regular expression excluding word in R

Question

I don't really know how to correctly find words using non-matching regular expression in R

Eg: the data includes:

x =  c("hail", "small hail", "wind hail",  "deep hail",  "thunderstorm hail", "tstm wind hail", "gusty wind hail", "late season hail", "non severe hail", "marine hail")

I want to find all obs having "hail" but not having "marine"

My attempt:

x[grep("[^(marine)] hail", x)]

-> I have only 5:

"small hail"      "wind hail"       "deep hail"       "tstm wind hail"  "gusty wind hail"

I don't know what happens with the other 4

Answer 1

Use lookaround assertions.

> x =  c("hail", "small hail", "wind hail",  "deep hail",  "thunderstorm hail", "tstm wind hail", "gusty wind hail", "late season hail", "non severe hail", "marine hail")
> x[grep("^(?=.*hail)(?!.*marine)", x, perl=TRUE)]
[1] "hail"              "small hail"        "wind hail"        
[4] "deep hail"         "thunderstorm hail" "tstm wind hail"   
[7] "gusty wind hail"   "late season hail"  "non severe hail" 

OR

Add \b boundaries if necessary. \b matches between a word character and a non-word character.

> x[grep("^(?=.*\\bhail\\b)(?!.*\\bmarine\\b)", x, perl=TRUE)]

• ^ Asserts that we are at the start.

• (?=.*hail) Positive lookahead which asserts that the match must contain the string hail

• (?!.*marine) Negative lookahead which asserts that the match won't contain the string marine.

• So the above regex would match the starting anchor or the start of the line only if both conditions are satisfied.

Answer 2

You want to use a lookahead assertion in this circumstance. The current implementation of your negated character class does not do what you expect, instead it matches the following:

[^(marine)]  # any character except: '(', 'm', 'a', 'r', 'i', 'n', 'e', ')'
 hail        # ' hail'

To fix this, you could simply do:

> x[grep('^(?!.*marine).*hail', x, perl=TRUE)]
# [1] "hail"              "small hail"        "wind hail"        
# [4] "deep hail"         "thunderstorm hail" "tstm wind hail"   
# [7] "gusty wind hail"   "late season hail"  "non severe hail"