How to extract strings from a full text using R?

An approach using str_extract and sub. The substitution removes any follow up sentences, if they exists. So the detected plaintiff and defendant can only be one sentence long (。 as the separator).

library(dplyr)
library(stringr)

tibble(text) %>% 
  mutate(plaintiff = sub("(。).*", "\\1", str_extract(text, "The plaintiff.*。")), 
         defendant = sub("(。).*", "\\1", str_extract(text, "The defendant.*。")), 
         law = grepl("《marriage law》", text)) %>% 
  print(Inf)
# A tibble: 1 × 4
  text                                                     plain…¹ defen…² law  
  <chr>                                                    <chr>   <chr>   <lgl>
1 "Ganluo County People's Court of X Province。The plaint… The pl… The de… TRUE 
# … with abbreviated variable names ¹​plaintiff, ²​defendant

full output

# A tibble: 1 × 4
  text                                                                          
  <chr>                                                                         
1 "Ganluo County People's …
  plaintiff                                                                  
  <chr>                                                                      
1 The plaintiff X, female, born on May, 1980, lives in X County, X Province。
  defendant                                                                
  <chr>                                                                    
1 The defendant X, male, born on May, 1971, lives in X County, X Province。
  law  
  <lgl>
1 TRUE 

extended data

text <- "Ganluo County People's Court of X Province。The plaintiff X, female, born on May, 1980, lives in X County, X Province。The defendant X, male, born on May, 1971, lives in X County, X Province。The plaintiff wuen weofioi woe fowie fowie fowei f。The defendant wuen weofioi woe fowie fowie fowei f。The plaintiff wuen weofioi woe fowie fowie fowei f。The defendant wuen weofioi woe fowie fowie fowei f。The plaintiff wuen weofioi woe fowie fowie fowei f。The plaintiff wuen weofioi woe fowie fowie fowei f。\nIt is a divorce dispute, according to 《marriage law》on June 21, 2016。"

Update

With the additional information you've provided, see if this works for you.

This assumed that there is only once sentence each for the plaintiff and the defendant. I've added .* at the end of the 'rovince' (as in Province). That is so that if Province is not the end of the sentence, it still captures the entire sentence. I left off the P so that if capitalization is inconsistent, it doesn't matter.

I've used [^。]+ to capture anything except a period so it can only capture one sentence.

It still assumes that the sentence begins with "The plaintiff" (or defendant).

If this does not work, you'll really need to provide several more examples of potential content.

library(tidyverse)

td3 <- data.frame(oText = text) %>% 
  extract(into = c('plaintiff', 'defendent'), remove = F, col = oText,
          regex = "^.*(The plaintiff[^。]+rovince.*。).*(The defendant[^。]+rovince.*。).*") %>% 
  mutate(law = str_detect(oText, 'marriage law'))

Originally...

How tight are the patterns you've shown here? Is the plaintiff always in the second sentence? Does the defendant's description always follow the plaintiff? Is punctuation always used?

Here's a method that works with this data. This method does not assume any given order, but it does assume punctuation was used.

In the regex used you see 'The plaintiff' (or defendant), followed by .*, which means followed by anything, then ?, which tells us that we want the first occurrence of the lookahead. The lookahead, or where we want the regex to stop looking, is documented in (?= ). You have oddly encoded 。at the end of the sentences (assuming this was translated).

If you have periods or another recognized special character in your real data, you'll have to escape it. In this regex, you saw that the period followed by the asterisk was coding for ...and anything else... so if you were looking for a period or an asterisk, you'd have to 'escape' it so that the regex process knows that you meant the character literally.

library(tidyverse)
library(stringi)

tdf <- data.frame(oText = text) %>% 
  mutate(plaintiff = stri_extract_first_regex(oText, 'The plaintiff.*?(?=(。))'),
         defendent = stri_extract_first_regex(oText, 'The defendant.*?(?=(。))'),
         law = str_detect(oText, 'marriage law'))

If the patterns are strict, you could probably use dplyr::separate to make this even easier.