Regular expression that both includes and excludes certain strings in R

Question

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<

Answer 1

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.

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

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