How to split letters with bracket and numbers in R? - r

Question

The string is s = '[12]B1[16]M5'
I want to split it as the following results with strsplit function in R:
let <- c('[12]B', '[16]M')
num <- c(1, 5)
Thanks a lot

Answer 1

You could use regular expression for your task.
s = '[12]B1[16]M22'
grx <- gregexpr("\\[.+?\\].+[[:digit:]]?", s)
let <- do.call(c, regmatches(s, grx))
#let
#[1] "[12]B" "[16]M"
If you want to get all chunks (let + num), you can tweak the patter as below. This facilitates extracting the numeric part.
grx <- gregexpr("\\[.+?\\].+([[:digit:]]+)", s)
out <- do.call(c, regmatches(s, grx))
num <- gsub(".+\\][[:alpha:]]+", "", out)
num
[1] "1" "22"

Answer 2

Using the stringr package:
library(stringr)
x <- '[12]B1[16]M2'
let <- unlist(str_extract_all(x, "\\[[0-9]{2}\\][A-Z]"))
x <- gsub(pattern = "\\[[0-9]{2}\\][A-Z]",
replacement = "",
x)
num <- unlist(str_extract_all(x, "[0-9]"))
the regular expression "\\[[0-9]{2}\\][A-Z]" can be broken down as
\\[ an opening bracket
[0-9]{2} a sequence of two consecutive digits
\\] a closing bracket
[A-Z] a sequence of exactly one upper case letter

Answer 3

1) strapply Create a regular expression, pat which matches the two parts and then extract each separately using strapply. The first capture group (first parenthesized portion of regular expression) consists of a left square bracket "\\[" the smallest string ".*?" until the right square bracket "\\]" followed by any character "." . The second capture group consists of one or more digits "\\d+".
library(gsubfn)
pat <- "(\\[.*?\\].)(\\d+)"
let <- strapply(s, pat, simplify = c)
num <- strapply(s, pat, ~ as.numeric(..2), simplify = c)
let
## [1] "[12]B" "[16]M"
num
## [1] 1 5
1a) Variation
This could also be expressed as this mapply producing a 2 component list:
mapply(strapply, s, pat, c(~ ..1, ~ as.numeric(..2)), simplify = "c",
SIMPLIFY = FALSE, USE.NAMES = FALSE)
## [[1]]
## [1] "[12]B" "[16]M"
##
## [[2]]
## [1] 1 5
2) gsub/read.table This uses no packages -- only gsub and read.table. pat is defined in (1). It returns a data frame with the results in two coiumns:
read.table(text = gsub(pat, "\\1 \\2\n", s), as.is = TRUE, col.names = c("let", "num"))
## let num
## 1 [12]B 1
## 2 [16]M 5
3) gsub/strsplit This is somewhat similar to (2) but uses strsplit rather than read.table. pat is from (1).
spl <- matrix(strsplit(gsub(pat, "\\1 \\2 ", s), " ")[[1]], 2)
let <- spl[1, ]
num <- as.numeric(spl[2, ])