72 reRegular expression operationsPython v275 documentation.docx

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72reRegularexpressionoperationsPythonv275documentation

7.2.re—Regularexpressionoperations—Pythonv2.7.5documentation

7.2.—Regularexpressionoperations

ThismoduleprovidesregularexpressionmatchingoperationssimilartothosefoundinPerl.BothpatternsandstringstobesearchedcanbeUnicodestringsaswellas8-bitstrings.

Regularexpressionsusethebackslashcharacter（'\'）toindicatespecialformsortoallowspecialcharacterstobeusedwithoutinvokingtheirspecialmeaning.ThiscollideswithPython’susageofthesamecharacterforthesamepurposeinstringliterals;forexample,tomatchaliteralbackslash,onemighthavetowrite'\\\\'asthepatternstring,becausetheregularexpressionmustbe\\,andeachbackslashmustbeexpressedas\\insidearegularPythonstringliteral.

ThesolutionistousePython’srawstringnotationforregularexpressionpatterns;backslashesarenothandledinanyspecialwayinastringliteralprefixedwith'r'.Sor"\n"isatwo-characterstringcontaining'\'and'n',while"\n"isaone-characterstringcontaininganewline.UsuallypatternswillbeexpressedinPythoncodeusingthisrawstringnotation.

Itisimportanttonotethatmostregularexpressionoperationsareavailableasmodule-levelfunctionsandmethods.Thefunctionsareshortcutsthatdon’trequireyoutocompilearegexobjectfirst,butmisssomefine-tuningparameters.

Seealso

∙MasteringRegularExpressions

∙BookonregularexpressionsbyJeffreyFriedl,publishedbyO’Reilly.ThesecondeditionofthebooknolongercoversPythonatall,butthefirsteditioncoveredwritinggoodregularexpressionpatternsingreatdetail.

7.2.1.RegularExpressionSyntax

Aregularexpression（orRE）specifiesasetofstringsthatmatchesit;thefunctionsinthismoduleletyoucheckifaparticularstringmatchesagivenregularexpression（orifagivenregularexpressionmatchesaparticularstring,whichcomesdowntothesamething）.

Regularexpressionscanbeconcatenatedtoformnewregularexpressions;ifAandBarebothregularexpressions,thenABisalsoaregularexpression.Ingeneral,ifastringpmatchesAandanotherstringqmatchesB,thestringpqwillmatchAB.ThisholdsunlessAorBcontainlowprecedenceoperations;boundaryconditionsbetweenAandB;orhavenumberedgroupreferences.Thus,complexexpressionscaneasilybeconstructedfromsimplerprimitiveexpressionsliketheonesdescribedhere.Fordetailsofthetheoryandimplementationofregularexpressions,consulttheFriedlbookreferencedabove,oralmostanytextbookaboutcompilerconstruction.

Abriefexplanationoftheformatofregularexpressionsfollows.Forfurtherinformationandagentlerpresentation,consulttheRegularExpressionHOWTO.

Regularexpressionscancontainbothspecialandordinarycharacters.Mostordinarycharacters,like'A','a',or'0',arethesimplestregularexpressions;theysimplymatchthemselves.Youcanconcatenateordinarycharacters,solastmatchesthestring'last'.（Intherestofthissection,we’llwriteRE’sinthisspecialstyle,usuallywithoutquotes,andstringstobematched'insinglequotes'.）

Somecharacters,like'|'or'（',arespecial.Specialcharacterseitherstandforclassesofordinarycharacters,oraffecthowtheregularexpressionsaroundthemareinterpreted.Regularexpressionpatternstringsmaynotcontainnullbytes,butcanspecifythenullbyteusingthe\numbernotation,e.g.,'\x00'.

Thespecialcharactersare:

∙'.'

∙（Dot.）Inthedefaultmode,thismatchesanycharacterexceptanewline.Iftheflaghasbeenspecified,thismatchesanycharacterincludinganewline.

∙'^'

∙（Caret.）Matchesthestartofthestring,andinmodealsomatchesimmediatelyaftereachnewline.

∙'$'

∙Matchestheendofthestringorjustbeforethenewlineattheendofthestring,andinmodealsomatchesbeforeanewline.foomatchesboth‘foo’and‘foobar’,whiletheregularexpressionfoo$matchesonly‘foo’.Moreinterestingly,searchingforfoo.$in'foo1\nfoo2\n'matches‘foo2’normally,but‘foo1’inmode;searchingforasingle$in'foo\n'willfindtwo（empty）matches:

onejustbeforethenewline,andoneattheendofthestring.

∙'*'

∙CausestheresultingREtomatch0ormorerepetitionsoftheprecedingRE,asmanyrepetitionsasarepossible.ab*willmatch‘a’,‘ab’,or‘a’followedbyanynumberof‘b’s.

∙'+'

∙CausestheresultingREtomatch1ormorerepetitionsoftheprecedingRE.ab+willmatch‘a’followedbyanynon-zeronumberof‘b’s;itwillnotmatchjust‘a’.

∙'?

'

∙CausestheresultingREtomatch0or1repetitionsoftheprecedingRE.ab?

willmatcheither‘a’or‘ab’.

∙*?

+?

?

?

∙The'*','+',and'?

'qualifiersareallgreedy;theymatchasmuchtextaspossible.Sometimesthisbehaviourisn’tdesired;iftheRE<.*>ismatchedagainst'

title

'.Adding'?

