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gsub()可以用于字段的删减、增补、替换和切割，可以处理一个字段也可以处理由字段组成的向量。
具体的使用方法为：gsub("目标字符", "替换字符", 对象)
在gsub函数中，任何字段处理都由将“替换字符”替换到“目标字符”这一流程中实现，令替换字符为''''可实现删除，令替换字符为"目标字符+增补内容"可实现增补，替换和切割也是使用类似的操作。
> text <- "AbcdEfgh . Ijkl MNM"
> gsub("Efg", "AAA", text) #将Efg改为AAA，区分大小写
[1] "AbcdAAAh . Ijkl MNM"
任何符号，包括空格、Tab和换行都是可以识别的
> gsub(" I", "i", text) #可识别空格
[1] "AbcdEfgh .ijkl MNM"
同时字符可以识别多个，进行批量置换
> gsub("M", "N", text)
[1] "AbcdEfgh . Ijkl NNN"
除此之外，gsub还有其他批量操作的方法
> gsub("^.* ", "a", text) #开头直到最后一个空格结束替换成a
[1] "aMNM"
> gsub("^.* I(j).*$", "\\1", text) #只保留一个j
[1] "j"
> gsub(" .*$", "b", text) #第一个空格直达结尾替换成b
[1] "AbcdEfghb"
> gsub("\\.", "\\+", text) #句号.和加号+是特殊的，要添加\\来识别
[1] "AbcdEfgh + Ijkl MNM"
| Syntax | Description | | \\d | Digit, 0,1,2 ... 9 | | \\D | Not Digit | | \\s | Space | | \\S | Not Space | | \\w | Word | | \\W | Not Word | | \\t | Tab | | \\n | New line | | ^ | Beginning of the string | | $ | End of the string | | \ | Escape special characters, e.g. \\ is "\", \+ is "+" | | | | Alternation match. e.g. /(e|d)n/ matches "en" and "dn" | | · | Any character, except \n or line terminator | | [ab] | a or b | | [^ab] | Any character except a and b | | [0-9] | All Digit | | [A-Z] | All uppercase A to Z letters | | [a-z] | All lowercase a to z letters | | [A-z] | All Uppercase and lowercase a to z letters | | i+ | i at least one time | | i* | i zero or more times | | i? | i zero or 1 time | | i{n} | i occurs n times in sequence | | i{n1,n2} | i occurs n1 - n2 times in sequence | | i{n1,n2}? | non greedy match, see above example | | i{n,} | i occures >= n times | | [:alnum:] | Alphanumeric characters: [:alpha:] and [:digit:] | | [:alpha:] | Alphabetic characters: [:lower:] and [:upper:] | | [:blank:] | Blank characters: e.g. space, tab | | [:cntrl:] | Control characters | | [:digit:] | Digits: 0 1 2 3 4 5 6 7 8 9 | | [:graph:] | Graphical characters: [:alnum:] and [:punct:] | | [:lower:] | Lower-case letters in the current locale | | [:print:] | Printable characters: [:alnum:], [:punct:] and space | | [:punct:] | Punctuation character: ! " # $ % & ' ( ) * + , - . / : ; < = > ? @ [ \ ] ^ _ ` { | } ~ | | [:space:] | Space characters: tab, newline, vertical tab, form feed, carriage return, space | | [:upper:] | Upper-case letters in the current locale | | [:xdigit:] | Hexadecimal digits: 0 1 2 3 4 5 6 7 8 9 A B C D E F a b c d e f |
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