Find patterns in strings and concatenate unique parts in R - regex

Question

Conciser having 2 strings s1 and s2. How is it possible to concatenate only those parts in these strings, which are unique, and leave non unique parts as is. Let / be the separator between concatenated parts. The desired result looks like this:
s1 <- "very big house"
s2 <- "very small house"
some_function(s1,s2)
"very big/small house" #// desired result.
Usually leading and tailing charters are the same for both strings and the differences appear only in the middle.
More examples of desired result:
# a
s1 <- "1b"; s2 <- "2b"
"1/2b" # <- new string
# b
s1 <- "a_1_b"; s2 <- "a_2_b"
"a_1/2_b" # <- new string
# c
s1 <- "a"; s2 <- "b"
"a/b" # <- new string

Answer 1

Here's a solution:
pasteMergePrefixAndSuffix <- function(vs1,vs2,sep=' ') {
## cycle string vectors to same length
vsl <- max(length(vs1),length(vs2));
vs1 <- rep(vs1,len=vsl);
vs2 <- rep(vs2,len=vsl);
## precompute character splits
ss1 <- strsplit(vs1,'');
ss2 <- strsplit(vs2,'');
## iterate over each pair of strings
sapply(seq_along(vs1),function(si) {
s1 <- vs1[si];
s2 <- vs2[si];
s1l <- length(ss1[[si]]);
s2l <- length(ss2[[si]]);
slmin <- min(s1l,s2l);
## handle trivial case of exact equality
if (s1==s2) return(s1);
## get prefix and suffix lengths
if (slmin==0L) { ## empty string cannot result in a prefix or suffix
pl <- sl <- 0L;
} else {
eq <- ss1[[si]][seq_len(slmin)]==ss2[[si]][seq_len(slmin)];
pl <- if (all(eq)) slmin else if (eq[1L]==T) which(!eq)[1L]-1L else 0L;
eq <- rev(ss1[[si]])[seq_len(slmin)]==rev(ss2[[si]])[seq_len(slmin)];
sl <- if (all(eq)) slmin else if (eq[1L]==T) which(!eq)[1L]-1L else 0L;
}; ## end if
## paste together prefix, sep-pasted middles, and suffix
m1 <- substr(s1,pl+1L,s1l-sl);
m2 <- substr(s2,pl+1L,s2l-sl);
paste0(substr(s1,1L,pl),paste(sep=sep,m1,m2),substr(s1,s1l-sl+1L,s1l));
});
}; ## end pasteMergePrefixAndSuffix()
Demo:
pasteMergePrefixAndSuffix('a','b');
## [1] "a b"
pasteMergePrefixAndSuffix('a','b','/');
## [1] "a/b"
s1 <- 'very big house'; s2 <- 'very small house'; pasteMergePrefixAndSuffix(s1,s2,'/');
## [1] "very big/small house"
s1 <- '1b'; s2 <- '2b'; pasteMergePrefixAndSuffix(s1,s2,'/');
## [1] "1/2b"
s1 <- 'a_1_b'; s2 <- 'a_2_b'; pasteMergePrefixAndSuffix(s1,s2,'/');
## [1] "a_1/2_b"
s1 <- 'ab'; s2 <- 'ab'; pasteMergePrefixAndSuffix(s1,s2,'/');
## [1] "ab"
s1 <- 'xab'; s2 <- 'ab'; pasteMergePrefixAndSuffix(s1,s2,'/');
## [1] "x/ab"
s1 <- 'ab'; s2 <- 'abx'; pasteMergePrefixAndSuffix(s1,s2,'/');
## [1] "ab/x"
s1 <- 'abx'; s2 <- 'ab'; pasteMergePrefixAndSuffix(s1,s2,'/');
## [1] "abx/"
s1 <- 'ab'; s2 <- 'xab'; pasteMergePrefixAndSuffix(s1,s2,'/');
## [1] "/xab"
s1 <- ''; s2 <- 'x'; pasteMergePrefixAndSuffix(s1,s2,'/');
## [1] "/x"
s1 <- 'x'; s2 <- ''; pasteMergePrefixAndSuffix(s1,s2,'/');
## [1] "x/"