What does it mean to replace slashes (/) by exclamation marks (!) in a substitute command?

Question

I recently saw a substitute command where the author had the / replaced by a ! like this: :s!abc!ABC!g

I tried to find some documentation on this usage but I couldn't find anything relevant. So I tried to experiment by myself and once again I couldn't figure out the real difference between the two signs. Here's what I found:

• It's not possible to mix ! and / in a command. For example :s!abc/ABC fails.
• It may be useful to use ! to avoid escaping a /in a pattern. For example if I want to replace </ with % I can use :%s!</!%!g instead of :%s/<\//%/g.
• It seems that in some case some regex won't work with / and works properly with ! but as I'm not really a regex expert i'm not sure of that.

So my question is simple: What is the advantage of using ! in a substitute command and when should I decide to use it instead of /?

Answer 1

From :help :global:

Instead of the '/' which surrounds the {pattern}, you can use any other single byte character, but not an alphabetic character, '', '"' or '|'. This is useful if you want to include a '/' in the search pattern or replacement string.

Also see :help pattern-delimiter.

As you already devised from your experimentation, this is to prevent the so-called "leaning toothpick syndrome". Consider this:

:%s/\/home\/martin\/test/\/home\/jake\/x/

versus:

:%s!/home/martin/test!/home/jake/x!

The second form is obviously a lot more readable. I prefer to use ! or | for this as it's the most similar to /, but other prefer @, #, ^, or something else.

This is the only reason you can change the delimiter; to make it more readable for us humans. The computer doesn't care.

Some other programs are even more flexible by the way, in GNU sed for example you can use x as delimiter if you wanted. sed sxaxbxg file is the same as sed s/a/b/g file, although I don't think that's especially readable.