Find word in a line and output in which line the word occurs / no. of times it occurred

I have a file: file.txt, which contains the following data in it.

 
    This is a file, my name is Karl, what is this process, karl is karl junior, file is a test file, file's name is file.txt
    My name is not Karl, my name is Karl Joey
    What is your name?
    Do you know your name and what it is?

When I'm running the following command to get line# of a word "is" in this file using the following command, it's giving me the output like:

 
$ grep -now "is" file.txt

 
    1:is
    is
    is
    is
    is
    is
    2:is
    is
    3:is
    4:is

What command can I run to get the following output:

 
    1:is
    1:is
    1:is
    1:is
    1:is
    1:is
    2:is
    2:is
    3:is
    4:is

OR

 
    is:1,1,1,1,1
    is:2,2
    is:3
    is:4

Is this a homework assignment?

NOPE

Your title for this this thread is "Find word in a line and output in which line the word occurs / no. of times it occurred", but neither of the outputs you say you want includes the "no. of times it occurred".

Did you give up on getting output like:

1:is:5
2:is:2
3:is:1
4:is:1

or:

is:1:5,2:2,3:1,4:1

or is the thread title just misleading?