CODEWITHSUNDEEP
regexr
Misha
Misha

Reputation: 3126

regex for the last one or two digits in r

Is it possible to use regex on the following vector:

u<-c("first1","sec2","thir33","fourth4","fifth25","sixth16",
     "seven7","eight8","nine9","ten10","eleven11")

to obtain: 

[1] "first.1" "sec.2" "thir3.3" "fourth.4" "fifth2.5" "sixth1.6" "seven.7" "eight.8"
[9] "nine.9" "ten.10" "eleven.11"

This is as close as I've gotten:

gsub("([A-Za-z]*)([1]{0,1})([0-9]$)","\\1\\.\\2\\3",u)
#[1] "first.1"   "sec.2"     "thir3.3"   "fourth.4"  "fifth2.5"  "sixth.16"  "seven.7"   "eight.8"   "nine.9"    "ten.10"   
#[11] "eleven.11"

Note the sixth element is incorrect: "sixth.16" should be "sixth1.6".

Upvotes: 0

Views: 526

Answers (3)

BenBarnes
BenBarnes

Reputation: 19454

Using DWin's answer as a jumping off point, you could gain some speed (assuming your real problem tackles a much longer vector) by knowing that elements 1:9, 10:99, 100:999 and so on should all be handled in the same respective way.

So, get some larger data

u<-c("first1","sec2","thir33","fourth4","fifth25","sixth16",
     "seven7","eight8","nine9","ten10","eleven11")
u[12:101981]<-NA

set.seed(1)
for(i in 12:101981)u[i]<-paste0(paste(sample(c(LETTERS,1:9),5),collapse=""),i)

lengthu<-length(u)
maxLength<-nchar(lengthu)
theStart<-10^(seq_len(maxLength)-1)
theEnd<-c(theStart[-1]-1,lengthu)

Then use sapply not over each element in u, but rather over a sequence of length maxLength

tempans<-sapply(seq_len(maxLength),function(x){
  sub(paste0("(^.*)(\\d{",x,"})"),"\\1.\\2",u[theStart[x]:theEnd[x]])
})
tail(unlist(tempans))
# [1] "DWY96.101976" "UWFCO.101977" "UR5L8.101978" "XBQ9V.101979" "48MTI.101980"
# [6] "75LIS.101981"

head(unlist(tempans))
# [1] "first.1"  "sec.2"    "thir3.3"  "fourth.4" "fifth2.5" "sixth1.6"

Upvotes: 1

IRTFM
IRTFM

Reputation: 263332

I don't see an internal regex method that would "know" or have access to the position in a vector, but can certainly pass it in and use its 'as.character' coerced value in a pattern.

sapply(seq_along(u), function(x) sub(
                                     paste("(^.+)(", as.character(x), "$)", sep=""),
                                     "\\1.\\2", u[x]) )
 [1] "first.1"   "sec.2"     "thir3.3"   "fourth.4"  "fifth2.5"  "sixth1.6"  "seven.7"   "eight.8"   "nine.9"   
[10] "ten.10"    "eleven.11"

Upvotes: 4

Blue Magister
Blue Magister

Reputation: 13363

This isn't particularly pretty, but you can do it in one step with:

gsub("([A-Za-z]+)(10|11)?(?:(\\d)(\\d))?([0-9]{0,1}?)$","\\1\\3\\.\\2\\4\\5",u)

Alternatively, you can break it up into a few steps. Take the single-digits first, then handle the 2-digit cases separately.

v <- gsub("([A-Za-z]+)(\\d)$","\\1.\\2",u)
v <- gsub("([A-Za-z]+)(10|11)$","\\1.\\2",v)
v <- gsub("([A-Za-z]+\\d)(\\d)$","\\1.\\2",v)

Upvotes: 1

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