R stringr str_extract 提取完整的匹配項

str_extract(string, pattern, group = NULL)

str_extract_all(string, pattern, simplify = FALSE)

str_match() 提取匹配組； stringi::stri_extract() 用於底層實現。

shopping_list <- c("apples x4", "bag of flour", "bag of sugar", "milk x2")
str_extract(shopping_list, "\\d")
#> [1] "4" NA  NA  "2"
str_extract(shopping_list, "[a-z]+")
#> [1] "apples" "bag"    "bag"    "milk"  
str_extract(shopping_list, "[a-z]{1,4}")
#> [1] "appl" "bag"  "bag"  "milk"
str_extract(shopping_list, "\\b[a-z]{1,4}\\b")
#> [1] NA     "bag"  "bag"  "milk"

str_extract(shopping_list, "([a-z]+) of ([a-z]+)")
#> [1] NA             "bag of flour" "bag of sugar" NA            
str_extract(shopping_list, "([a-z]+) of ([a-z]+)", group = 1)
#> [1] NA    "bag" "bag" NA   
str_extract(shopping_list, "([a-z]+) of ([a-z]+)", group = 2)
#> [1] NA      "flour" "sugar" NA     

# Extract all matches
str_extract_all(shopping_list, "[a-z]+")
#> [[1]]
#> [1] "apples" "x"     
#> 
#> [[2]]
#> [1] "bag"   "of"    "flour"
#> 
#> [[3]]
#> [1] "bag"   "of"    "sugar"
#> 
#> [[4]]
#> [1] "milk" "x"   
#> 
str_extract_all(shopping_list, "\\b[a-z]+\\b")
#> [[1]]
#> [1] "apples"
#> 
#> [[2]]
#> [1] "bag"   "of"    "flour"
#> 
#> [[3]]
#> [1] "bag"   "of"    "sugar"
#> 
#> [[4]]
#> [1] "milk"
#> 
str_extract_all(shopping_list, "\\d")
#> [[1]]
#> [1] "4"
#> 
#> [[2]]
#> character(0)
#> 
#> [[3]]
#> character(0)
#> 
#> [[4]]
#> [1] "2"
#> 

# Simplify results into character matrix
str_extract_all(shopping_list, "\\b[a-z]+\\b", simplify = TRUE)
#>      [,1]     [,2] [,3]   
#> [1,] "apples" ""   ""     
#> [2,] "bag"    "of" "flour"
#> [3,] "bag"    "of" "sugar"
#> [4,] "milk"   ""   ""     
str_extract_all(shopping_list, "\\d", simplify = TRUE)
#>      [,1]
#> [1,] "4" 
#> [2,] ""  
#> [3,] ""  
#> [4,] "2" 

# Extract all words
str_extract_all("This is, suprisingly, a sentence.", boundary("word"))
#> [[1]]
#> [1] "This"        "is"          "suprisingly" "a"           "sentence"   
#>