r ngram extraction with regex

Question

Karl Broman's post: https://kbroman.wordpress.com/2015/06/22/randomized-hobbit-2/ got me playing with regex and ngrams just for fun. I attempted to use regex to extract 2-grams. I know there are parsers to do this but am interested in the regex logic (i.e., it was a self challenge that I failed to meet).

Below I give a minimal example and the desired output. The problem in my attempt is 2 fold:

1. The grams (words) get eaten up and aren't available for the next pass. How can I make them available for the second pass? (e.g., I want like to be available for like toast after it's already been consumed previously in I like)

2. I couldn't make the space between words non-captured (notice the trailing white space in my output even though I used (?:\\s*)). How can I not capture trailing spaces on the nth (in this case second) word? I know this could be done simply with: "(\\b[A-Za-z']+\\s)(\\b[A-Za-z']+)" for a 2-gram but I want to extend the solution to n-grams. PS I know about \\w but I don't consider underscores and numbers as word parts, but do consider ' as a word part.

MWE:

library(stringi)

x <- "I like toast and jam."

stringi::stri_extract_all_regex(
    x,
    pattern = "((\\b[A-Za-z']+\\b)(?:\\s*)){2}"
)

## [[1]]
## [1] "I like "    "toast and "

Desired Output:

## [[1]]
## [1] "I like"  "like toast"    "toast and"  "and jam"

Answer 1

Here's one way using base R regex. This can be easily extended to handle arbitrary n-grams. The trick is to put the capture group inside a positive look-ahead assertion, eg., (?=(my_overlapping_pattern))

x <- "I like toast and jam."
pattern <- "(?=(\\b[A-Za-z']+\\b \\b[A-Za-z']+\\b))"
matches<-gregexpr(pattern, x, perl=TRUE)
# a little post-processing needed to get the capture groups with regmatches
attr(matches[[1]], 'match.length') <- as.vector(attr(matches[[1]], 'capture.length')[,1])
regmatches(x, matches)

# [[1]]
# [1] "I like"     "like toast" "toast and"  "and jam"

Answer 2

Actually, there is an app for that: the quanteda package (for the quantitative analysis of textual data). My coauthor Paul Nulty and I are working hard to improve this, but it easily handles the use case you describe.

install.packages("quanteda")
require(quanteda)
x <- "I like toast and jam."
> ngrams(x, 2)
## [[1]]
## [1] "i_like"     "like_toast" "toast_and"  "and_jam"   
ngrams(x, n = 2, concatenator = " ", toLower = FALSE)
## [[1]]
## [1] "I like"     "like toast" "toast and"  "and jam"   

No painful regexes required!