Finding all groups of contiguous words in string - ruby

Question

I need to find all groups of two contiguous words in a string, but only of words that have 2-3 chars of length. So far I've come with this:
'toolong fee fi fo fum toolong verylong aa bb'.scan(/\b[a-z]{2,3}\s+\b[a-z]{2,3}/)
=> ["fee fi", "fo fum", "aa bb"]
But I want something like this:
=> ["fee fi", "fi fo", "fo fum", "aa bb"]
Any help greatly appreciated.

Answer 1

You need to use lookahead along with capturing group in-order to do overlapping matches.
> 'toolong fee fi fo fum toolong verylong aa bb'.scan(/(?=\b([a-z]{2,3}\s+[a-z]{2,3})\b)/)
=> [["fee fi"], ["fi fo"], ["fo fum"], ["aa bb"]]
> 'toolong fee fi fo fum toolong verylong aa bb'.scan(/\b(?=([a-z]{2,3}\s+[a-z]{2,3})\b)/).flatten
=> ["fee fi", "fi fo", "fo fum", "aa bb"]

Answer 2

The logical way is to consume the first 3 ltr word, then lookahead for the
next one.
Since you want both words together, you'd capture each one then join
them together after each match. \b([a-z]{2,3})(?=(\s+[a-z]{2,3})\b)
\b
( [a-z]{2,3} ) # (1)
(?=
( # (2 start)
\s+
[a-z]{2,3}
) # (2 end)
\b
)
The next logical way (though, not intuitive) is to lookahead for the
combined 2 words, then consume the first one to advance the match
position. (?=\b(([a-z]{2,3})\s+[a-z]{2,3})\b)\2
This way lets you just grab group 1 without the need to join.
(?=
\b
( # (1 start)
( [a-z]{2,3} ) # (2)
\s+
[a-z]{2,3}
) # (1 end)
\b
)
\2