如何使用R中的RegEx替换字符串中的字符

Question

I am trying to replace a certain character in string but as this pattern is recurring I have thought to replace this iteratively using RegExes if it's even possible. For example consider a link http://www.bbc.co.uk/food/ingredients/by/letter/a

I want to replace the last 'a' with 'b', then a 'c', a 'd' and so-on till 'z' iteratively. The RegEx to work with this is as simple as:

http:\\/\\/www\\.bbc\\.co\\.uk\\/food\\/ingredients\\/by\\/letter\\/[az]

this would validate the string but I'd like to automate the process of link generation. I am quite new to 'R' and I would like to know if something like this is possible.

Answer 1

You can create such a list of links as follows with paste0 :

l <- paste0("http://www.bbc.co.uk/food/ingredients/by/letter/", letters)

which results in the following list:

> l
 [1] "http://www.bbc.co.uk/food/ingredients/by/letter/a"
 [2] "http://www.bbc.co.uk/food/ingredients/by/letter/b"
 [3] "http://www.bbc.co.uk/food/ingredients/by/letter/c"
 [4] "http://www.bbc.co.uk/food/ingredients/by/letter/d"
 [5] "http://www.bbc.co.uk/food/ingredients/by/letter/e"
 [6] "http://www.bbc.co.uk/food/ingredients/by/letter/f"
 [7] "http://www.bbc.co.uk/food/ingredients/by/letter/g"
 [8] "http://www.bbc.co.uk/food/ingredients/by/letter/h"
 [9] "http://www.bbc.co.uk/food/ingredients/by/letter/i"
[10] "http://www.bbc.co.uk/food/ingredients/by/letter/j"
[11] "http://www.bbc.co.uk/food/ingredients/by/letter/k"
[12] "http://www.bbc.co.uk/food/ingredients/by/letter/l"
[13] "http://www.bbc.co.uk/food/ingredients/by/letter/m"
[14] "http://www.bbc.co.uk/food/ingredients/by/letter/n"
[15] "http://www.bbc.co.uk/food/ingredients/by/letter/o"
[16] "http://www.bbc.co.uk/food/ingredients/by/letter/p"
[17] "http://www.bbc.co.uk/food/ingredients/by/letter/q"
[18] "http://www.bbc.co.uk/food/ingredients/by/letter/r"
[19] "http://www.bbc.co.uk/food/ingredients/by/letter/s"
[20] "http://www.bbc.co.uk/food/ingredients/by/letter/t"
[21] "http://www.bbc.co.uk/food/ingredients/by/letter/u"
[22] "http://www.bbc.co.uk/food/ingredients/by/letter/v"
[23] "http://www.bbc.co.uk/food/ingredients/by/letter/w"
[24] "http://www.bbc.co.uk/food/ingredients/by/letter/x"
[25] "http://www.bbc.co.uk/food/ingredients/by/letter/y"
[26] "http://www.bbc.co.uk/food/ingredients/by/letter/z"

Answer 2

Extract the directory name (ie everything before /a) and then append each letter using the built-in variable letters :

u <- "http://www.bbc.co.uk/food/ingredients/by/letter/a" # input string
file.path(dirname(u), letters)

which gives this character vector:

 [1] "http://www.bbc.co.uk/food/ingredients/by/letter/a"
 [2] "http://www.bbc.co.uk/food/ingredients/by/letter/b"
 [3] "http://www.bbc.co.uk/food/ingredients/by/letter/c"
 [4] "http://www.bbc.co.uk/food/ingredients/by/letter/d"
 [5] "http://www.bbc.co.uk/food/ingredients/by/letter/e"
 [6] "http://www.bbc.co.uk/food/ingredients/by/letter/f"
 [7] "http://www.bbc.co.uk/food/ingredients/by/letter/g"
 [8] "http://www.bbc.co.uk/food/ingredients/by/letter/h"
 [9] "http://www.bbc.co.uk/food/ingredients/by/letter/i"
[10] "http://www.bbc.co.uk/food/ingredients/by/letter/j"
[11] "http://www.bbc.co.uk/food/ingredients/by/letter/k"
[12] "http://www.bbc.co.uk/food/ingredients/by/letter/l"
[13] "http://www.bbc.co.uk/food/ingredients/by/letter/m"
[14] "http://www.bbc.co.uk/food/ingredients/by/letter/n"
[15] "http://www.bbc.co.uk/food/ingredients/by/letter/o"
[16] "http://www.bbc.co.uk/food/ingredients/by/letter/p"
[17] "http://www.bbc.co.uk/food/ingredients/by/letter/q"
[18] "http://www.bbc.co.uk/food/ingredients/by/letter/r"
[19] "http://www.bbc.co.uk/food/ingredients/by/letter/s"
[20] "http://www.bbc.co.uk/food/ingredients/by/letter/t"
[21] "http://www.bbc.co.uk/food/ingredients/by/letter/u"
[22] "http://www.bbc.co.uk/food/ingredients/by/letter/v"
[23] "http://www.bbc.co.uk/food/ingredients/by/letter/w"
[24] "http://www.bbc.co.uk/food/ingredients/by/letter/x"
[25] "http://www.bbc.co.uk/food/ingredients/by/letter/y"
[26] "http://www.bbc.co.uk/food/ingredients/by/letter/z"

Answer 3

Another option:

library(stringi)
v <- "http://www.bbc.co.uk/food/ingredients/by/letter/a"
stri_replace_last(v, letters, regex = "[a-z]")