Vim Regex Capture Groups [bau -> byau : ceu -> cyeu] - regex

Question

I have a list of words:
bau
ceu
diu
fou
gau
I want to turn that list into:
byau
cyeu
dyiu
fyou
gyau
I unsuccessfully tried the command:
:%s/(\w)(\w\w)/\1y\2/g
Given that this doesn't work, what do I have to change to make the regex capture groups work in Vim?

Answer 1

One way to fix this is by ensuring the pattern is enclosed by escaped parentheses:
:%s/\(\w\)\(\w\w\)/\1y\2/g
Slightly shorter (and more magic-al) is to use \v, meaning that in the pattern after it all ASCII characters except '0'-'9', 'a'-'z', 'A'-'Z' and '_' have a special meaning:
:%s/\v(\w)(\w\w)/\1y\2/g
See:
:help \(
:help \v

Answer 2

You can also use this pattern which is shorter:
:%s/^./&y
%s applies the pattern to the whole file.
^. matches the first character of the line.
&y adds the y after the pattern.

Answer 3

If you don't want to escape the capturing groups with backslashes (this is what you've missed), prepend \v to turn Vim's regular expression engine into very magic mode:
:%s/\v(\w)(\w\w)/\1y\2/g

Answer 4

You also have to escape the Grouping paranthesis:
:%s/\(\w\)\(\w\w\)/\1y\2/g
That does the trick.

Answer 5

In Vim, on a selection, the following
:'<,'>s/^\(\w\+ - \w\+\).*/\1/
or
:'<,'>s/\v^(\w+ - \w+).*/\1/
parses
Space - Commercial - Boeing
to
Space - Commercial
Similarly,
apple - banana - cake - donuts - eggs
is parsed to
apple - banana
Explanation
^ : match start of line
\-escape (, +, ) per the first regex (accepted answer) -- or prepend with \v (#ingo-karkat's answer)
\w\+ finds a word (\w will find the first character): in this example, I search for a word followed by - followed by another word)
.* after the capturing group is needed to find / match / exclude the remaining text
Addendum. This is a bit off topic, but I would suggest that Vim is not well-suited for the execution of more complex regex expressions / captures. [I am doing something similar to the following, which is how I found this thread.]
In those instances, it is likely better to dump the lines to a text file and edit it "in place"
sed -i ...
or in a redirect
sed ... > out.txt
In a terminal (or BASH script, ...):
echo 'Space Sciences - Private Industry - Boeing' | sed -r 's/^((\w+ ){1,2}- (\w+ ){1,2}).*/\1/'
Space Sciences - Private Industry
cat in.txt
Space Sciences - Private Industry - Boeing
sed -r 's/^((\w+ ){1,2}- (\w+ ){1,2}).*/\1/' ~/in.txt > ~/out.txt
cat ~/out.txt
Space Sciences - Private Industry
## Caution: if you forget the > redirect, you'll edit your source.
## Subsequent > redirects also overwrite the output; use >> to append
## subsequent iterations to the output (preserving the previous output).
## To edit "in place" (`-i` argument/flag):
sed -i -r 's/^((\w+ ){1,2}- (\w+ ){1,2}).*/\1/' ~/in.txt
cat in.txt
Space Sciences - Private Industry
sed -r 's/^((\w+ ){1,2}- (\w+ ){1,2}).*/\1/'
(note the {1,2}) allows the flexibility of finding {x,y} repetitions of a word(s) -- see https://www.gnu.org/software/sed/manual/html_node/Regular-Expressions.html .
Here, since my phrases are separated by -, I can simply tweak those parameters to get what I want.