[R] R functions to deal with regular expression-优快云博客

本文详细介绍了R语言中基本的正则表达式函数（如nchar、tolower、toupper、chartr、paste）及复杂函数（如grep、grepl、regexpr、gregexpr、regexec、sub、gsub）的使用方法，并通过实例展示了如何进行字符串搜索、匹配、替换等操作。

Basic functions
Complex functions

1. Basic functions

The basic functions of R to deal with regular expression is nchar, tolower, toupper, chartr, paste.

nchar gives the number of characters of each element in a character vector.

> temp <- c("Hello", "Kitty", "word")
> nchar(temp)
[1] 5 5 4

tolower, toupper is translate lower and upper cases.

> tolower(temp)
[1] "hello" "kitty" "word" 
> toupper(temp)
[1] "HELLO" "KITTY" "WORD"

chartr(old, new, x) translates each character in x that is specified in old to the corresponding character specified in new

> chartr("HKw", "ABs", temp)
[1] "Aello" "Bitty" "sord"

paste concatenates vectors after converting to character

> paste ("A", 1:3, sep = "")
[1] "A1" "A2" "A3"
> paste ("A", 1:3, sep = "*")
[1] "A*1" "A*2" "A*3"
> paste(c("A", "B"), 1:7)
[1] "A 1" "B 2" "A 3" "B 4" "A 5" "B 6" "A 7"
> paste("A", 1:5, sep="", collapse = "-") 
[1] "A1-A2-A3-A4-A5"
> paste("A", 1:5, sep="", collapse = "*") 
[1] "A1*A2*A3*A4*A5"

2.Complex functions

grep searches for match and return their subscript(place in the vector).
grepl searches for match and return the logical value for each element in the vector if it matches.

> temp1 <- c("abs", "abd", "bss")
> grep("s$", temp1)
[1] 1 3
> grepl("s$", temp1)
[1]  TRUE FALSE  TRUE
> temp1[grep("s$", temp1)]
[1] "abs" "bss"
> temp1[grepl("s$", temp1)]
[1] "abs" "bss"

regexpr、gregexpr、regexec search for march and return the place of matching characters in each element in the vector, but the format of their return is different.

> regexpr("s$", temp1)
[1]  3 -1  3
attr(,"match.length")
[1]  1 -1  1
attr(,"useBytes")
[1] TRUE

###############################
> gregexpr("s$", temp1)
[[1]]
[1] 3
attr(,"match.length")
[1] 1
attr(,"useBytes")
[1] TRUE

[[2]]
[1] -1
attr(,"match.length")
[1] -1
attr(,"useBytes")
[1] TRUE

[[3]]
[1] 3
attr(,"match.length")
[1] 1
attr(,"useBytes")
[1] TRUE

##########################################
> regexec("s$", temp1)
[[1]]
[1] 3
attr(,"match.length")
[1] 1

[[2]]
[1] -1
attr(,"match.length")
[1] -1

[[3]]
[1] 3
attr(,"match.length")
[1] 1

sub、gsub search for match and replace them. But sub only replace the first match in each element (replace for all element but only the first match). gsub replace all the matches(replace all matches for all elements)

> temp2 <- c("HelloHello", "Kitty", "Hello", "word")
> sub("Hello", "Hi", temp2)
[1] "HiHello" "Kitty"   "Hi"      "word"   
> gsub("Hello", "Hi", temp2)
[1] "HiHi"  "Kitty" "Hi"    "word"