I mostly use String#gsub (
http://www.ruby-doc.org/core-2.1.0/String.html#method-i-gsub ) . I am
familiar with it also.

But today I found Kernel#gsub (
http://www.ruby-doc.org/core-2.1.1/Kernel.html#method-i-gsub ), which I
have never used it.

What is the use-case of this method ?

I tried to explore this as below :

(arup~>~)$ echo 'matz' | ruby -n -e 'puts $_.gsub(/./,"*")'
****
(arup~>~)$ echo 'matz' | ruby -n -e '$_.gsub(/./,"*") ; puts $_'
matz
(arup~>~)$ echo 'matz' | ruby -np -e '$_.gsub(/./,"*")'
matz

Docs is saying - **Equivalent to $_.gsub..., except that $_ will be
updated if substitution occurs.** But in my case, it seems **$_** has
not been updated.

The reason is `String#gsub` has been called. Not `Kernel#gsub`. See
below :

(arup~>~)$ echo 'matz' | ruby -n -e 'p $_.method(:gsub)'
#<Method: String#gsub>


What am I missing in the doc ? How to call `Kernel#gsub` ?