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84
InEnglish Wearelookingfora1,followedbyaforward-slash,followedbyone-or-morenumerical
digits,andthenanotherforward-slashandthesequence
2012
.
Usingacapturinggroup,wewrapthenon-yearpartofthedateinparentheses. Andthenweuse
thebackreference
$1
toretrievethevaluetouseinthereplacement:
Find
(1/\d+)/2012
Replace
$1/2013
InSublimeText2,thisiswhattheoperationlookslike:
SublimeText2
Rememberthatsomevariationsofregex,includingtheoneusedinTextWrangler,thebackreference
involvesabackslashinsteadofadollarsign:
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85
AbackreferenceinTextWrangler’sFindandReplace
Fixingmisshapendatesisoneofmymostcommondailyusesforregexes. Datesusuallycontainan
obviouspattern,e.g.
9/5/2010
and
4-7-99
.Togetthemintoadifferentformat–e.g.
9/5/10
and
04/07/1999
–canoftenbeaccomplishedwithregextechniques.
Exercise: Changetheorder ofdateparts
InAmericanday-to-dayusage,datesareoftenformattedasmonth-day-year:
03-09-2001
11-14-2012
05-22-1978
However,programmerspreferdatestobeinyear-month-dayformat:
2001-03-09
2012-11-14
1978-05-22
Why? A common operation is to “find the earliest/latest date.” To sort date strings, the
simplestmethodfora program istodo itan alphabetical-fashion. Think ofhowyousortwords
alphabetically:
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86
Apples
Oracles
Oranges
Ore
Themostimportantletteristheleft-most one:
A
comesbefore
O
inthealphabet,so allwordsthat
beginwith
A
willcomebeforeanywordthatstartswith
O
.Butwhatifthewordsstartwiththesame
letter? Thenyoucomparethesecondletters,andsoforth.
Thisis thesamewithdates. In the American human-readable format,
03-09-2001
wouldcome
before
05-22-1978
,as
03
comesearlierinthenumericalscalethandoes
05
.Butchronologically,of
course,
05-22-1978
isseveraldecadesbefore
03-09-2001
.Intheprogrammer-friendlyversion,the
“biggest”partsofthedatecomefirst,i.e. theyear,thenthemonthandday. Thismakessortingeasy,
becausethealphabetical-orderofthedatetextisthesameasthechronologicalorderoftheactual
dates.
So, given the human readable version of the dates, write a regex that converts them to the
programmerfriendlyversion.
Answer
Find (\d+)-(\d+)-(\d+)
Replace $3-$1-$2
Exercise: Zero-padthedates
Theaboveexerciseisn’tcompletelyrealistic,however,becausethehuman-readabledatesareusually
writteninamoreshorthandway:
3-9-2001
11-14-2012
5-22-1978
ThemonthofMarch,inotherwords,iswrittensimplyas
3
,notas
03
Whyisthatextrazerothere? Ifyouthinkofthecomputer-sortablescenario,thoseextrazerosare
vital. Considertheissueoftwodateswiththesameyear. Inhumanreadableformat:
11-20-2002
3-20-2002
Inmachine-readableformat:
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87
2002-11-20
2002-3-20
Ifa programsortedtheabovedates,
2002-11-20
wouldcomebefore
2002-3-20
,becausetheleft-
most
1
iscomparedagainstthe
3
.Inotherwords,thetensplaceof
11
isbeingcomparedtotheones
placeof
3
,whichisnotwhatwewant.
With“zero-padding”,though,thisisn’taproblem:
2002-03-20
2002-11-20
Giventhefollowingdates(inhumanreadableformat):
1-12-1999
4-6-2002
3-3-1973
12-5-2004
Addzeroeswheretheyareneeded:
01-12-1999
04-06-2002
03-03-1973
12-05-2004
Answer
YourfirstinstinctmaybethatthisrequirestworegexFind-and-Replaceoperations. However,we
candoitwithone:
Find \b(\d)\b
Replace 0$1
InEnglish For every single-digit surrounded by word-boundaries, capture the digit. In the
replacementtext,adda
0
beforethecapturedsingledigit.
