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Lazinessandgreediness
104
Withthestaroperator,wecanrepeatacatch-allpatternforeithersituation. Thispattern–
.*?
neednotbecapturing,sinceallwecareaboutisthe
href
attribute:
Find
<a.*?href="(.+?)".*?>(.+?)</a>
Replace
[\2](\1)
Note: Whiletheregexpatternswe’velearnedareveryflexible,donotthinkthatthere’samasterful
regexthatdealswithalltheedgecasesfloatingoutontheWeb. Theexercisesabovearejustmeant
tobesimpleexamplesandtheymayworkforsimplereallifescenarios,suchaswhenalltheHTML
you’redealingwithcomesfromonesource.
Ifyou’rewondering,“Well,thenhowarewesupposedtoparseHTML?”That’ssomethingyouwant
tolearnalittleprogrammingandscriptingfor.LibrariessuchasNokogiriforRuby²²andBeautiful
SoupforPython²³handletheheavyliftingofcorrectlyparsingHTML.(Ifyou’rewondering,“Why
parseHTMLinthefirstplace?” Webscraping –theautomaticextractionofdataanduseful bits
fromwebsite–isacommonusecase.)
Thedifferencebetweenlazinessandgreedinessissubtle,butit’simportanttounderstanditwhen
you’retryingtodoacatch-allpatternthatyou’dprefernotcatcheverything. Infact,thelazyversion
ofaregexwillgenerallybemoreusefulthanthegreedyversionwhenitcomestoreallifedirtytext
processing.
²²
http://nokogiri.org/
²³
http://www.crummy.com/software/BeautifulSoup/
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Lookarounds
Thischapter coversa regextechniquethatallows usto test if a pattern exists without actually
replacingit.
Considerthefollowingtext:
100 BROADWAY ST. NEW W YORK NY 10006 UNITED STATES
Turnitto:
100 BROADWAY ST., NEW YORK, NY, 10006, UNITED D STATES
TKDOTOD
New York NY 10006 United States
Brooklyn NY 11035 United States
Queens NY 12006 United States
Andconsiderthedesiredtransformation: puttingacommaafterthecityname:
New York, NY 10006 United States
Brooklyn, NY 11035 United States
Queens, NY 12006 United States
Theregexissimpleenoughwithtwocapturinggroups:
Find:
([A-Za-z ]+) ([A-Z]{2} \d{5})
Replace:
\1, \2
All wedo with thesecond partof thepattern – e.g.
NY 10006
–isecho itback, after the first
backreferencehasacommaappendedtoit. Yetitseemsourregexneedstoconfirmthatthesecond
partexists.
Lookaroundsallowustotestforthatexistencewithoutusinganothercapturinggroupjusttoecho
these“tested”characters.
It’spossibletogetbywithoutknowinglookaroundsusingeverythingwe’velearnedsofar.Sodon’t
sweatitifitseemstoocomplicated. Lookaroundsarejustausefultechniqueandshedsomelight
ontheinnerworkingsoftheregexengine,somethingthisbookdoesnotspendmuchtimethinking
about.
Positive lookahead
Apositivelookaheadisdenotedwithaquestion-mark-and-equalssigninsideofparentheses:
105
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Lookarounds
106
cat(?=s)
–willmatchaninstanceof
cat
thatisimmediatelyfollowedbyan
s
character.
Thecatisoutsidewhiletheothercatsareinside
However,forthepurposesofFind-and-Replace,only
cat
willbematchedandreplaced.
Inthegiventext:
Howmuchwoodcanawoodchuckchuckifwoodchuckcouldchuckwood
ThisFind-and-Replacepattern:
Find
wood(?=chuck)
Replace
stone
Resultsin: >Howmuchwoodcanastonechuckchuckifstonechuckcouldchuckwood
Exercise
Deletethecommasthatareusedasnumberdelimitersbelow:
Original:
1,000dachshunds,ofthebrown-coloredvariety,ran12,000laps.
Fixed:
1000dachshunds,ofthebrown-coloredvariety,ran12000laps
Answer
Find
,(?=\d+)
Replace (withnothing)
Noticehowthereplacementdidn’taffectthenumbersmatchedwith
\d+
. Thepositivelookahead
onlyverifiesthatthosenumbersexistaheadofacomma;theregexengineonlycaresaboutreplacing
thatcomma.
Comparethattohavingtousecapturinggroups:
Find
,(\d+)
Replace
$1
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Lookarounds
107
Negative lookahead
Anegativelookaheadworksthesameasthepositivevariation,exceptthatitlooksforthespecified
patterntonotexist. Insteadofanequalssign,weuseanexclamationmark:
cat(!=s)
–willmatchaninstanceof
cat
thatisnotimmediatelyfollowedbyan
s
character.
Inthegiventext:
Howmuchwoodcanawoodchuckchuckifwoodchuckcouldchuckwood
ThisFind-and-Replacepattern:
Find
wood(?!chuck)
Replace
stone
Resultsin: >Howmuchstonecanawoodchuckchuckifwoodchuckcouldchuckstone
Exercise
Replacethecommasthatareusedtoseparatesentenceclauseswithdouble-hyphens.Donotreplace
thecommasusedasnumber-delimiters:
Original:
1,000dachshunds,ofthebrown-coloredvariety,ran12,000laps.