'afterthequalifiermakesitperformthematchinnon-greedyorminimalfashion;asfewcharactersaspossiblewillbematched.Using.*?

inthepreviousexpressionwillmatchonly'

'.

∙{m}

∙SpecifiesthatexactlymcopiesofthepreviousREshouldbematched;fewermatchescausetheentireREnottomatch.Forexample,a{6}willmatchexactlysix'a'characters,butnotfive.

∙{m,n}

∙CausestheresultingREtomatchfrommtonrepetitionsoftheprecedingRE,attemptingtomatchasmanyrepetitionsaspossible.Forexample,a{3,5}willmatchfrom3to5'a'characters.Omittingmspecifiesalowerboundofzero,andomittingnspecifiesaninfiniteupperbound.Asanexample,a{4,}bwillmatchaaaaborathousand'a'charactersfollowedbyab,butnotaaab.Thecommamaynotbeomittedorthemodifierwouldbeconfusedwiththepreviouslydescribedform.

∙{m,n}?

∙CausestheresultingREtomatchfrommtonrepetitionsoftheprecedingRE,attemptingtomatchasfewrepetitionsaspossible.Thisisthenon-greedyversionofthepreviousqualifier.Forexample,onthe6-characterstring'aaaaaa',a{3,5}willmatch5'a'characters,whilea{3,5}?

willonlymatch3characters.

∙'\'

∙Eitherescapesspecialcharacters（permittingyoutomatchcharacterslike'*','?

',andsoforth）,orsignalsaspecialsequence;specialsequencesarediscussedbelow.Ifyou’renotusingarawstringtoexpressthepattern,rememberthatPythonalsousesthebackslashasanescapesequenceinstringliterals;iftheescapesequenceisn’trecognizedbyPython’sparser,thebackslashandsubsequentcharacterareincludedintheresultingstring.However,ifPythonwouldrecognizetheresultingsequence,thebackslashshouldberepeatedtwice.Thisiscomplicatedandhardtounderstand,soit’shighlyrecommendedthatyouuserawstringsforallbutthesimplestexpressions.

∙[]

∙Usedtoindicateasetofcharacters.Inaset:

∙Characterscanbelistedindividually,e.g.[amk]willmatch'a','m',or'k'.

∙Rangesofcharacterscanbeindicatedbygivingtwocharactersandseparatingthembya'-',forexample[a-z]willmatchanylowercaseASCIIletter,[0-5][0-9]willmatchallthetwo-digitsnumbersfrom00to59,and[0-9A-Fa-f]willmatchanyhexadecimaldigit.If-isescaped（e.g.[a\-z]）orifit’splacedasthefirstorlastcharacter（e.g.[a-]）,itwillmatchaliteral'-'.

∙Specialcharacterslosetheirspecialmeaninginsidesets.Forexample,[（+*）]willmatchanyoftheliteralcharacters'（','+','*',or'）'.

∙Characterclassessuchas\wor\S（definedbelow）arealsoacceptedinsideaset,althoughthecharacterstheymatchdependsonwhetherormodeisinforce.

∙Charactersthatarenotwithinarangecanbematchedbycomplementingtheset.Ifthefirstcharacterofthesetis'^',allthecharactersthatarenotinthesetwillbematched.Forexample,[^5]willmatchanycharacterexcept'5',and[^^]willmatchanycharacterexcept'^'.^hasnospecialmeaningifit’snotthefirstcharacterintheset.

∙Tomatchaliteral']'insideaset,precedeitwithabackslash,orplaceitatthebeginningoftheset.Forexample,both[（）[\]{}]and[]（）[{}]willbothmatchaparenthesis.

∙'|'

∙A|B,whereAandBcanbearbitraryREs,createsaregularexpressionthatwillmatcheitherAorB.AnarbitrarynumberofREscanbeseparatedbythe'|'inthisway.Thiscanbeusedinsidegroups（seebelow）aswell.Asthetargetstringisscanned,REsseparatedby'|'aretriedfromlefttoright.Whenonepatterncompletelymatches,thatbranchisaccepted.ThismeansthatonceAmatches,Bwillnotbetestedfurther,evenifitwouldproducealongeroverallmatch.Inotherwords,the'|'operatorisnevergreedy.Tomatchaliteral'|',use\|,orencloseitinsideacharacterclass,asin[|].

∙（...）

∙Matcheswhateverregularexpressionisinsidetheparentheses,andindicatesthestartandendofagroup;thecontentsofagroupcanberetrievedafteramatchhasbeenperformed,andcanbematchedlaterinthestringwiththe\numberspecialsequence,describedbelow.Tomatchtheliterals'（'or'）',use\（or\）,orenclosetheminsideacharacterclass:

[（][）].

∙（?

...）

∙Thisisanextensionnotation（a'?

'followinga'（'isnotmeaningfulotherwise）.Thefirstcharacterafterthe'?

'determineswhatthemeaningandfurthersyntaxoftheconstructis.Extensionsusuallydonotcreateanewgroup;（?

P...）istheonlyexceptiontothisrule.Followingarethecurrentlysupportedextensions.

∙（?

iLmsux）

∙（Oneormorelettersfromtheset'i','L','m','s','u','x'.）Thegroupmatchestheemptystring;theletterssetthecorrespondingflags:

（ignorecase）,（localedependent）,（multi-line）,（dotmatchesall）,（Unicodedependent）,and（verbose）,fortheentireregularexpression.（Theflagsaredescribedin.）Thisisusefulifyouwishtoinclu