regex - Substitute/remove after nth occurrence of substring in string

Question

I'd like a regex for sub in R to substitute the characters in a string which follow the nth occurrence of ";" in that string, where n is a variable number passed to the regex.

  stringA="a; b; c; d; e; f; g; h; i; j;"

    stringB<-sub("^(;){4}.*", "", stringA)
##---------------^My attempt at a regular expression here-------

Desired output:

stringB
    "a; b; c; d;"

Answer 1

You can use the following regex:

^((?:[^;]*;){4}).*

It matches:

• ^ - start of string
• ((?:[^;]*;){4}) - (Group 1) captures a substring comprising 4 (or any number you pass with s variable) occurrences of
  • [^;]* - 0 or more symbols other than ;
  • ; - a literal semi-colon
• .* - 0 or more characters, as many as possible

Using backreference \1 in the replacement pattern we restore the leading substring in the result.

See IDEONE demo (here, the limit threshold is passed as a string):

stringA="a; b; c; d; e; f; g; h; i; j;"
s <- "4"
stringB <- sub(sprintf("^((?:[^;]*;){%s}).*", s), "\1", stringA)
stringB
##  "a; b; c; d;"

Or, if you pass an integer value

s <- 4
sub(sprintf("^((?:[^;]*;){%d}).*", s), "\1", stringA)

See another demo