加载中…R 中删除特殊字符
(2011-07-11 11:15:19)
标签：
杂谈 |
分类： R软件学习 |
Dear R-users --
I'm using R 1.3.0 on a PC running SuSE Linux 7.1. I'm confused by the following behavior from the gsub() function. Am I doing something wrong?
## A string of characters
string<-c("q","w","e","(",")","q","w","e")
## Use gsub to replace `q' with `A'
gsub("q","A",string)
[1] "A" "w" "e" "(" ")" "A" "w" "e" # Works fine
## Replace `(' [open-parenthesis symbol] with `A'
gsub("(","A",string)
Error in gsub(pattern, replacement, x, ignore.case, extended) : invalid regular expression
## But NB! -- Replace `)' [close parenthesis symbol] with `A'
gsub(")","A",string)
[1] "q" "w" "e" "(" "A" "q" "w" "e" # Works fine!
## Further experimentation reveals the following: ## Replace open-paren with close-paren does not work
gsub("(",")",string)
Error in gsub(pattern, replacement, x, ignore.case, extended) : invalid regular expression
## ... But replace close paren with open paren works fine!
gsub(")","(",string)
[1] "q" "w" "e" "(" "(" "q" "w" "e"
My understanding is that parentheses are not special characters is regexp syntax. And of course I don't understand at all why the open paren symbol doesn't work but the close paren symbol does. It seems to dislike it only when it appears as the first argument to the function.
Parentheses *are* special characters in POSIX 1003.2 regexps and one of the examples in help(gsub) uses them. Also gsub("\\(","A",string) does do want you want, as you would expect if ( were special. Close paren OTOH is not special except after an unmatched open paren.
-thomas
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