正则表达式表通常被用来检索、替换那些符合某个模式(规则)的文本。在我看来,正则表达式的主要用途有两种:①查找特定的信息②查找并编辑特定的信息,也就是我们经常用的替换。。比如我们要在Word,记事本等里面使用快捷键Ctrl+F,进行查找一个特定的字符,或者替换一个字符,这就使用了正则表达式。
正则表达式的功能非常强大,尤其是在文本数据进行处理中显得更加突出。R中的grep、grepl、sub、gsub、regexpr、gregexpr等函数都使用正则表达式的规则进行匹配。这几个函数原型如下:
grep(pattern, x, ignore.case = FALSE, perl = FALSE, value = FALSE,
fixed = FALSE, useBytes = FALSE, invert = FALSE)
grepl(pattern, x, ignore.case = FALSE, perl = FALSE,
fixed = FALSE, useBytes = FALSE)
sub(pattern, replacement, x, ignore.case = FALSE, perl = FALSE,
fixed = FALSE, useBytes = FALSE)
gsub(pattern, replacement, x, ignore.case = FALSE, perl = FALSE,
fixed = FALSE, useBytes = FALSE)
regexpr(pattern, text, ignore.case = FALSE, perl = FALSE,
fixed = FALSE, useBytes = FALSE)
gregexpr(pattern, text, ignore.case = FALSE, perl = FALSE,
fixed = FALSE, useBytes = FALSE)
regexec(pattern, text, ignore.case = FALSE, perl = FALSE,
fixed = FALSE, useBytes = FALSE)
这里是对参数进行一个解释说明。
接下来我们对这几个函数谈谈他们的不同点。
现在来举几个例子。
首先使用[]中括号的功能,来查找一下看有没有do组合的单词。
text<-c("Don't","aim","for","success","if","you","want","it","just","do","what","you","love",
"and","believe","in","and","it","will","come","naturally")
#查找含有DO组合的单词
grep("[Dd]o",text)#不区分大小写
grep("[D]o",text)#D要大写
grep("[d]o",text)#D小写
运行结果如下:
> text<-c("Don't","aim","for","success","if","you","want","it","just","do","what",
"you","love","and","believe","in","and","it","will","come","naturally")
> 数据分析培训
> #查找含有DO组合的单词
> grep("[Dd]o",text)#不区分大小写
[1] 1 10
> grep("[D]o",text)#D要大写
[1] 1
> grep("[d]o",text)#D小写
[1] 10
邮箱匹配:
#邮箱匹配:
text2<-c("704232753@qq.com is my email address.")
grepl("[0-9.*]+@[a-z.*].[a-z.*]",text2)
结果如下
> text2<-c("704232753@qq.com is my email address.")
> grepl("[0-9.*]+@[a-z.*].[a-z.*]",text2)
[1] TRUE
说明可以查找到了。
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