Question

I have string and character vector. I would like to find all strings in character vector matching as much as possible characters from beging of string.
For example:

s <- "abs"
vc <- c("ab","bb","abc","acbd","dert")

result <- c("ab","abc")

String s should be matched exactly up to first K characters. I want match for as much as possible (max K<=length(s)).
Here there is no match for “abs” (grep(“abs”,vc)), but for “ab” there are two matches (result <-grep(“ab”,vc)).

Answer 1

Another interpretation:

s <- "abs"
# Updated vc
vc <- c("ab","bb","abc","acbd","dert","abwabsabs")

st <- strsplit(s, "")[[1]]
mtc <- sapply(strsplit(substr(vc, 1, nchar(s)), ""), 
              function(i) {
                m <- i == st[1:length(i)]
                sum(m * cumsum(m))})

vc[mtc == max(mtc)]
#[1] "ab"        "abc"       "abwabsabs"

# Another vector vc
vc <- c("ab","bb","abc","acbd","dert","absq","abab")
....
vc[mtc == max(mtc)]
#[1] "absq"

Since we are considering only beginnings of strings, in the first case the longest match was "ab", even though there is "abwabsabs" which has "abs".

Edit: Here is a “single pattern” solution, possibly it could be more concise, but here we go…

vc <- c("ab","bb","abc","acbd","dert","abwabsabs")
(auxOne <- sapply((nchar(s)-1):1, function(i) substr(s, 1, i)))
#[1] "ab"   "a"
(auxTwo <- sapply(nchar(s):2, function(i) substring(s, i)))
#[1] "s" "bs" 
l <- attr(regexpr(
  paste0("^((",s,")|",paste0("(",auxOne,"(?!",auxTwo,"))",collapse="|"),")"),
  vc, perl = TRUE), "match.length")
vc[l == max(l)]
#[1] "ab"        "abc"       "abwabsabs"