Vim: how to undo a regex with a new regex?

Posted by Victor Tihomirov on Stack Overflow See other posts from Stack Overflow or by Victor Tihomirov
Published on 2013-11-03T03:10:29Z Indexed on 2013/11/03 3:53 UTC
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regex

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vim

my I have here a text that has been encrypted in Vi this way:

1,$s/\(.\)\1/&&/g
1,$s/\(.\)\(.\)\2\2\2/\2\2\2\1\2/g
1,$s/\(.\)\(.\)\(.\)\(.\)/\3\1\4\2/g
1,$s/\(...\)\(...\)/\2\1/g

The task is to write a new regex for each regex in order to undo the the changes, so-called decrypting of the text.

I figured out how to undo the last 2 changes:

:1,$s/\(...\)\(...\)/\2\1/g
:1,$s/\(.\)\(.\)\(.\)\(.\)/\2\4\1\3/g

I'd like you to explain me what do the first two regex do. What does \1 or \2\2\2 mean before the forward slash? I assume that after the slash they change place (but wouldn't it be enough to just write \2\1 instead of \2\2\2\1\2(what does it mean?)? I just don't get it). And what does & or && mean?

Thank you

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