Vi magic

I was trying to replace the pipe character with a comma in a file using vi. I tried using

:%s/\|/,/g

to replace all pipes with commas in the following string.

|a|b|c|d

But this didn’t work and resulted in a comma before each letter as follows.

,|,a,|,b,|,c,|,d

Searching on the internet, I found a suggestion that worked.

:%s/\v\|/,/g

this correctly replaced the pipes to commas as expected.

a,b,c,d

After some digging through the vi manual, I found out that this was because vi has a special meaning for ‘\|’ when used in patterns.

In a vi substitution pattern, some characters are taken literally and gain special meaning if preceded by a backslash. At the same time, there are some other characters that have a special meaning by default and need to be preceded by a backslash for a literal match.

This is governed by the ‘magic’ option in vi.

The pipe character is non-magic by default which means it need not be escaped for a literal search. So the expression

:%s/|/,/g

would have also worked in this case.

The expression

:%s/\|/,/g

escaped the pipe character and therefore made it “magic”. Pipe, in its special form is treated as a separator. So in this case, the expression reads

“Find nothing and replace with a comma i.e., add a comma before every character.”

Using \v makes every following character to be treated as special or magical. Therefore

:%s/\v|/,/g

makes the pipe character from non-magical to magical.

But escaping pipe with a slash makes it non-magical again.

:%s/\v\|/,/g

The expression, therefore, now reads as

“Replace pipe character with comma”

which works as intended.

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