Fixed:
1000dachshunds–ofthebrown-coloredvariety–ran12000laps
Answer
Find
,(?!\d+)
Replace
--
Positive lookbehind
Thelookbehindiswhatyouexpect: matchapatternonlyif agivenpatterndoesnotimmediately
precedeit. Thefollowingpatternmatchesan
s
characterthatisprecededby
cat
:
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Lookarounds
108
`(?<=cat)s`
Giventhisphrase:
Howmuchwoodcanawoodchuckchuckifwoodchuckcouldchuckwood
ThefollowingFind-and-Replace:
Find
(?<=wood)chuck
Replace
duck
Resultsin: >Howmuchwoodcanawoodduckchuckifwoodduckcouldchuckwood.
Thelimits oflookbehinds
Themaincaveatwithlookbehindsisthatarenotsupportedacrossallregexflavors. Andevenfor
theregexflavorsthatdosupportlookbehinds,theyonlysupportalimitedsetofregexsyntax.
Essentially,youmayrunintoproblemswithlookbehindsthatattempttomatchapatternofvariable
length.
Thisisgenerallykosher
(?<=\d),
Thislikelywillnot work
(?<=\d+)
Thedifferenceisthatthefirstpatternisjustonedigit. Thelattercouldbeonedigitorathousand.
Theregexenginecan’thandlethatvariability.Regular-expressions.infohasanexcellentexplainer²⁴.
Inother words,uselookbehindswithcaution. We’llexaminesomealternativesattheendofthis
chapter.
Exercise
Giventhefollowingdatelistings:
²⁴
http://www.regular-expressions.info/lookaround.html
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Lookarounds
109
3/12/2010
4/6/2001
11/17/2009
Replacethefour-digityearswithtwo-digits:
3/12/2010
4/6/2001
11/17/2009
Answer
Find
(?<=/)\d{2}(?=\d{2})
Replace (withnothing)
Negative lookbehind
Sameconceptandlimitsasthepositivevariation,withthedifferencethatwe’reverifyingthenon-
existenceofaprecedingpattern. Thesyntaxissimilarexceptanexclamationmarktakestheplace
oftheequalssign.
Thefollowingpatternmatchesan
s
characterthatisnotprecededby
cat
:
`(?<!cat)s`
Inthegiventext:
Howmuchwoodcanawoodchuckchuckifwoodchuckcouldchuckwood
ThefollowingFind-and-Replace:
Find
(?<!wood)chuck
Replace
shuck
Resultsin:
Howmuchwoodcanawoodchuckshuckifwoodchuckcouldshuckwood
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Lookarounds
110
Exercise
Giventhefollowingtext:
Charlesspent20.5atthechatusserie.Hethenwithdrew40.5fromtheATM.
Replacetheperiodsusedasdecimalpointswithcommas:
Charlesspent20,5atthechatusserie.Hethenwithdrew40,5fromtheATM.
Answer
Find
(?<![ˆ\d])\.
Replace
,
The importance of zero-width
I’veskipped
Thatissofetch. Hecanfetchthecat.
Regexes in Real Life
#FromTexttoDatatoVisualization(TODO)
Ifyouworkwithdataandbelievedataissomethingthatcomesfromaspreadsheetordatabase,then
thisiswhereregularexpressionscanbecomeanindispensable.
Why learn Excel?
Butwhydata? Whyspreadsheets?
(todo)
The limits of Excel (todo)
Delimitation
Comma-separatedvalues(CSV)
CSVfilesusecommastoseparatethedatafields.Thus,whenyouopenaCSVfileusingExcel,Excel
usescommastodeterminewherethecolumnsare.
Note: Ifyoudon’thaveExcel,youmay beabletofollowalongwithanotherspreadsheetprogram,
likeGoogleDrive. However,thepointofthisnextstepisatrivialdemonstration,becausethepoint
ofthischapteristoshowyouhowtomanagedata_without_Excel. Soallwe’redoinghereisjusta
point-and-clickexercise.
1. Savethefollowingtextasatextfile;youcanuse
.txt
asthefileextension:
111
Lookarounds
112
City,Country
Albuquerque,USA
Istanbul,Turkey
Paris,France
Hamburg,Germany
2. Modernspreadsheetprogramswillsurmisethatthetextfileisdelimited. Excel,forexample,
willpop-upitsTextImportWizard:
Openinga.txtfileinExcel2011
3. Althoughit’spretty obvioushere, Excelthenasksyouto tell itwhatdelimitercharacter it
shoulduse.BychoosingComma,wecanseeapreviewofhowthecolumnswillbearranged:
Lookarounds
113
SelectingadelimiterinExcel’sTextImportWizard
4. Afterthat,wehaveatextfileinspreadsheet-manipulableformat:
Thetextasaspreadsheet
Exercise: MakeCSVdata fromanaddresslist
Soturning CSV-delimitedtextto spreadsheetdataiseasy. butwhatifwewantto getturn non-
delimitedtextintoaspreadsheet? Wewouldhavetoconvertthattextintoadelimitedformat. And
thisiswhereregexescomein.
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