R语言学习笔记（二十一）：字符串处理中的元字符

元字符有自己的特殊含义

states <- row.names(USArrests)
# 提取字符串子集
substr(x = states, start = 1, stop = 4)
abbreviate(states, minlength = 5)
# 计算字符串长度
state_chars <- nchar(states)
# 字符串匹配
grep(pattern = "w", x = states, value = T)
# 匹配大写和小写

• [ ]内的任意字符将被匹配

grep(pattern = "[wW]", x = states, value = T)
grep(pattern = "w", ignore.case = T, x = states, value = T)
tolower(states)     # 变为小写
toupper(states)     # 变为大写
library("stringr")
# 含a的数目
str_count(states, "a")

• \对元字符进行转义

strsplit("strsplit.also.uses", split = ".")
strsplit("strsplit.also.uses", split = "\\.")
str_extract_all("me credit card: 334", pattern = "\\d")

• ^匹配字符串的开头，将^置于character class 的首位表达的意思是取反义。如[ˆ5] 表示匹配除了“5” 以外的所有字符。

test_vector <- c("123","456","321")
str_extract_all(test_vector, "3")
str_extract_all(test_vector, "^3")
str_extract_all(test_vector, "[^3]")

• $匹配字符串的结尾。但将它置于character class 内则消除了它的特殊含义。如[akm$]将匹配'a','k','m' 或者'$'。

str_extract_all(test_vector, "3$")
str_extract_all(test_vector, "[3$]")

• .匹配除换行符以外的任意字符。

str_extract_all(string = c("regular.exp\n","\n"), pattern =".")

• | 或者

str_extract_all(string = "we23", pattern ="b|w|3")

• ?此符号前的字符(组) 是可有可无的，并且最多被匹配一次

str_extract_all(string = c("abc","bc","ac"),pattern = "ab?c")

• ( )表示一个字符组，括号内的字符串将作为一个整体被匹配

str_extract_all(string = c("abc","ac","cde"),pattern = "(ab)c")

• *此符号前的字符(组) 将被匹配零次或多次

str_extract_all(string = c("abab","abc","ac"),pattern = "(ab)*")

• +前面的字符(组) 将被匹配一次或多次

str_extract_all(string = c("abbab","abc","ac"),pattern = "ab+")

• {n,m} 重复n次到m次

str_extract_all(string = c("abababab","ababc","abc"),pattern = "(ab){2}")
str_extract_all(string = c("abababab","ababc","abc"),pattern = "(ab){2,}")
str_extract_all(string = c("abababab","ababc","abc"),pattern = "(ab){2,3}")