherhand,requiresthelowercaseletterbat leastonce.ThisexpressionmatchesbandBobinadditiontotheexamplematchesforb.in theprecedingparagraph.Theregularexpressionb*requireszeroormorebs,soit matchesanystring.Thatseemstobefairlyuseless,butitmakesmoresenseaspartofa largerregularexpression.Bob*y,forexample,matchesallofBoy,Boby,andBobbybutnot Boboby. Assertions  Severalso-calledassertionsareusedtoanchorpartsof thepatterntowordorstringboundaries.The^assertionmatchesthestartofa string,sotheregularexpression^foolmatchesfoolandfoolhardybut nottomfooleryorAprilfool.Table1.5liststheassertions. Table1.5  Perl'sRegular-ExpressionAssertions Assertion Matches Example Matches Doesn'tMatch ^ Startofstring ^fool foolish tomfoolery $ Endofstring fool$ Aprilfool foolish \b Wordboundary be\bside beside beside \B Nonwordboundary be\Bside beside beside Atoms  The.(period)thatyousawinb.earlierinthischapterisan exampleofaregular-expressionatom.Atomsare,asthenamesuggests,the fundamentalbuildingblocksofaregularexpression.AfulllistofatomsappearsinTable 1.6. Table1.6  Perl'sRegular-ExpressionAtoms Atom Matches Example Matches Doesn'tMatch period(.) Anycharacterexceptnewline b.b bob bb Listofcharactersinbrackets Anyoneofthosecharacters ^[Bb] Bob,bob Rbob Regularexpressioninparentheses Anythingthatregularexpressionmatches ^a(b.b)c$ abobc abbc Quantifiers  Aquantifierisamodifierforanatom.Itcanbe usedtospecifythataparticularatommustappearatleastonce,asinb+.Theatom quantifiersarelistedinTable1.7. Table1.7  Perl'sRegular-ExpressionAtomQuantifiers Quantifier Matches Example Matches Doesn'tMatch * Zeroormoreinstancesoftheatom ab*c ac,abc abb + Oneormoreinstancesoftheatom ab+c abc ac ? Zerooroneinstancesoftheatom ab?c ac,abc abbc {n} ninstancesoftheatom ab{2}c abbc abbbc {n,} Atleastninstancesoftheatom ab{2,}c abbc,abbbc abc {nm} Atleastn,mostminstancesoftheatom ab{2,3}c abbc abbbbcat SpecialCharacters  Severalspecialcharactersaredenotedby backslashedletters,with\nbeingespeciallyfamiliartoCprogrammers,perhaps. Table1.8liststhespecialcharacters. Table1.8  Perl'sRegular-ExpressionSpecialCharacters Symbol Matches Example Matches Doesn'tMatch \d Anydigit b\dd b4d bad \D Nondigit b\Dd bdd b4d \n Newline ¡@   ¡@ \r Carriagereturn ¡@   ¡@ \t Tab ¡@   ¡@ \f Formfeed ¡@   ¡@ \s White-spacecharacter ¡@   ¡@ \S Non-white-spacecharacter ¡@   ¡@ \w Alphanumericcharacter a\wb a2b a^b \W Nonalphanumericcharacter a\Wb aa^b aabb BackslashedTokens  Itisessentialthatregularexpressionsbe capableofusingallcharacters,sothatallpossiblestringsthatoccurintherealword canbematched.Withsomanycharactershavingspecialmeanings,amechanismisrequired thatallowsyoutorepresentanyarbitrarycharacterinaregularexpression. Thismechanismisabackslash(\),followedbyanumericquantity.This quantitycantakeanyofthefollowingformats: Singleordoubledigit:matchedquantitiesafteramatch.Thesematched quantitiesarecalledbackreferencesandareexplainedinthefollowingsection. Two-orthree-digitoctalnumber:thecharacterwiththatnumberascharacter code,unlessit'spossibletointerpretitasabackreference. x,followedbytwohexadecimaldigits:thecharacterwiththatnumberas itscharactercode.\x3e,forexample,is>. c,followedbyasinglecharacter:thecontrolcharacter.\cG, forexample,matchesCtrl+G. Anyothercharacter:thecharacteritself.\&,forexample,matches the&character. Matching Nowyou'rereadytostartputtingallthatinformationtogetherwithsomerealpattern matching.Thematchoperatornormallyconsistsoftwoforwardslasheswitharegular expressioninbetween,anditnormallyoperatesonthecontentsofthe$_ variable.Soif$_isserendipity,/^ser/,/end/,and/^s.*y$/ arealltrue. Matchingon$_  The$_operatorisspecial;seeChapter13,"SpecialVariables,"forfulldetails.Inmany ways,$_isthedefaultcontainerfordatathatisbeingreadinbyPerl.The<> operator,forexample,getsthenextlinefromSTDINandstoresitin$_. Sothefollowingcodesnippetallowsyoutotypelinesoftextandtellsyouwhenyour linematchesoneoftheregularexpressions: $prompt="EntersometextorpressCtrl+Dtostop:"; print$prompt; while(<>){ /^[aA]/&&print"StartswithaorA."; /[0-9]$/&&print"Endswithadigit."; /perl/&&print"Yousaidit!"; print$prompt; } BoundMatches  Matchingdoesn'talwayshavetooperateon$_, althoughthisdefaultbehaviorisquiteconvenient.Aspecialoperator,=~, evaluatestoeithertrueorfalse,dependingonwhetheritsfirstoperandmatchesonits secondoperand.So$filename=~/dat$/istrueif$filenamematcheson/dat$/. Youcanuse=~inconditionalsintheusualway,asfollows: ?$filename=~/dat$/&&die"Can'tuse.datfiles.