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:help change.txt mentions 2- or 3- letter :substitute commands, such as :sg, :sgn and so on. I am guessing that these :s[xx] correspond to flags, but am not sure how to use them.
For example, I would think that :%sg/foo/bar would substitute all matches of foo, including multiple occurences on the same line. However, that commands gives me an error --- "trailing characters".
What is the appropriate syntax for :s[x[x]]?
Actually the behavior of the substitution command :s if you don't provide any arguments is to repeat the last substitution you did. So if you have a line
foo Foo foF
And use :s/f/X/ you will get
Xoo Foo foF
Now if you use :s (without any argument) you will reapply the same command and get as a result:
Xoo Foo XoF
The 2 and 3 letters substitution commands work the same but adding some flags at the end of the original substitution. So now if you have the previous line and use :sgi you will get the equivalent of :s/f/X/gi which will result in
Xoo Xoo XoX
That explains why you get E488 Trailing characters the short substitutes commands don't take an argument as they only apply the last :s command.
statox has given a complete answer. I tried to find this in the documentation, searching for :h :sg, :h :sgn, etc. But that only provided a table of 2-letter and 3-letter substitute commands.
It turns out that the behavior in statox's answer is documented much higher up in the help file. To find it, refer to :h :& or the paragraph after :h :s. Quoting from the help (emphasis mine):
:[range]s[ubstitute] [flags] [count]
:[range]&[&][flags] [count]Repeat last :substitute with same search pattern and substitute string, but without the same flags. You may add [flags], see |:s_flags|.
Note that after:substitutethe '&' flag can't be used, it's recognized as a pattern separator.
The space between:substituteand the 'c', 'g', 'i', 'I' and 'r' flags isn't required, but in scripts it's a good idea to keep it to avoid confusion.
