R / stringr: split string, but keep the delimiters in the output

Question

I tried to search for the solution, but it appears that there is no clear one for R.
I try to split the string by the pattern of, let's say, space and capital letter and I use stringr package for that.

x <- "Foobar foobar, Foobar foobar"
str_split(x, " [:upper:]")

Normally I would get:

[[1]]
[1] "Foobar foobar," "oobar foobar"  

The output I would like to get, however, should include the letter from the delimiter:

[[1]]
[1] "Foobar foobar," "Foobar foobar"

Probably there is no out of box solution in stringr like back-referencing, so I would be happy to get any help.

Answer 1

You may split with 1+ whitespaces that are followed with an uppercase letter:

> str_split(x, "\\s+(?=[[:upper:]])")
[[1]]
[1] "Foobar foobar," "Foobar foobar" 

Here,

• \\s+ - 1 or more whitespaces
• (?=[[:upper:]]) - a positive lookahead (a non-consuming pattern) that only checks for an uppercase letter immediately to the right of the current location in string without adding it to the match value, thus, preserving it in the output.

Note that \s matches various whitespace chars, not just plain regular spaces. Also, it is safer to use [[:upper:]] rather than [:upper:] - if you plan to use the patterns with other regex engines (like PCRE, for example).