Exclude everything after the second occurrence of a certain string

Question

I have the following string

string <- c('a - b - c - d',
            'z - c - b',
            'y',
            'u - z')

I would like to subset it such that everything after the second occurrence of ' - ' is thrown away.

The result would be this:

> string
[1]  "a - b" "z - c" "y"     "u - z"

I used substr(x = string, 1, regexpr(string, pattern = '[^ - ]*$') - 4), but it excludes the last occurrence of ' - ', which is not what I want .

Answer 1

Note that you cannot use a negated character class to negate a sequence of characters. [^ - ]*$ matches any 0+ chars other than a space (yes, it matches -, too, because the - created a range between a space and a space) followed by the end of the string marker ($).

You may use a sub function with the following regex:

^(.*? - .*?) - .*

to replace with \1. See the regex demo.

R code:

> string <- c('a - b - c - d', 'z - c - b', 'y', 'u - z')
> sub("^(.*? - .*?) - .*", "\\1", string)
[1] "a - b" "z - c" "y"     "u - z"

Details:

• ^ - start of a string
• (.*? - .*?) - Group 1 (referred to with the \1 backreference in the replacement pattern) capturing any 0+ chars lazily up to the first space, hyphen, space and then again any 0+ chars up to the next leftmost occurrence of space, hyphen, space
• - - a space, hyphen and a space
• .* - any zero or more chars up to the end of the string.