Definition
Escapecharactersarepartoftheformalgrammarofmanyprogramminglanguages,dataformats,andcommunicationprotocols.Foragivenalphabet,thepurposeofanescapecharacteristostartacharactersequence,sothatthecharactersequenceatthebeginningoftheescapecharacterhasadifferentsemanticsthanwhenthecharactersequenceappearsalone.Therefore,thesequenceofcharactersatthebeginningoftheescapecharacteriscalledanescapesequence.
Escapesequencesusuallyhavetwofunctions.Thefirstistoencodeasyntacticentity,suchasadevicecommandorspecialdatathatcannotbedirectlyrepresentedbythealphabet.Thesecondfunction,alsocalledcharacterreference,isusedtoindicatecharactersthatcannotbeenteredbythekeyboardinthecurrentcontext(suchasthecarriagereturninastring),orcharactersthathaveundesiredmeaninginthecurrentcontext(suchasCThedoublequotecharacter"inthelanguagestringcannotappeardirectlyandmustbeexpressedbyanescapesequence).Inthelattercase,theescapesequenceisacombinationoftheescapecharacteritselfandaquotedcharacterLetter(digraph)situation.
Controlcharacters
Escapecharactersarenotcontrolcharacters;controlcharactersarenotescapecharacters.Ifthedefinitionofcontrolcharactersisanon-graphiccharacter,orCharacterswithspecialmeaningtooutputdevices(printers,textterminals),thentheescapecharactersforthesedevicesarealsocontrolcharacters.However,theescapecharactersusedinprogrammingaregraphiccharacters,sotheyarenotcontrolcharacters.Onthecontrary,mostASCIIcontrolcharactersalonehavecontrolfunctions,sotheyarenotescapecharacters.
Charactertable
Allescapecharactersandtheircorrespondingmeanings:
Escapecharacter | Meaning | ASCIIcodevalue(decimal) |
\a | Bell(BEL) | 007 |
\b | Backspace(BS),changethecurrentpositionMovetothepreviouscolumn | 008 |
\f | Pagefeed(FF),movethecurrentpositiontothebeginningofthenextpage | 012 |
\n | Linefeed(LF),movethecurrentpositiontothebeginningofthenextline | 010 |
\r | Carriagereturn(CR),movethecurrentpositiontothislineBeginning | 013 |
\t | HorizontalTabulation(HT)(skiptothenextTABposition) | 009 |
\v | VerticalTabulation(VT) | 011 |
\\ | representsabackslashcharacter``\' | 092 |
\' | representsasinglequote(apostrophe)character | 039 |
\" | representsoneDoublequotecharacter | 034 |
\? | representsaquestionmark | 063 |
\0 | Nullcharacter(NUL) | 000 |
\ddd | Anycharacterrepresentedby1to3octalnumbers | Threeoctalnumbers |
\xhh | Anycharacterrepresentedbyhexadecimal | Hexadecimal |
Note:
1.Distinguish,Slash:"/"andbackslash:"\",herearenotinterchangeable
2.\xhhhexadecimalescapedoesnotlimitthenumberofcharacters'\x000000000000F'=='\xF'
Characterconstant
ThevaluerepresentedbythecharacterconstantintheClanguageisthevaluethattheinttypecancontain.WecanuseASCIIexpressionstorepresentacharacterconstant,orusesinglequoteswithbackslashestorepresentescapecharacters.
'A','\x2f','\013';
where:\xmeansthefollowingcharacterisahexadecimalnumber,\0meansthefollowingcharacterisOctalnumber.Forexample,17indecimalisrepresentedbyhexadecimal
'\x11',andrepresentedbyoctalis'\021';
The\x,\n,\aandsoonareallcalledescapecharacters,whichtellthecompilerthattheyneedtobeprocessedinaspecialway.
Composition
EscapeString(EscapeString),thatis,CharacterEntity(CharacterEntity)isdividedintothreeparts:thefirstpartisan&symbol,calledampersandinEnglish;thesecondpartistheentity(Entity)nameor#plustheentity(Entity)number;thethirdpartisasemicolon.
Forexample,todisplaythelessthansign(
Theadvantageofusingtheentitynameisthatitiseasiertounderstand.Whenyoulookatlt,youprobablyguessthemeaningoflessthan,butitsThedisadvantageisthatnotallbrowserssupportthelatestEntityname.Entitynumbercanbehandledbyvariousbrowsers.
Tips:Entitynameiscasesensitive.
