這篇文章講文字與字串資料的處理。關於文字的處理，我將會按照我自己對其他程式語言的掌握程度來盡可能縮減這篇文章的內容。關於特殊字符輸入方式（轉義）等基本內容，我不會贅述。
stringr 套件
stringr 套件是 tidyverse 套件系統提供的一套用於處理文字和字串的套件。
合併文字或字串
使用 string_c() 函式合併文字或字串。使用引數 sep 設定合併的中間字元。
str_c("medical", "statistics")## [1] "medicalstatistics"str_c("medical", "statistics", sep = " ")## [1] "medical statistics"若是遇到缺失值 NA，則仍回傳 NA，若要改變遇到缺失值 NA，回傳列印 NA，可以使用加用函式 str_replace_na()。
x.char <- c("bio", NA, "statistics")str_c("pre-", x.char, "-end")## [1] "pre-bio-end" NA "pre-statistics-end"str_c("pre-", str_replace_na(x.char), "-end")## [1] "pre-bio-end" "pre-NA-end" "pre-statistics-end"合併字串向量，使用引數 collpse。
char.vec <- c("I", "love", "biostatistics")str_c(char.vec, collapse = ", ")## [1] "I, love, biostatistics"取出子串
使用 str_sub() 函式取字串的子串。
char.vec <- c("I", "love", "medical", "statistics")str_sub(char.vec, start = 1, end = 3)## [1] "I" "lov" "med" "sta"移除空白，加入空白，截斷文字
套件 stringr 內的函式 str_trim() 與 str_pad() 可以對文字或字串向量內的首尾之空白（white space）移除，或是加入。
其定義如下：
str_trim(string, side = c("both", "left", "right"))str_pad(string, width, side = c("left", "right", "both"), pad = " ")str_trunc(string, width, side = c("right", "left", "center"), ellipsis = "...")str_trim() 函式用於移除字串兩端的空白，其中 side 變數的取值：both 代表兩端，left 代表僅左邊，right 代表僅右邊。
str_pad() 函式用於向字串兩端增加空白，引數中 width 表示增加後字串的長度，side 與上個函式相同，pad 表示以什麼字元填充空白。
str_trunc() 用於創造省略號。width 引數表示包括省略號之後字串的長度，side 引數取 left、right、center。
veg.vec <- c("apple ", " eggplant ", " banana")str_trim(veg.vec, side = c("both"))## [1] "apple" "eggplant" "banana"str_trim(veg.vec, side = c("left"))## [1] "apple " "eggplant " "banana"str_trim(veg.vec, side = c("right"))## [1] "apple" " eggplant" " banana"veg.vec <- c("apple ", " eggplant ", " banana")str_pad("a", width = 15, side = c("both"), pad = " ")## [1] " a "str_pad("a", width = 15, side = c("both"), pad = c("_"))## [1] "_______a_______"str_pad(veg.vec, width = 15, side = c("both"))## [1] " apple " " eggplant " " banana "str_pad(veg.vec, width = 15, side = c("left"))## [1] " apple " " eggplant " " banana"str_pad(veg.vec, width = 15, side = c("right"))## [1] "apple " " eggplant " " banana "str_pad(veg.vec, width = 15, side = c("both"), pad = c("_"))## [1] "____apple _____" "__ eggplant ___" "____ banana____"char.vec <- c("I love biostatistics")str_trunc(char.vec, width = 10, side = c("center"))## [1] "I lo...ics"str_trunc(char.vec, width = 10, side = c("left"))## [1] "...tistics"str_trunc(char.vec, width = 10, side = c("right"))## [1] "I love ..."特定形式文字與字串處理
偵測函式
str_detect() 函式用於偵測字串向量是否包含特定形式文字，回傳邏輯向量。說得通俗，就是判斷指定的字串中是否含有指定的子串。str_count() 函式用於偵測子串在指定的字串中出現了多少次。
str_detect(string, pattern, negate = FALSE)str_count(string, pattern = "")引數說明：
- string：指定的字串。
- pattern：要判斷的子串。
- negate：若為TRUE，同時回傳沒有配對成功的邏輯向量。
確認位置函式
函式 str_locate() 用於查找配對成功的字串之第一次位置，回傳矩陣，包含開始和末端的位置。另外函式 str_locate_all() 尋找配對成功的字串之所有位置，回傳列表。
char.vec <- c("statistics", "biostatistics", "probability", "distribution")str_locate(char.vec, pattern = "ti")## start end## [1,] 4 5## [2,] 7 8## [3,] NA NA## [4,] 9 10str_locate_all(char.vec, pattern = "ti")## [[1]]## start end## [1,] 4 5## [2,] 7 8#### [[2]]## start end## [1,] 7 8## [2,] 10 11#### [[3]]## start end#### [[4]]## start end## [1,] 9 10確認索引函式
函式 str_subnet() 尋找字串向量內配對成功之第一次的元素內容。函式 str_which() 則尋找字串向量內配對成功之第一次索引。
