Regular expression that both includes and excludes certain strings in R

r, regex

Solution

Using a regular expression, you could do the following.

x <- c('I like apples', 'I really like apples', 
       'I like apples and oranges', 'I like oranges and apples',
       'I really like oranges and apples but oranges more')

x[grepl('^((?!.*orange).)*apple.*$', x, perl=TRUE)]
# [1] "I like apples"        "I really like apples"

The regular expression looks ahead to see if there's no character except a line break and no substring `orange` and if so, then the dot `.` will match any character except a line break as it is wrapped in a group, and repeated (`0` or more times). Next we look for `apple` and any character except a line break (`0` or more times). Finally, the start and end of line anchors are in place to make sure the input is consumed.

UPDATE: You could use the following if performance is an issue.

x[grepl('^(?!.*orange).*$', x, perl=TRUE)]

Problem

I am trying to use R to parse through a number of entries. I have two requirements for the the entries I want back. I want all the entries that contain the word `apple` but don't contain the word `orange`. For example: - I like apples - I really like apples - I like apples and oranges I want to get entries 1 and 2 back. How could I go about using R to do this? Thanks.

Original source