正则表达式：从多个匹配的括号中提取数字

Question

How do I match the year such that it is general for the following examples.

a <- '"You Are There" (1953) {The Death of Socrates (399 B.C.) (#1.14)}'
b <- 'Þegar það gerist (1998/I) (TV)'

I have tried the following, but did not have the biggest success.

gsub('.+\\(([0-9]+.+\\)).?$', '\\1', a)

What I thought it did was to go until it finds a (, then it would make a group of numbers, then any character until it meets a ). And if there are several matches, I want to extract the first group.

Any suggestions to where I go wrong? I have been doing this in R.

Answer 1

You could use

library(stringr)

strings <- c('"You Are There" (1953) {The Death of Socrates (399 B.C.) (#1.14)}', 'Þegar það gerist (1998/I) (TV)')

years <- str_match(strings, "\\((\\d+(?: B\\.C\\.)?)")[,2]
years
# [1] "1953" "1998"

The expression here is

\(               # (
(\d+             # capture 1+ digits
    (?: B\.C\.)? # B.C. eventually
)

Note that backslashes need to be escaped in R .

Answer 2

Your pattern contains .+ parts that match 1 or more chars as many as possible, and at best your pattern could grab last 4 digit chunks from the incoming strings.

You may use

^.*?\((\d{4})(?:/[^)]*)?\).*

Replace with \\1 to only keep the 4 digit number. See the regex demo .

Details

• ^ - start of string
• .*? - any 0+ chars as few as possible
• \\( - a (
• (\\d{4}) - Group 1: four digits
• (?: - start of an optional non-capturing group
  • / - a /
  • [^)]* - any 0+ chars other than )
• )? - end of the group
• \\) - a ) (OPTIONAL, MAY BE OMITTED)
• .* - the rest of the string.

See the R demo :

a <- c('"You Are There" (1953) {The Death of Socrates (399 B.C.) (#1.14)}', 'Þegar það gerist (1998/I) (TV)', 'Johannes Passion, BWV. 245 (1725 Version) (1996) (V)')
sub("^.*?\\((\\d{4})(?:/[^)]*)?\\).*", "\\1", a) 
# => [1] "1953" "1998" "1996"

Another base R solution is to match the 4 digits after ( :

regmatches(a, regexpr("\\(\\K\\d{4}(?=(?:/[^)]*)?\\))", a, perl=TRUE))
# => [1] "1953" "1998" "1996"

The \\(\\K\\d{4} pattern matches ( and then drops it due to \\K match reset operator and then a (?=(?:/[^)]*)?\\\\)) lookahead ensures there is an optional / + 0+ chars other than ) and then a ) . Note that regexpr extracts the first match only.