Using regex to selectively extract substrings in R

user_1700047 0
Viewed 68

Suppose I have the following strings:

string <- c(
  "DATE_OF_BIRTH_B1",
  "HEIGHT_BABY2",
  "WEIGHT_BABY_3",
  "OTHER_CONDITION_4",
  "OTHER_OPERATION_5"
)

How can I use regex in gsub() to extract:

  • Everything except the trailing underscore up until the number suffixes in the first three strings;
  • Nothing from the last two strings.

In other words, my expected gsub() output is:

"DATE_OF_BIRTH_B", "HEIGHT_BABY", "WEIGHT_BABY"

I managed to use gsub("(.+_B[A-Z]*)_?[0-9]", "\\1", string) to extract the desired substrings from the first three strings, but it failed to excluded the last two strings.

Could anyone help to correct and improve my regex, with a bit of explanation? Many thanks!

r regex
2 Answers

Remove OTHER or the suffix.

gsub("^OTHER.*|_?[0-9]+$", "", string)
#> [1] "DATE_OF_BIRTH_B"
#> [2] "HEIGHT_BABY"    
#> [3] "WEIGHT_BABY"    
#> [4] ""               
#> [5] ""

Or, if you specifically want capture groups, use a non-greedy capture.

gsub("(OTHER.*)?(.*?)_?[0-9]", "\\2", string)
#> [1] "DATE_OF_BIRTH_B"
#> [2] "HEIGHT_BABY"    
#> [3] "WEIGHT_BABY"    
#> [4] ""               
#> [5] ""
user_17519610

If you expect gsub (or sub, usually, in this case, you really should use a sub since you only expect a single replacement operation) to return a result of the replacement or an empty string, you need to follow this technique:

sub("...(<what_you_want_to_extract>)...|.+", "\\1", x)

That is, your regex is before | alternation operator that is followed with .+ that matches any one or more chars as many as possible.

So, in your case, assuming your regex is just what you need and meets all your requirements, you can use

> res <- sub("(.+_B[A-Z]*)_?[0-9]|.+", "\\1", string)
> res
[1] "DATE_OF_BIRTH_B" "HEIGHT_BABY"     "WEIGHT_BABY"     ""                ""

If you need to remove empty items, just use

> res[nzchar(res)]
[1] "DATE_OF_BIRTH_B" "HEIGHT_BABY"     "WEIGHT_BABY"
user_38329700
Related