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Specificandlimitedrepetition
54
Cleaning messily-spaced data
Sometimes you come across a text or PDF file that contains data that you want to put in a
spreadsheet:
Red Apples
Green Oranges
Blue Pears
$10
$50
$100
IfyouattempttopastethatintoExcel,however,youendupwith:
ThedataisjustabunchoftextcrammedintoasingleExcelcell
ThatmaylookOK,butthat’snotwhatwewant. Wewanteachdatapointto occupyitsowncell,
likeso:
EachdatapointisseparatedbyacolumninExcel
Inorder forthisto happen, Excelneedssomehelp. Wehavetotell itthatwheretherearethree
spacesormore that’swhereadatapointends. Theeasiestwaytodo thatistoreplace thoseextra
spaceswithasymbol–i.e. adelimiter–thatExcelwillinterpretasacolumnbreak.
The most common symbol is a comma. Using the curly braces notation, we can replace the
consecutivespaceswithacomma:
Find
{3,}
Replace
,
Whichconvertsthetextto:
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Specificandlimitedrepetition
55
Red Apples,Green Oranges,Blue Pears
$10,$50,$100
Ifyousavethattextasa.CSV(comma-separatedvalues)file,Excelwillseethosecommasandwill
knowthatthosecommasaremeanttorepresentcolumnbreaks.
Whatifwedidn’tknowhowtousecurlybracesandinsteadusedthe
+
operator?Thenwewould’ve
putcommasineveryspace:
Theredareasrepresentthematchedwhitespace
Thisofcoursemeanscommasbetweeneachword:
Red,Apples,Green,Oranges,Blue,Pears
$10,$50,$100
AndifExcelinterpretseachcommaasacolumnbreak,weendupwith:
Extra,unwantedcolumnbreaks
Whenyoudon’tneedaplus,thecurlybracesareanicealternative. We’llbeusingthemalotwhen
weneedtoperformmoreprecisematching.
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Anchors: A way to trim emptiness
Ina[previouschapter]{#plus-operator},welearnedhow tomatchconsecutivenewlinecharacters.
Thisishandywhenwewanttoremoveblanklinesfromatextfile.
Anotherannoyingwhitespaceproblemoccurswhenthereispaddingbetweenthebeginningofthe
lineandthefirstactualtextcharacter.Forexample,giventhesehaikus¹⁶:
dowhatyoufeellikesincetheworkisabandonedthelawdoesn’tcare
ifyouusethiscodeyouandyourchildren’schildrenmustmakeyoursourcefree
Iwantthemtobeformattedsothattheleadingwhitespaceisremovedandthelinesareflushagainst
theleft-margin:
dowhatyoufeellikesincetheworkisabandonedthelawdoesn’tcare
ifyouusethiscodeyouandyourchildren’schildrenmustmakeyoursourcefree
These“licensehaikus”arecourtesyofAaronSwartz¹⁷
The caretas starting anchor
Weneedawaytomatchthebeginningoftheline. Thinkofitasaword-boundary‐remember
\b
?–exceptthatwewantaboundaryfortheentireline.
Tohavearegexincludethebeginningoftheline,usethecaret:
ˆ
Thefollowingregex:
^hello
–willmatchtheword
hello
if
hello
occursatthebeginningoftheline,asinthefollowingexample:
hellogoodbye
However,
ˆhelloˆ
willnotmatchthe
hello
inthefollowingline:
goodbyehello
¹⁶
http://www.aaronsw.com/weblog/000360
¹⁷
http://www.aaronsw.com/weblog/000360
56
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Anchors:Awaytotrimemptiness
57
So,inordertoremoveunneededspacesatthebeginningofaline:
InEnglish We’relookingtomatchallwhitespacecharactersthatoccuratthebeginningofaline.
Wewanttoreplacethosecharacterswithnothing(i.e. wewanttodeletethem).
Find
ˆ +
Replace (withnothing)
Thisregexonly affectslinesthatbegin with one-or-morewhitespace characters. For thoselines,
thosewhitespacecharactersareremoved.
Exercise: Addacommon directorytoa listof files
Asadigitalphotographer,Irunintothisscenarioallthetime:I’vetakenabunchofphotosandwant
tosendthemtosomeoneacrosstheworld.Butwithmegapixelsbeingsoplentifultoday,thesefiles
intheirrawformatcanweighinasmuchas10MBto20MB–toobigtosendasemailattachments.
