How to insert a word using bash between repeated characters in a file?

Question

bash

Hello!
Is available:
a certain file with the text inside.
the desired piece of text:
" mydomain.ru/blablablabla "
Need:
insert "www" in each place where the domain is specified, so that it is " www.mydomain.ru/blablabla ".
I know that using sed, but I don’t know how exactly, there is absolutely no time to thoroughly manual sed :(
Thank you!

Answer 1

sed 's/mydomain\.ru/www.mydomain.ru/g'