[R]字符串函数

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摘自：http://www.biostatistic.net/thread-195-1-1.html

用于字符串分割的函数：
如
strsplit('123abcdefgabcdef','ab')

[[1]]
[1] "123"   "cdefg" "cdef"

#字符串连接：
paste() #paste(..., sep = " ", collapse = NULL)

#字符串分割：
strsplit() #strsplit(x, split, extended = TRUE, fixed = FALSE, perl = FALSE)

#计算字符串的字符数：
nchar()

#字符串截取：
substr(x, start, stop)
substring(text, first, last = 1000000)
substr(x, start, stop) <- value
substring(text, first, last = 1000000) <- value
###########例子说明
substr("abcdef",2,4)
substring("abcdef",1:6,1:6)## strsplit is more efficient ...
substr(rep("abcdef",4),1:4,4:5)
x <- c("asfef", "qwerty", "yuiop[", "b", "stuff.blah.yech")
substr(x, 2, 5)substring(x, 2, 4:6)
substring(x, 2) <- c("..", "+++")
x

###########

#字符串替换及大小写转换：
chartr(old, new, x)
tolower(x)
toupper(x)
casefold(x, upper = FALSE)

字符完全匹配
grep()
字符不完全匹配
agrep()
字符替换
gsub()
#以上这些函数均可以通过perl=TRUE来使用正则表达式。
     grep(pattern, x, ignore.case = FALSE, extended = TRUE,
          perl = FALSE, value = FALSE, fixed = FALSE, useBytes = FALSE)

     sub(pattern, replacement, x,
         ignore.case = FALSE, extended = TRUE, perl = FALSE,
         fixed = FALSE, useBytes = FALSE)

     gsub(pattern, replacement, x,
          ignore.case = FALSE, extended = TRUE, perl = FALSE,
          fixed = FALSE, useBytes = FALSE)

     regexpr(pattern, text, ignore.case = FALSE, extended = TRUE,
             perl = FALSE, fixed = FALSE, useBytes = FALSE)

     gregexpr(pattern, text, ignore.case = FALSE, extended = TRUE,
              perl = FALSE, fixed = FALSE, useBytes = FALSE)
See Also:

     regular expression (aka 'regexp') for the details of the pattern
     specification.

     'glob2rx' to turn wildcard matches into regular expressions.

     'agrep' for approximate matching.

     'tolower', 'toupper' and 'chartr' for character translations.
     'charmatch', 'pmatch', 'match'. 'apropos' uses regexps and has
     nice examples.

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