Exercise: Padtheyear
Attheturnofthecentury,alotofdatasystemsrecordeddatesas:
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05-14-89
12-03-98
03-15-99
02-01-00
07-07-01
03-08-02
01-22-12
Thereasoningwasthatit’s“obvious”that
98
refersto
1998
and
02
refersto
2002
.Ofcourse,itwon’t
beso obviousaswegetfartherintothe21stcentury. So writearegexthatcovertsthetwo-digit
yearsintofour-digits:
05-14-1989
12-03-1998
03-15-1999
02-01-2000
07-07-2001
03-08-2002
01-22-2012
Answer
Thisisalittlebittricky,notleastofwhichbecauseusing aregexhereisnotrecommendedinreal
life. Ifyou’reworkingwithabigdataset,youmighthavedatesthatinclude
1902
alongwith
2002
.
Andsoyoudon’twanttouseadumbregextodealwiththatambiguity,you’lllikelyneedtowrite
aprogrammingscriptwithsomelogicanderror-handling.
However,sometimesyoujustneedaquickanddirtysolutionifyouknowyourdatadoesn’tcontain
thoseambiguities. Inthiscase,we’llneedto usetworegexoperations, oneeachforpre-2000and
post-2000dates.
Forpre-2000:
Find ([89]\d)$
Replace 19$1
Forpost-2000:
Find ([01]\d)$
Replace 20$1
Exercise
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"What in the world?" Mary asked.
"Hello!" James yelled.
"Goodbye!" Mary responded.
Re-arrangethelinessothespeakerscomebeforethedialogue.
Mary asked"What in n the e world?"
James yelled"Hello!"
Mary responded"Goodbye!"
Answer
Let’sbreakdownthepatternintoitscomponentpieces:
1. Westartwithaquotationmark
2. Wethenwanttocaptureallthetext(thedialogue)thatisnotaquotationmark.
3. Thenwewanttocaptureallthetext(e.g. “Jamesyelled”)untilthesentence-endingperiod.
Note: Rememberthatthedot-characterisnotaspecialcharacterwhenusedinsidebrackets.
Find
"([ˆ"]+)" ([ˆ.]+)\.
Replace
\2: "\1"
Thenotationisalittleconfusing atfirst. In thefirstpartofthesentence, we’rerepresentingthe
patternwiththreequotationmarks:
"([^"]+)"
Butread outthe pattern to yourself: we are capturing everything that is nota quotation mark
betweentwoquotationmarks.
Themiddlequotationmarkisusedtoindicate,insidethenegativecharacterset,thatwedonotwant
aquotationmarkinthatcontext. Weneedthatthirdquotationmarktosettheboundsofthefirst
capturedgroup.
Asforthatawkwardliteraldot-characterattheend,ifwedidn’tincludeitinthepattern,itwould
notbereplaced. Tryleavingitout:
Find
"([ˆ"]+) ([ˆ.]+)
Replace
\2: "\1"
Andyou’llhaveanawkwardtrailingperiodinyourlines:
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Mary asked"What in n the e world?".
James yelled"Hello!".
Mary responded"Goodbye!".
Using parentheses without capturing
Whatifwewanttouseparenthesesjustasawaytomaintainorderofoperations?
Tomakeasetofparenthesesnon-capturing,simplyaddaquestion-markandcolonimmediately
aftertheleft-brace:
(?:this-pattern-isnt-captured)(this is captured)
Note: Tore-emphasize,thesesymbolsmustshowuponeaftertheotherinthatexactorder.
