Create a look around expression and add it to an existing regular expression

Lookarounds allow you to position a regular expression to more specificity.

Usage

create_lookaround(pattern, lookaround_pattern, position, negate = FALSE)

Arguments

pattern: String. Pattern that you wish to add a look around to
lookaround_pattern: String. Pattern that you wish to look for.
position: String. One of 'before', 'after', 'ahead', or 'behind'. Capitalization doesn't matter
negate: Logical. allows you to exclude cases where look around is detected.

Value

Returns a string that can be used as a regular expression

Examples


# Here is a string with three patterns of digits
text <- "cars123ruin456cities789"

# To extract the first one we can use this pattern
stringr::str_extract(text, "\\d{3}")
#> [1] "123"
# or
create_lookaround("\\d{3}", "cars", "before") |>
stringr::str_extract(string=text)
#> [1] "123"

# To exclude the first one we can write
create_lookaround("\\d{3}", "cars", "before", negate=TRUE) |>
stringr::str_extract_all(string=text)
#> [[1]]
#> [1] "456" "789"
#> 

# To extract the second one we can write
 create_lookaround("\\d{3}", "ruin", "before") |>
stringr::str_extract(string=text)
#> [1] "456"

# or

create_lookaround("\\d{3}", "cities", "after") |>
stringr::str_extract(string=text)
#> [1] "456"