Find overlapping Regexp matches

Question

I want to find all matches within a given string including overlapping matches. How could I achieve it?

# Example
"a-b-c-d".???(/\w-\w/)  # => ["a-b", "b-c", "c-d"] expected

# Solution without overlapped results
"a-b-c-d".scan(/\w-\w/) # => ["a-b", "c-d"], but "b-c" is missing

Wiktor Stribiżew · Accepted Answer · 2016-12-08T07:43:03.307

11

Use capturing inside a positive lookahead:

"a-b-c-d".scan(/(?=(\w-\w))/).flatten
 # => ["a-b", "b-c", "c-d"]

See Ruby demo

edited Dec 08 '16 at 07:43

answered Dec 07 '16 at 22:37

Wiktor Stribiżew

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Works perfectly, also with complex patterns. `"abaca".scan(/(?=(\w)(?:(?!\1)(\w))\1)/) #=> [["a", "b"], ["a", "c"]]`. – sschmeck Dec 07 '16 at 22:50
The inspect seems not only unnecessary, but actually harmful to the end result. – Eli Sadoff Dec 08 '16 at 03:14
2

...or positive lookbehind: `"a-b-c-d".scan(/(?<=(\w-\w))/).flatten`. – Cary Swoveland Dec 08 '16 at 05:18
3

Guys, `inspect` is only used in the above snippet **to print out demo results** in the online demo. No one tells to use `inspect` in real code. I remove it to avoid further confusion. – Wiktor Stribiżew Dec 08 '16 at 07:40

score 4 · Answer 2 · edited Dec 08 '16 at 07:41

4

I suggest a non-regex solution:

"a-b-c-d".delete('-').each_char.each_cons(2).map { |s| s.join('-') }
  #=> ["a-b", "b-c", "c-d"]

or

"a-b-c-d".each_char.each_cons(3).select.with_index { |_,i| i.even? }.map(&:join)
  #=> ["a-b", "b-c", "c-d"]

or

enum = "a-b-c-d".each_char
a = []
loop do
  a << "%s%s%s" % [enum.next, enum.next, enum.peek]
end
a #=> ["a-b", "b-c", "c-d"]

edited Dec 08 '16 at 07:41

Wiktor Stribiżew

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answered Dec 08 '16 at 05:05

Cary Swoveland

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1

Your last technique is very very neat. – Sagar Pandya Dec 08 '16 at 06:28
1

Nice answer, but I was explicitly asking for a regexp. The pattern above was just the simplest one I found to explain my problem. The pattern I actually use, is `(\w)(?:(?!\1)(\w))\1`. – sschmeck Dec 08 '16 at 07:03

Find overlapping Regexp matches

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