Ergo:
(?:x)
isnotatallthesameas:
( ?:x)
Giventhistext:
1. J.R. Ewing
2. A.C. Thompson
3. T.A.L. Glass
Andwantingtochangeitto:
1. Ewing
2. Thompson
3. Smith
Thepatternweseektomatchis:
• Anumberfollowedbyaperiod(captured)
• Aseriesofuppercaseletters,eachfollowedbyaperiod(butnotcaptured)
• Astringofword-characters(captured)
ThisistheFind-and-Replace:
Find (\d+.) ([A-Z].)+(\w+)
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91
Replace \1\3
Notethatwedon’tusethesecondbackreference,whichcontainstheinitials(e.g.
A.C.
,
T.A.L.
). It
seemskindofwastefultoevenstoreitifwedon’tneedit. Soweneedawaytogroupapatternwe
wanttorepeat–
[A-Z]\.
–withoutcapturingit.
Thisiswherewecouldusenon-capturingparentheses:
Find (\d+.) (?:[A-Z].)+(\w+)
Replace \1\2
The onlychangeistheaddition of
?:
to thesecond-setofparentheses in theFindfield. In the
Replacefield, thereisno longera3rdbackreferencetouse,asonlytwoofthethreeparenthetical
groupsarecapturing.
Capturinggroupsallowustotransformtextinverypowerfulways. Andwithgreatpower,comes
greatpotentialtousethembadly. We’llfindoutinlaterchaptersthatparentheseshaveseveralother
uses,andsoitbehoovesustousecapturinggroupsonlywhenweneedto,elseourregexpatterns
becomeanincomprehensibletangleofparentheses.
Thisisagoodtimetoreadotherinformationsourcesaboutregexes.Regular-expressions.info²¹in
particularhasagreatsummaryofthenuancesofcapturedgroups.
²¹
http://www.regular-expressions.info/brackets.html
Optionality and alternation
Thetwoconceptsinthischapterareactuallymorestraightforwardthanwhatwe’vecoveredinpast
chapter. I liketo think ofthem aswaysto accommodatehumanerror andinconsistency. Were
phonenumbersenteredwithorwithoutdashes? Didanauthoruse“J.F.K.”or“JFK”torefertoJohn
FitzgeraldKennedy? DidyourBritishandAmericanwritersuse“grey”and“gray”throughoutthe
text?
Theoptionalityandalternationoperatorshelpuscreatepatternswhenwedon’tknow for sureif
certaincharactersexistinthetext.
Alternation with the pipe character
Thepipecharacter–
|
–isusedtomatcheitherpatternontheleftandrightofit:
`a|b|c`
–willmatcheither
a
,
b
,or
c
.Thisofcourseisequivalentto
[abc]
However,thepipecharactercanbeusedtomatchgroupsofcharactersaswell:
`happy|\d+`
Theabovepatternwouldmatcheither
happy
oraconsecutivestringofnumbers.
Exercise: There, there
The simplest use of alternation can save you from having to run multiple Find operations.
Consideringthattheusesfor“their”,“they’re”,and“there”arecommonlymixedup,writearegex
thatwouldallowyoutoquicklylocateanditeratethrougheachofthehomonyms.
Answer
Find
they're|their|there
92
Optionalityandalternation
93
MatchinghomonymsinSublimeText2
Precedencewithalternation
The alternation operator (the pipe character) has a low priority when it comes to priority of
operations.
Forexample,thepattern
gra|ey
willmatch
gra
or
ey
.Itwouldnotmatch
grey
nor
gray
To match
grey
or
gray
,wewouldhaveto useparenthesesto groupoperationsto our preferred
order:
gr(a|e)y
Here,the“preferred”orderisthattheregexengine,whenitreachesthethirdcharacterinthepattern,
looksforeitheran
a
oran
e
.Withouttheparentheses–
gra|ey
–theregexengineconsidersonlyalternatingbetweenthestrings
gra
and
ey
Exercise: Whois/are
In a particular document, you’ve noticed that the writer has consistently, but incorrectly used
“who’s”and“whois”–forexample:
Shewantstoknowwho’stheagentsresponsibleforthis.
Writearegexthatwilldoamass-replaceofallinstancesof
who's
and
who are
–butuseparentheses
tocreateapatternthatisnotsimply,
who's|who is
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