Question

I am trying to extract records from a data.frame using grepl. Here are some example cases.

a <- c('This is a healthcare facility', 'this is a hospital', 'this is a hospital district', 'this is a district health service')

I wish to extract all records that have hospital but not district. I have come unstuck when district and hospital occur in the same string. I tried using the dollowing:

str_match(string=a,pattern='hospital|^district' )

How do I limit district but still include hospital in this example?

Thanks.

Answer 1

R supports Perl-compatible regular expressions, which allow negative lookahead assertions, so in principle, you can write:

str_match(string=a, pattern='^(?!.*district).*hospital', perl=TRUE)

(which matches “start-of-string, followed by a point in the string that is not followed by .*district, followed by .*hospital“). That said, I’m really not sure if putting this condition into a single regex is the best way to do it; there may be a more R-ish way.