Extract matching words from strings in order

February 13, 2023

If I have two strings that look like this:

x <- "Here is a test of words and stuff."
y <- "Here is a better test of words and stuff."

Is there an easy way to check the words from left to right and create a new string of matching words and then stop when the words no longer match so the output would look like:

> "Here is a"

I don’t want to find all matching words between the two strings but rather just the words that match in order. So "words and stuff." is in both string but I don’t want that to be selected.

>Solution :

You could write a helper function to do the check for you

common_start<-function(x, y) {
  i <- 1
  last <- NA
  while (i <= nchar(x) & i <= nchar(x)) {
    if (substr(x,i,i) == substr(y,i,i)) {
      if (grepl("[[:space:][:punct:]]", substr(x,i,i), perl=T)) {
        last <- i
      }
    } else {
      break;
    }
    i <- i + 1
  }
  if (!is.na(last)) {
    substr(x, 1, last-1)
  } else {
    NA
  }
}

and use that with your sample stirngs