CODEWITHSUNDEEP
rregex
Rtist
Rtist

Reputation: 4205

Replace the last occurence of a string (and only it) using regular expression

I have a string, let say MyString = "aabbccawww". I would like to use a gsub expression to replace the last "a" in MyString by "A", and only it. That is "aabbccAwww". I have found similar questions on the website, but they all requested to replace the last occurrence and everything coming after. I have tried gsub("a[^a]*$", "A", MyString), but it gives "aabbccA". I know that I can use stringi functions for that purpose but I need the solution to be implemented in a part of a code where using such functions would be complicated, so I would like to use a regular expression. Any suggestion?

Upvotes: 17

Views: 9171

Answers (3)

PixelMaster
PixelMaster

Reputation: 954

While akrun's answer should solve the problem (not sure, haven't worked with \1 etc. yet), you can also use lookouts:

a(?!(.|\n)*a)

This is basically saying: Find an a that is NOT followed by any number of characters and an a. The (?!x) is a so-called lookout, which means that the searched expression won't be included in the match.

You need (.|\n) since . refers to all characters, except for line breaks.

For reference about lookouts or other regex, I can recommend http://regexr.com/.

Upvotes: 4

Sotos
Sotos

Reputation: 51582

You can use stringi library which makes dealing with strings very easy, i.e.

library(stringi)

x <- "aabbccawww"
stri_replace_last_fixed(x, 'a', 'A')
#[1] "aabbccAwww"

Upvotes: 15

akrun
akrun

Reputation: 887028

We can use sub to match 'a' followed by zero or more characters that are not an 'a' ([^a]*), capture it as group ((...)) until the end of the string ($) and replace it with "A" followed by the backreference of the captured group (\\1)

sub("a([^a]*)$", "A\\1", MyString)
#[1] "aabbccAwww"

Upvotes: 14

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