R: Find the last dot in a string

Question

In R, is there a better/simpler way than the following of finding the location of the last dot in a string?

x <- "hello.world.123.456"
g <- gregexpr(".", x, fixed=TRUE)
loc <- g[[1]]
loc[length(loc)]  # returns 16

This finds all the dots in the string and then returns the last one, but it seems rather clumsy. I tried using regular expressions, but didn't get very far.

score 80 · Accepted Answer · edited Nov 10 '17 at 12:41

80

Does this work for you?

x <- "hello.world.123.456"
g <- regexpr("\\.[^\\.]*$", x)
g

\. matches a dot
[^\.] matches everything but a dot
* specifies that the previous expression (everything but a dot) may occur between 0 and unlimited times
$ marks the end of the string.

Taking everything together: find a dot that is followed by anything but a dot until the string ends. R requires \ to be escaped, hence \\ in the expression above. See regex101.com to experiment with regex.

edited Nov 10 '17 at 12:41

Flo

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answered Mar 07 '11 at 01:19

Vincent

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a '.' matches every possible character, to match a literal '.' you need to escape it with a '\' and unfortunatly, you need to escape this '\' with another '\'. So finally your expression looks like '\\.' – CousinCocaine Apr 16 '14 at 09:28
@Vincent, is it there a document or a package where all the symbols and text paterns are explained in detail? – R18 Apr 18 '23 at 10:10

mdsumner · Answer 2 · 2011-03-07T02:55:41.097

31

How about a minor syntax improvement?

This will work for your literal example where the input vector is of length 1. Use escapes to get a literal "." search, and reverse the result to get the last index as the "first":

 rev(gregexpr("\\.", x)[[1]])[1]

A more proper vectorized version (in case x is longer than 1):

 sapply(gregexpr("\\.", x), function(x) rev(x)[1])

and another tidier option to use tail instead:

sapply(gregexpr("\\.", x), tail, 1)

edited Mar 07 '11 at 02:55

answered Mar 07 '11 at 01:12

mdsumner

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9 more to go,but all i wanted to say is: slick. – Matt Bannert Mar 07 '11 at 08:39

score 7 · Answer 3 · answered Mar 07 '11 at 01:37

7

Someone posted the following answer which I really liked, but I notice that they've deleted it:

regexpr("\\.[^\\.]*$", x)

I like it because it directly produces the desired location, without having to search through the results. The regexp is also fairly clean, which is a bit of an exception where regexps are concerned :)

answered Mar 07 '11 at 01:37

Hong Ooi

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yeah, that was me. I thought the previous solution worked so I deleted it. Maybe I shouldn't have :) – Vincent Mar 07 '11 at 02:11

score 2 · Answer 4 · answered Jul 09 '20 at 07:58

There is a slick stri_locate_last function in the stringi package, that can accept both literal strings and regular expressions.

To just find a dot, no regex is required, and it is as easy as

stringi::stri_locate_last_fixed(x, ".")[,1]

If you need to use this function with a regex, to find the location of the last regex match in the string, you should replace _fixed with _regex:

stringi::stri_locate_last_regex(x, "\\.")[,1]

Note the . is a special regex metacharacter and should be escaped when used in a regex to match a literal dot char.

See an R demo online:

x <- "hello.world.123.456"
stringi::stri_locate_last_fixed(x, ".")[,1]
stringi::stri_locate_last_regex(x, "\\.")[,1]

R: Find the last dot in a string

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