char.vec <- c("statistics", "biostatistics", "probability", "distribution")str_subset(char.vec, pattern = "ti")## [1] "statistics" "biostatistics" "distribution"str_which(char.vec, pattern = "ti")## [1] 1 2 4取出函式
函式 str_extract() 尋找配對成功的字串之第 1 次位置，回傳字串向量。另外函式 str_extract_all() 尋找配對成功的字串之所有位置，回傳所有字串向量形成列表。引數 simplify = TRUE 簡化成文字矩陣。
char.vec <- c("statistics", "biostatistics", "probability", "distribution")str_extract(char.vec, pattern = "ti")## [1] "ti" "ti" NA "ti"str_extract_all(char.vec, pattern = "ti")## [[1]]## [1] "ti" "ti"#### [[2]]## [1] "ti" "ti"#### [[3]]## character(0)#### [[4]]## [1] "ti"配對函式
函式 str_match() 使用在群組尋找特定形式文字或字串，若尋到找配對成功的字串之第 1 次位置，回傳文字矩陣，第一欄位為完全配對成功的文字，其餘欄位為群組內個別配對成功的文字。另外函式 str_match_all() 尋找配對成功的字串之所有位置。
char.vec <- c("statistics", "biostatistics", "probability", "distribution")str_match(char.vec, pattern = "(a|ti)")## [,1] [,2]## [1,] "a" "a"## [2,] "a" "a"## [3,] "a" "a"## [4,] "ti" "ti"str_match_all(char.vec, pattern = "(a|ti)")## [[1]]## [,1] [,2]## [1,] "a" "a"## [2,] "ti" "ti"## [3,] "ti" "ti"#### [[2]]## [,1] [,2]## [1,] "a" "a"## [2,] "ti" "ti"## [3,] "ti" "ti"#### [[3]]## [,1] [,2]## [1,] "a" "a"#### [[4]]## [,1] [,2]## [1,] "ti" "ti"替代置換函式
函式 str_match() 使用在群組尋找特定形式文字或字串，若尋找到配對成功的字串之第1次位置，則使用其他特定字串替代置換。引數 replacement 設定新的替代字串置換原有尋找特定形式文字或字串。另外函式 str_replace_all() 尋找配對成功的字串之所有位置，同時使用其他特定字串替代置換。
char.vec <- c("statistics", "biostatistics", "probability", "distribution")str_replace(char.vec, pattern = "ti", replacement = "--")## [1] "sta--stics" "biosta--stics" "probability" "distribu--on"str_replace_all(char.vec, pattern = "b", replacement = "+++")## [1] "statistics" "+++iostatistics" "pro+++a+++ility" "distri+++ution"分割函式
函式 str_split() 使用在群組尋找特定形式文字或字串，若尋找到配對成功的字串之第 1 次位置，則從特定形式文字或字串分割字串向量，回傳分割結果為列表物件。其中引數 n 設定回傳物件的數目，simplify = TRUE 回傳物件簡化成文字矩陣。另外函式 str_split_fixed() 回傳物件簡化成文字矩陣且欄位（column）數目為 n。str_split_n() 回傳物件簡化成文字向量，長度為 n。
char.vec <- c("a b c", "d e", "bio-statistics required-courses")str_split(char.vec, pattern = " ", n = Inf, simplify = FALSE)## [[1]]## [1] "a" "b" "c"#### [[2]]## [1] "d" "e"#### [[3]]## [1] "bio-statistics" "required-courses"str_split(char.vec, pattern = " ", n = Inf, simplify = TRUE)## [,1] [,2] [,3]## [1,] "a" "b" "c"## [2,] "d" "e" ""## [3,] "bio-statistics" "required-courses" ""str_split_fixed(char.vec, pattern = " ", n = 2)## [,1] [,2]## [1,] "a" "b c"## [2,] "d" "e"## [3,] "bio-statistics" "required-courses"str_split_fixed(char.vec, pattern = "-", n = 2)## [,1] [,2]## [1,] "a b c" ""## [2,] "d e" ""## [3,] "bio" "statistics required-courses"群組尋找
有些時候在尋找特定形式的文字與字串，須要尋找不只一種特定的形式。例如，同時尋找 b 或 ti, 可以輸入 b|ti。
char.vec <- c("statistics", "biostatistics", "probability", "distribution")str_replace(char.vec, pattern = "b|ti", replacement = "--")## [1] "sta--stics" "--iostatistics" "pro--ability" "distri--ution"str_replace_all(char.vec, pattern = "b|ti", replacement = "+++")## [1] "sta+++s+++cs" "+++iosta+++s+++cs" "pro+++a+++ility" "distri+++u+++on"起始符號 ^ 可以尋找字串的起始具有特定形式，尾端符號 $ 可以尋找字串的尾端具有特定形式。例如 ^b 尋找字串的起始具有b，或 n$ 尋找字串的尾端具 n。
