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.