How to extract sentences containing specific person names using R - regex

Question

I am using R to extract sentences containing specific person names from texts and here is a sample paragraph:
Opposed as a reformer at Tübingen, he accepted a call to the University of Wittenberg by Martin Luther, recommended by his great-uncle Johann Reuchlin. Melanchthon became professor of the Greek language in Wittenberg at the age of 21. He studied the Scripture, especially of Paul, and Evangelical doctrine. He was present at the disputation of Leipzig (1519) as a spectator, but participated by his comments. Johann Eck having attacked his views, Melanchthon replied based on the authority of Scripture in his Defensio contra Johannem Eckium.
In this short paragraph, there are several person names such as:
Johann Reuchlin, Melanchthon, Johann Eck. With the help of openNLP package, three person names Martin Luther, Paul and Melanchthon can be correctly extracted and recognized. Then I have two questions:
How could I extract sentences containing these names?
As the output of named entity recognizer is not so promising, if I add "[[ ]]" to each name such as [[Johann Reuchlin]], [[Melanchthon]], how could I extract sentences containing these name expressions [[A]], [[B]] ...?

Answer 1

Using `strsplit` and `grep`, first I set made an object `para` which was your paragraph.
toMatch <- c("Martin Luther", "Paul", "Melanchthon")
unlist(strsplit(para,split="\\."))[grep(paste(toMatch, collapse="|"),unlist(strsplit(para,split="\\.")))]
> unlist(strsplit(para,split="\\."))[grep(paste(toMatch, collapse="|"),unlist(strsplit(para,split="\\.")))]
[1] "Opposed as a reformer at Tübingen, he accepted a call to the University of Wittenberg by Martin Luther, recommended by his great-uncle Johann Reuchlin"
[2] " Melanchthon became professor of the Greek language in Wittenberg at the age of 21"
[3] " He studied the Scripture, especially of Paul, and Evangelical doctrine"
[4] " Johann Eck having attacked his views, Melanchthon replied based on the authority of Scripture in his Defensio contra Johannem Eckium"
Or a little cleaner:
sentences<-unlist(strsplit(para,split="\\."))
sentences[grep(paste(toMatch, collapse="|"),sentences)]
If you are looking for the sentences that each person is in as separate returns then:
toMatch <- c("Martin Luther", "Paul", "Melanchthon")
sentences<-unlist(strsplit(para,split="\\."))
foo<-function(Match){sentences[grep(Match,sentences)]}
lapply(toMatch,foo)
[[1]]
[1] "Opposed as a reformer at Tübingen, he accepted a call to the University of Wittenberg by Martin Luther, recommended by his great-uncle Johann Reuchlin"
[[2]]
[1] " He studied the Scripture, especially of Paul, and Evangelical doctrine"
[[3]]
[1] " Melanchthon became professor of the Greek language in Wittenberg at the age of 21"
[2] " Johann Eck having attacked his views, Melanchthon replied based on the authority of Scripture in his Defensio contra Johannem Eckium"
Edit 3: To add each persons name, do something simple such as:
foo<-function(Match){c(Match,sentences[grep(Match,sentences)])}
EDIT 4:
And if you wanted to find sentences that had multiple people/places/things (words), then just add an argument for those two such as:
toMatch <- c("Martin Luther", "Paul", "Melanchthon","(?=.*Melanchthon)(?=.*Scripture)")
and change perl to TRUE:
foo<-function(Match){c(Match,sentences[grep(Match,sentences,perl = T)])}
> lapply(toMatch,foo)
[[1]]
[1] "Martin Luther"
[2] "Opposed as a reformer at Tübingen, he accepted a call to the University of Wittenberg by Martin Luther, recommended by his great-uncle Johann Reuchlin"
[[2]]
[1] "Paul"
[2] " He studied the Scripture, especially of Paul, and Evangelical doctrine"
[[3]]
[1] "Melanchthon"
[2] " Melanchthon became professor of the Greek language in Wittenberg at the age of 21"
[3] " Johann Eck having attacked his views, Melanchthon replied based on the authority of Scripture in his Defensio contra Johannem Eckium"
[[4]]
[1] "(?=.*Melanchthon)(?=.*Scripture)"
[2] " Johann Eck having attacked his views, Melanchthon replied based on the authority of Scripture in his Defensio contra Johannem Eckium"
EDIT 5: Answering your other question:
Given:
sentenceR<-"Opposed as a reformer at [[Tübingen]], he accepted a call to the University of [[Wittenberg]] by [[Martin Luther]], recommended by his great-uncle [[Johann Reuchlin]]"
gsub("\\[\\[|\\]\\]", "", regmatches(sentenceR, gregexpr("\\[\\[.*?\\]\\]", sentenceR))[[1]])
Will give you the words inside the double brackets.
> gsub("\\[\\[|\\]\\]", "", regmatches(sentenceR, gregexpr("\\[\\[.*?\\]\\]", sentenceR))[[1]])
[1] "Tübingen" "Wittenberg" "Martin Luther" "Johann Reuchlin"

Answer 2

Here's a considerably simpler method using two packages quanteda and stringi:
sents <- unlist(quanteda::tokenize(txt, what = "sentence"))
namesToExtract <- c("Martin Luther", "Paul", "Melanchthon")
namesFound <- unlist(stringi::stri_extract_all_regex(sents, paste(namesToExtract, collapse = "|")))
sentList <- split(sents, list(namesFound))
sentList[["Melanchthon"]]
## [1] "Melanchthon became professor of the Greek language in Wittenberg at the age of 21."
## [2] "Johann Eck having attacked his views, Melanchthon replied based on the authority of Scripture in his Defensio contra Johannem Eckium."
sentList
## $`Martin Luther`
## [1] "Opposed as a reformer at Tübingen, he accepted a call to the University of Wittenberg by Martin Luther, recommended by his great-uncle Johann Reuchlin."
##
## $Melanchthon
## [1] "Melanchthon became professor of the Greek language in Wittenberg at the age of 21."
## [2] "Johann Eck having attacked his views, Melanchthon replied based on the authority of Scripture in his Defensio contra Johannem Eckium."
##
## $Paul
## [1] "He studied the Scripture, especially of Paul, and Evangelical doctrine."