\n"; Acorrespondingoperator,!~,hastheoppositesense.!~istrueif thefirstoperatordoesnotmatchonthesecond,asfollows: $ENV{'PATH'}!~/perl/&&warn"Notsureifperlisinyourpath"; AlternativeDelimiters  Thematchoperatorcanusecharactersother than//-ausefulpointifyou'retryingtomatchacomplexexpressionthat involvesforwardslashes.Amoregeneralformofthematchoperatorthan//ism//. Ifyouusetheleadingm,youcanuseanycharactertodelimittheregular expression.Forexample, $installpath=~m!^/usr/local!||warn"Thepathyouhavechosenisodd.\n"; warnsthat"Thepathyouhavechosenisodd.\n"ifthevariable$installpath startswith/usr/local. MatchOptions  Youcanapplyseveraloptionalswitchestothematch operator(either//orm//)toalteritsbehavior.Theseoptionsare listedinTable1.9. Table1.9  Perl'sMatch-OperatorOptionalSwitches Switch Meaning g Performglobalmatching i Performcase-insensitivematching o Evaluatetheregularexpressiononetimeonly Thegswitchcontinuesmatchingevenafterthefirstmatchhasbeenfound. Thisswitchisusefulwhenyouareusingbackreferencestoexaminethematchedportionsof astring,asdescribedinthe"Backreferences"sectionlaterinthischapter. Theiswitchforcesacase-insensitivematch. Finally,theoswitchisusedinsideloopsinwhichagreatdealofpattern matchingistakingplace.ThisswitchtellsPerlthattheregularexpression(thematch operator'soperand)istobeevaluatedonetimeonly.Theswitchcanimproveefficiency whentheregularexpressionisfixedforalliterationsoftheloopthatcontainsit. Backreferences  Aswementionedinthe"BackslashedTokens" sectionearlierinthischapter,patternmatchingproducesquantitiesthatareknownasbackreferences. Thesequantitiesarethepartsofyourstringinwhichthematchsucceeded.Youneedto tellPerltostorethembysurroundingtherelevantpartsofyourregularexpressionwith parentheses,andyoucanrefertothemafterthematchas\1,\2,andsoon.The followingexampledetermineswhethertheusertypedthreeconsecutivefour-letterwords: while(<>){ /\b(\S{4})\s(\S{4})\s(\S{4})\b/&&print"Gosh,yousaid$1$2$3!\n"; } Thefirstfour-letterwordliesbetweenawordboundary(\b)andsomewhite space(\s),andconsistsoffournon-white-spacecharacters(\S).If thereisamatchontheexpression\b(\S{4})\s-ifafour-letterwordisfound-the matchingsubstringisstoredinthespecialvariable\1,andthesearch continues.Whenthesearchiscomplete,youcanrefertothebackreferencesas$1,$2, andsoon. Whatifyoudon'tknowinadvancehowmanymatchestoexpect?Performthematchinan arraycontext;Perlreturnsthematchesinanarray.Considerthisexample: @hits=("YonYonson,Wisconsin"=~/(\won)/g); print"Matchedon",join(',',@hits),".\n"; We'llstartattherightsideandworkbackward.Theregularexpression(\won) meansthatwematchanyalphanumericcharacterfollowedbyonandstoreallthree characters.Thegoptionafterthe//operatormeansthatwewanttodo thisfortheentirestring,evenafterwefindamatch.The=~operatormeans thatwecarryoutthisoperationonagivenstring(YonYonson,Wisconsin). Finally,thewholethingisevaluatedinanarraycontext,soPerlreturnsthearrayof matches,[email protected]: MatchedonYon,Yon,son,con. Substitution Whenyougetthehangofpatternmatching,you'llfindthatsubstitutionsarequite straightforwardandverypowerful.Thesubstitutionoperatoriss///,which resemblesthematchoperatorbuthasthreeratherthantwoslashes.Justasyoucando withthematchoperator,youcansubstituteanyothercharacterfortheforwardslashes, andyoucanusetheoptionali,g,andoswitches. Thepatterntobereplacedgoesbetweenthefirstandseconddelimiters,andthe replacementpatterngoesbetweenthesecondandthirddelimiters.Thissimpleexample changes$housefromhenhousetodoghouse: $house="henhouse"; $house=~s/hen/dog/; Noticethatitisn'tpossibletousethe=~operatorwithaliteral stringasyoucanwhenmatching,becauseyoucan'tmodifyaliteralconstant.Instead, storethestringinavariableandmodifythatvariable. FromHere... YouhavereachedtheendofyourwhirlwindtourofPerl.YousawhowPerl'sdeceptively simpleconstructscanbeusedtowritedeceptivelysimpleprograms,andyougotabrief lookatthebasicelementsofthelanguage.Atminimum,youshouldhaveaclearideaof howthelanguageworks,andyoushouldknowwheretogoformoreinformationonPerlas theneedarisesthroughouttherestofthisbook. ThisbooknowmovesontoWebmatters,butlookinthefollowingplacesformore informationaboutPerl: RefertoPartVofthisbookforcomprehensiveinformationonPerlspecialvariables, operators,andbuilt-infunctions. AlsorefertoPartVtolearnhowtousemodulesandlibrariestocompartmentalizeyour codeforgreaterrobustnessandextensibility. ConsiderbuyingabookthatdealsindetailwiththePerllanguage.Thedefinitivework isthe"Camelbook,"citedatthebeginningofthischapter. ¡@