We have many `\foo{something}`. As the string inside the curly braces contains odd number of character, we want to add a space after the string. The result will be `\foo{something }`. We only want odd number of characters only. For example, in `\foo{string}` the string inside the curly braces has even number of character, then we do not add a space.

What I have tried:

1. a dot means a character, two dots means two characters
2. asterisk means repeat 0 or more times
3. Then, `[..]*.` means 1, 3, 5 ... characters.
4. Then, `\foo{[..]*.}` will be my matching sequence.
5. Then, `:%s/\foo{[..]*.}/\foo{[..]*. }/g` will add the space we want.

But we failed.