Note:Thesamesymbolcanbequotedintwoways:"EntityName"and"EntityNumber".Theadvantageof"EntityName"isthatitiseasytoremember,butitdoesnotguaranteethatallbrowserscanrecognizeitsmoothly.It,andthe"entitynumber"doesnothavesuchconcerns,butitisreallyinconvenienttoremember.
Programminglanguage
SuchasHTMLescapecharacter,javaescapecharacter,xmlescape,Oracleescape,sqlescape,sqlserverescape,phpescape,asp,vb,javascript,etc.,aswellasthepercentsignintheURL.
Forexample,HTML<>&"©istheescapecharacterof&,",©;respectively
XMLhasonly5escapecharacters:<>&"'
Function
EscapeSequence(EscapeSequence)isalsocalledCharacterEntity.InHTML,thedefinitionofescapeTherearetworeasonsforstrings:Thefirstreasonisthatsymbolslike""havebeenusedtorepresentHTMLtags,sotheycannotbeuseddirectlyassymbolsintext.InordertousethesesymbolsinHTMLdocuments,theyneedtobedefinedItsescapedstring.Whentheinterpreterencountersthistypeofstring,itwillinterpretitasarealcharacter.Whenenteringanescapedstring,strictlyfollowtherulesoflettercase.ThesecondreasonisthatsomeCharactersarenotdefinedintheASCIIcharacterset,sotheyneedtoberepresentedbyescapedstrings.
Infact,inallprogramminglanguages,therearebasicallytworeasonsforhavingescapecharacters:1.UseescapecharacterstorepresentThecharactersdefinedinthecharacterset,suchasthecontrolcharactersinASCllandthecharacterssuchascarriagereturnandlinefeed,havenoready-madetextcodes.Therefore,theycanonlyberepresentedbyescapecharacters.Second,certainspecificcharactersareusedintheeditinglanguageDefinedasspecial-purposecharacters.Becausethesecharactersaredefinedasspecial-purposecharacters,theylosetheiroriginalmeaning.Forexample,inHtml,
Itcanalsobeseenfromtheabovethatescapingisnothingmorethantwosituations:1:Convertordinarycharacterstospecialpurposes,generallyusedinprogramminglanguagestorepresentcharactersthatcannotbedirectlydisplayed,suchasthebackkey,enterkey,etc.2:Usedtoconvertthecharacterswithspecialmeaningbacktotheiroriginalmeaningrighteous.Generallyusedinregularexpressions.Therearealsosomescriptinglanguagesthatareweaklytyped,somelanguagessuchashtmlarenotprogramminglanguages,butmarkuplanguages,andsomelanguageshaveonlyonetypesuchasshellscriptinglanguages.Thestringsintheselanguagesarenotquoted"",ortheycanbeomittedQuotationmarks"",sosometimesitisnecessarytoescapecharacterstoindicatethatacharacter'sidentityatthistimeisanordinarycharacter,ratherthanametacharacterwithspecialmeaning.
Inaddition,forthesafetyofthewebsite.Beforedataiswrittentothedatabase,somesensitivecharactersareescapedusingescapecharacters(functions).Thiscanpreventsomepeoplewithulteriormotivesfromusingspecialsymbolinjectionattacks.
Showspaces
Usually,HTMLwillautomaticallycutoffextraspaces.Nomatterhowmanyspacesyouadd,itwillbetreatedasonespace.Forexample,ifyouadd10spacesbetweentwowords,HTMLwillcut9spacesandonlykeepone.Inordertoaddaspaceinthewebpage,youcanuse''toindicateaspace.
url
Inwebdevelopment,whenpassingavalueintheaddressbarofthebrowserthroughtheEnglishquestionmark(?).Thebrowserdistinguishesthenumberofparametersafterthequestionmarkthrough"&".Ifthereisan"&"inthevalueparameter,anerrorwillappearonthereceivingpage,similartothefollowingrequestpath:/next.jsp?param1=hendhs89&furej¶m2=sss
ParametersThereisanescapecharacter"&"inparam1,whichwillcausetheparameterreceivingerroroftherequestedpage.
Afterencodingbyjava.net.URLEncoder.encode(param1)beforepassingthevalue,theescapecharactercanbeconvertedtohexadecimal;
Representsaspace | %2B |
Theplussign+or%20shouldbeusedinstead | %20 |
SeparateddirectoriesAndsubdirectories | %2F |
SeparatetheactualURLandtheparameters | %3F |
Specifyspecialcharacters | %25 |
meansbookmark | %23 |
TheseparatorbetweentheparametersspecifiedintheURL | %26 |
ThevalueofthespecifiedparameterintheURL | %3D |
ThevalueofthespecifiedparameterintheURL | %21 |
