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Replace sequences of the same characters with a single character

 $ echo heeeeeeelllo | sed 's/\(.\)\1\+/\1/g'

— by Janos on Dec. 11, 2013, 7:58 p.m.

Explanation

That is, this will output "helo".

The interesting thing here is the regular expression in the s/// command of sed:

  • \(.\) -- capture any character
  • \1 -- refers to the last captured string, in our case the previous character. So effectively, \(.\)\1 matches pairs of the same character, for example aa, bb, ??, and so on.
  • \+ -- match one or more of the pattern right before it
  • ... and we replace what we matched with \1, the last captured string, which is the first letter in a sequence like aaaa, or bbbbbbb, or cc.