char.vec <- c("statistics", "biostatistics", "probability", "distribution")str_replace(char.vec, pattern = "^b", replacement = "--")## [1] "statistics" "--iostatistics" "probability" "distribution"str_replace_all(char.vec, pattern = "n$", replacement = "+++")## [1] "statistics" "biostatistics" "probability" "distributio+++"有些時候需要尋找字串前後具有特定形式的文字與字串，例如，尋找在 ti 之前的字元，在 p 之後的字元等等。使用小括號 () 代表特定形式的前後順序。輸入 a(?=c) 表示在 a 之後有 c 字元，輸入 a(?!c) 表示在 a 之後無 c 字元，輸入 (?<=b)a 表示在 a 之前有 b 字元，輸入 (?<!b)a 表示在 a 之前無 b 字元。
char.vec <- c("statistics", "biostatistics", "probability", "distribution")str_replace(char.vec, pattern = "t(?=i)", replacement = "--")## [1] "sta--istics" "biosta--istics" "probability" "distribu--ion"str_replace(char.vec, pattern = "t(?!i)", replacement = "--")## [1] "s--atistics" "bios--atistics" "probabili--y" "dis--ribution"str_replace(char.vec, pattern = "(?<=i)o", replacement = "--")## [1] "statistics" "bi--statistics" "probability" "distributi--n"str_replace_all(char.vec, pattern = "(?<=t)i", replacement = "--")## [1] "stat--st--cs" "biostat--st--cs" "probability" "distribut--on"str_replace(char.vec, pattern = "(?<!t)i", replacement = "--")## [1] "statistics" "b--ostatistics" "probab--lity" "d--stribution"str_replace_all(char.vec, pattern = "(?<!t)i", replacement = "--")## [1] "statistics" "b--ostatistics" "probab--l--ty" "d--str--bution"尋找連續重複的子串
| stringr 輸入 | 意義 |
|---|---|
| a? | zero or one |
| a* | zero or more |
| a+ | one or more |
| a{n} | exactly n |
| a{n,} | n or more |
| a{n,m} | between n and m |
x.vec <- c(".a.aa.aaa.aaaa")str_replace(x.vec, pattern = "a?", replacement = "-")## [1] "-.a.aa.aaa.aaaa"str_replace(x.vec, pattern = "a*", replacement = "-")## [1] "-.a.aa.aaa.aaaa"str_replace(x.vec, pattern = "a+", replacement = "-")## [1] ".-.aa.aaa.aaaa"str_replace(x.vec, pattern = "a{2}", replacement = "-")## [1] ".a.-.aaa.aaaa"str_replace(x.vec, pattern = "a{2,}", replacement = "-")## [1] ".a.-.aaa.aaaa"str_replace(x.vec, pattern = "a{2,3}", replacement = "-")## [1] ".a.-.aaa.aaaa"char.vec <- c("statistics", "biostatistics", "probability", "distribution")str_replace(char.vec, pattern = "i?", replacement = "-")## [1] "-statistics" "-biostatistics" "-probability" "-distribution"str_replace(char.vec, pattern = "i*", replacement = "-")## [1] "-statistics" "-biostatistics" "-probability" "-distribution"str_replace(char.vec, pattern = "i+", replacement = "-")## [1] "stat-stics" "b-ostatistics" "probab-lity" "d-stribution"str_replace(char.vec, pattern = "i{2}", replacement = "-")## [1] "statistics" "biostatistics" "probability" "distribution"str_replace(char.vec, pattern = "i{2}", replacement = "-")## [1] "statistics" "biostatistics" "probability" "distribution"str_replace(char.vec, pattern = "i{2,3}", replacement = "-")## [1] "statistics" "biostatistics" "probability" "distribution"萬用字元
R Lang 尋找特定形式的文字與字串，可以使用程式語言通用的正規表示，在使用套件 stringr 輸入時有些差異。
差異大致如下：
- 正規表示為單“/”表示的，使用“//”。例如正規表示
\!表示驚嘆號!，在stringr中表示作\\!。 - 在上面的規則上，有幾個特例
- 斜線“\”，正規表示作
\\，stringr表示作\\\\。 - 製表元，正規表示作
\t，stringr仍表示作\t。
- 斜線“\”，正規表示作
- 新增表示
| stringr表示 | 真實文字與字串 |
|---|---|
| [:digit:] | digits |
| [:alpha:] | letters |
| [:lower:] | lowercase letters |
| [:upper:] | uppercase letters |
| [:alnum:] | letters and numbers |
| [:punct:] | punctuation |
| [:graph:] | letters, numbers and punctuation |
| [:space:] | space characters |
| [:blank:] | space and tab, without new line |
| . | every character exceppt a new line |