R gsub stringi, stringr misunderstanding : I don't seem to understand gsub or stringr.
Example:

 

     > a<- "a book"
    
    > gsub(" ", ".", a)

    [1] "a.book"

Okay. BUT:

    > a<-"a.book"
    
    > gsub(".", " ", a)

    [1] "      "

I would of expected 
>"a book"

I'm replacing the full stop with a space. 

Also: `srintr`: `str_replace(a, ".", " ")` returns:

`" .book"` 

and `str_replace_all(a, ".", " ")` returns

`"      "`

I can use `stringi`: `stri_replace(a, " ", fixed=".")`:

`"a book"`

I'm just wondering why gsub (and str_replace) don't act as I'd have expected. They work when replacing a space with another character, but not the other way around. 