元字符有自己的特殊含义
[ ]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)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.expn","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,}")
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