Andthisclientdoesn’thaveasharedDropbox.
SotheeasiestwaytosendthemistouploadthemtoanFTPorwebserver. Onmycomputer,Ido
thisbyopeningthefolder,selectingallthefiles,anddraggingthemtomyserver:
Movingfiles
Theclientcanthendownloadthefilesattheir leisure. Whatwas
dog.jpg
inmylocalharddrive
cannowbefoundat:
http://www.example.com/images/client-joe/dog.jpg
ButhowdoItelltheclientthis? Well,first,Iselectallthefilenamesonmylocalharddriveanddo
acopy-and-pasteintoatexteditor. Igetsomethinglike:
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Anchors:Awaytotrimemptiness
58
dog.jpg
cat.jpg
mouse.jpg
Afterthisit’seasyenoughtojustcopyandpastetheremotepath–
http://www.example.com/images/client-joe/dog.jpg
–3orsotimes.Butwhydothatwhen,withregularexpressions,wecandoitinonefellswoop?
Usethecaretanchortoprependtheremotepathtothefilenames:
http://www.example.com/images/client-joe/dog.jpg
http://www.example.com/images/client-joe/cat.jpg
http://www.example.com/images/client-joe/mouse.jpg
Answer
Simply“replace”thecaretwiththeremotefilepath:
Find
ˆ
Replace
http://www.example.com/images/client-joe/
Technically,nothinggetsreplaced,perse.Thecaretsimplymarksthebeginningofthelineandthe
replacementactioninsertsthedesiredtextthere.
Thisisaquicktimesaverwhenyoujusthavetoaddsomethingtoadozenorsolines.
The dollar sign as the ending anchor
Ifthere’sawaytomatchthebeginning ofaline,thentheremustbeawayto matchtheendofa
line.
Tohavearegexincludetheendoftheline,usethedollar-sign:
bye$
–thiswillmatchtheword
bye
ifitoccursattheendoftheline,asinthefollowingexample:
world,goodbye
However,itwillnotmatchthe
bye
inthefollowingline:
goodbyeworld
Exercise: Addacommon filenametoa listof directories
Thisisavariationtothepreviousfilepathproblem. Wehavealistofdirectories:
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Anchors:Awaytotrimemptiness
59
http://example.com/events/
http://example.com/people/
http://example.com/places/
Andweneedareferencetoacommonfilename:
http://example.com/events/index.html
http://example.com/people/index.html
http://example.com/places/index.html
Answer
Find
$
Replace
index.html
Strippingend-of-linecharacters
Comma-delimitedfiles(also knownasCSVs,comma-separatedvalues) isacommontextformat
fordata. InorderforaprogramlikeExceltoarrangeCSVdataintoaspreadsheet,itusesthecommas
todeterminewherethe“columns”are.
Forexample:
Item,Quantity,In Season?
Apples,50,Yes
Oranges,109,No
Pears,12,Yes
Kiwis,72,Yes
InExcel,thedatalookslikethis:
CSVinExcel
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Anchors:Awaytotrimemptiness
60
CSVisa popular formatandyou’llrun into itifyougetinto thehabitofrequesting data from
governmentalagencies.Unfortunately,itwon’talwaysbeperfect.
Acommonproblem–albeittrivial–willoccurwhenexportingdatafromExceltoCSV.Ifthelast
columnismeanttobeemptybutisn’t(notalldatabasesarewellmaintained),everylineintheCSV
filewillhaveatrailingcomma:
Item,Quantity,In Season?,
Apples,50,Yes,
Oranges,109,No,
Pears,12,Yes,
Kiwis,72,Yes,
Thisusuallywon’tcauseproblems, especially in modern spreadsheetssuch asExcelandGoogle
Docs. Butolder databaseimportprogramsmightprotest. Andifyou’reevenalittleOCD, those
superfluouscommaswillbotheryou. Solet’swipethemoutwithasingleregex.
Inefficient method: remove and replace newlines If you fully grok newline characters, you
realizeyoucanfixtheproblembytargetingthepatternof:acommafollowedbyanewlinecharacter:
Find
,\n
Replace
\n
Thisalmost works,exceptyouwon’tcatchtheverylastline:
Item,Quantity,In Season?
