What is the \& pattern in Vim's Regex

Question

I recently came across the branch specifier in Vim regex builtins. Vim's help section on \\& contains this:

A branch is one or more concats, separated by "\&".  It matches the last
concat, but only if all the preceding concats also match at the same
position.  Examples:
      "foobeep\&..." matches "foo" in "foobeep".
      ".*Peter\&.*Bob" matches in a line containing both "Peter" and "Bob"

It's not clear how it is used and what it is used for. A good explanation of what it does and how it is used would be great.

To be clear this is not the & (replace with whole match) used in a substitution, this is the \\& used in a pattern.

Example usage:

/\c\v([^aeiou]&\a){4}

Used to search for 4 consecutive consonants (Taken from vim tips).

Answer 1

Explanation:

\\& is to \\| , what the and operator is to the or operator. Thus, both concats have to match, but only the last will be highlighted.

Example 1:

(The following tests assume :setlocal hlsearch .)

Imagine this string:

foo foobar

Now, /foo will highlight foo in both words. But sometimes you just want to match the foo in foobar . Then you have to use /foobar\\&foo .

That's how it works anyway. Is it often used? I haven't seen it more than a few times so far. Most people will probably use zero-width atoms in such simple cases. Eg the same as in this example could be done via /foo\\zebar .

Example 2:

/\\c\\v([^aeiou]&\\a){4} .

\\c - ignore case

\\v - "very magic" (-> you don't have to escape the & in this case)

(){4} - repeat the same pattern 4 times

[^aeiou] - exclude these characters

\\a - alphabetic character

Thus, this, rather confusing, regexp would match xxxx , XXXX , wXyZ or WxYz but not AAAA or xxx1 . Putting it in simple terms: Match any string of 4 alphabetic characters that doesn't contain either 'a', 'e', 'i', 'o' or 'u'.

Answer 2

\\& can be used to match a line containing two (or more) words in any order. For example,

/.*one\\&.*two\\&.*three

will find lines containing one , two and three in any order. The .* is necessary because each branch must start matching in the same place.

Note, the last branch is the one that participates in any substitution. For example, applying the following substitution:

s/.*one\\&.*two\\&.*three/<&>/

on the line

The numbers three, two, and one

results in

<The numbers three>, two, and one