Extracting first word after a specific expression in R - r

Question

I have a column that contains thousands of descriptions like this (example) :
Description
Building a hospital in the city of LA, USA
Building a school in the city of NYC, USA
Building shops in the city of Chicago, USA
I'd like to create a column with the first word after "city of", like that :
Description
City
Building a hospital in the city of LA, USA
LA
Building a school in the city of NYC, USA
NYC
Building shops in the city of Chicago, USA
Chicago
I tried with the following code after seeing this topic Extracting string after specific word, but my column is only filled with missing values
library(stringr)
df$city <- data.frame(str_extract(df$Description, "(?<=city of:\\s)[^;]+"))
df$city <- data.frame(str_extract(df$Description, "(?<=of:\\s)[^;]+"))
I took a look at the dput() and the output is the same than the descriptions i see in the dataframe directly.

Answer 1

Solution
This should make the trick for the data you showed:
df$city <- str_extract(df$Description, "(?<=city of )(\\w+)")
df
#> Description city
#> 1 Building a hospital in the city of LA, USA LA
#> 2 Building a school in the city of NYC, USA NYC
#> 3 Building shops in the city of Chicago, USA Chicago
Alternative
However, in case you want the whole string till the first comma (for example in case of cities with a blank in the name), you can go with:
df$city <- str_extract(df$Description, "(?<=city of )(.+)(?=,)")
Check out the following example:
df <- data.frame(Description = c("Building a hospital in the city of LA, USA",
"Building a school in the city of NYC, USA",
"Building shops in the city of Chicago, USA",
"Building a church in the city of Salt Lake City, USA"))
str_extract(df$Description, "(?<=the city of )(\\w+)")
#> [1] "LA" "NYC" "Chicago" "Salt"
str_extract(df$Description, "(?<=the city of )(.+)(?=,)")
#> [1] "LA" "NYC" "Chicago" "Salt Lake City"
Documentation
Check out ?regex:
Patterns (?=...) and (?!...) are zero-width positive and negative
lookahead assertions: they match if an attempt to match the ...
forward from the current position would succeed (or not), but use up
no characters in the string being processed. Patterns (?<=...) and
(?<!...) are the lookbehind equivalents: they do not allow repetition
quantifiers nor \C in ....