Apples,50,Yes
Oranges,109,No
Pears,12,Yes
Kiwis,72,Yes,
Whywasn’tthattrailingcommafromthefinallinedeleted? Thepatternweusedlookedonlyfor
commasfollowedbyanewlinecharacter. So ifthatfinallineisthefinal line, thereisno other
newlinecharacter. Sowehavetodeletethatcommamanually.
Efficientmethod: leave thenewlinesalone Besidesthatnaggingmanualdeletionstep(which
quickly becomes annoying if you’recleaning dozensof files), our Find-and-Replace justseems
alittlewasteful. Allwewant isto deletethetrailing comma, butweend upalso deleting (and
reinserting)anewlinecharacter.
Anchors:Awaytotrimemptiness
61
Usingthe
$
,however,requiresnoextrareplacementaction.Itsimplyassertsthatthepatterncontains
theend-of-the-line,whichisn’tanactualcharacter,perse,inthetext.
Inotherwords,the
$
onlydetectstheend-of-the-line,andsonoreplacementisactuallydoneatthat
position.
Try it out yourself. In your Find field, match the
$
operator and then replace itwith nothing:
nothingwillhappentothetext(comparethistoreplacing
\n
withnothing).
Solet’sstripthetrailingcommasusingthe
$
operator:
Find
,$
Replace (withnothing)
InEnglish Wewanttofindthecommarightbeforetheend-of-the-line
Escapingspecial characters
Ifthecaretandthedollarsigndenotethebeginningandtheendoftheline, respectively,thenit
followsthatthefollowingpatternsdon’tmakemuchsense:
abc^Bye
Hello$world
Howwould
abc
comebeforethebeginningofthelineyetbeonthesamelineas
Bye
?Orhowcould
world
beonthesamelineas
Hello
andtheendofthelinecharacter,
$
?
So,withwhatweknowsofar,itmakessensethatthecaretanddollarsigntypicallyserveasbookends
foraregexpattern. However,thefollowingregexmaybeconfusing:
He gave me \$10+
Thisregexmatchesthetextinboldbelow:
Hegaveme$100yesterday
Thekeycharacterhereisourfriendthebackslash. We’veseenhowthebackslash,whenpreceding
theletters
b
and
n
,givethemspecialmeaning:awordboundaryandnewlinecharacter,respectively.
Whathappenswhenthebackslashprecedesanalready specialcharacter,suchasthedollar sign?
Thenit’sjustaliteraldollarsign,asweseeintheaboveexample.
Exercise: Removeleadingdollarsigns Giventhislistofdollaramountsinwhichthedollarsign
ismistakenlyrepeated:
Anchors:Awaytotrimemptiness
62
$$100
$$200
$$399
Removetheleadingdollarsignwitharegexthatcontainsthecaret.
$100
$200
$399
Answer
Find
ˆ\$
Replace (withnothing)
Anchorcharactersarenotonlyprettyeasytoremember,they’reveryusefulinmanyregexpatterns.
I use it frequently for data-cleaning, where a text file may contain unwanted spaces or junk
charactersatthebeginningorendofeachline.
Matching any letter, any number
Ofcourse, we’llneed to work withactual alphabeticalletters, numbers, and symbolsother than
whitespaceandnewlines. Regularexpressionshaveasetofshortcutsandsyntaxforthispurpose.
The numericcharacter class
Usingourfriendthebackslash,wecanturnaliteralletterintoaspecialcharactertomatchnumbers.
Whentheletter
d
isprecededbyabackslash,thepatternnolongermatchestheliteralletter
d
,but
anynumericaldigitfrom
0
to
9
:
\d+
Matches:
Thereare10010sheep
Wereferto
\d
asa“characterclass”or“characterset”becauseitaffectsasetofcharacters. Inthis
case,
\d
matchestheset(i.e.“class”)ofcharactersthatarenumbers.
Let’stryoutthe
\d
inasimpleexercise.
Exercise: Redactthenumbers
Giventhisphrase:
TherobberywasreportedonJanuary12, 2013. At9:40,thesuspectisallegedtohave
enteredthepizzeriaat120Broadwayandstolen9bagsoforegano.
Replaceallthenumberswiththeletter
X
63
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