p>您可以尝试以下代码删除在开始或结束处出现的逗号。

 >数据 +，Siem Reap，FC，11 Wall Street，New York，NY，，Addis Ababa， b $ b> 
 [1]1600 Pennsylvania Avenue，Washington DC
 [2]（1）
 [1] ]暹粒，FC
 [3]11华尔街，纽约，纽约
 [4]亚的斯亚贝巴，FC
 pre> 

 模式说明：
 

• （？在regex （？称为正后备。在我们的例子中，它断言什么前面的逗号必须是一个行开始 ^ 。


• | 逻辑OR运算符通常用于合并（即ORing）两个正则表达式。


• ，（？= $） Lookahead要求逗号后面的行必须是行尾 $ 。因此，它匹配行末尾的逗号。

I have (sometimes incomplete) data on addresses that looks like this:

data <- c("1600 Pennsylvania Avenue, Washington DC", 
          ",Siem Reap,FC,", "11 Wall Street, New York, NY", ",Addis Ababa,FC,")  

I need to remove the first and/or last character if either one of them are a comma.

So far, I have:

for(i in 1:length(data)){
  lastchar <- nchar(data[i])
  sec2last <- nchar(data[i]) - 1
  if(regexpr(",",data[i])[1] == 1){
    data[i] <- substr(data[i],2, lastchar)
  }
  if(regexpr(",",data[i])[1] == nchar(data[i])){
    data[i] <- substr(data[i],1, sec2last)
  }
}

data

which works for the first character, but not the last character. How can I modify the second if statement or otherwise accomplish my goal?

You could try the below code which remove the comma present at the start or at the end,

> data <- c("1600 Pennsylvania Avenue, Washington DC", 
+           ",Siem Reap,FC,", "11 Wall Street, New York, NY", ",Addis Ababa,FC,")
> gsub("(?<=^),|,(?=$)", "", data, perl=TRUE)
[1] "1600 Pennsylvania Avenue, Washington DC"
[2] "Siem Reap,FC"                           
[3] "11 Wall Street, New York, NY"           
[4] "Addis Ababa,FC" 

Pattern explanation:

• (?<=^), In regex (?<=) called positive look-behind. In our case it asserts What precedes the comma must be a line start ^. So it matches the starting comma.
• | Logical OR operator usually used to combine(ie, ORing) two regexes.
• ,(?=$) Lookahead aseerts that what follows comma must be a line end $. So it matches the comma present at the line end.