Looking on the internet deeply has found these results:

no worries is the most popular phrase on the web.

not a problem 12,620,000 results on the web	no worries 1164,000,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
May 27, 2013 ... ... someone born before 1980, just keep telling him "No problem" when they ask you to do something that is most certainly NOT a problem. Jul 3, 2012 ... Many people seem to have adopted the expression "Not a problem" in place of " You're welcome." For example, if I thank someone in email, ... Apr 20, 2007 ... I'd normally say "it's no problem", but now that I've heard some chap saying "it's not a problem" I'm no longer sure whether "it's no problem" is ... Jan 12, 2015 ... [It] is not [a problem][to do that]. (we added the pronoun "it". A pronoun, speaking simply, is a word that is used as a stand in for a noun.	Do what you want, no worries... If it's your job, no worries. It's OK, no worries. (hurried) Okay, no worries, love. But no worries, Shane thinks he can reattach it. I do know, and, again, no worries. And if you are looking for a Multilingual WordPress theme, no worries thanks to the support of the plugin WPML. But no worries, we can repurpose the wheel.

not a problem

12,620,000 results on the web

no worries

1164,000,000 results on the web

More popular!

May 27, 2013 ... ... someone born before 1980, just keep telling him "No problem" when they ask you to do something that is most certainly NOT a problem.
Jul 3, 2012 ... Many people seem to have adopted the expression "Not a problem" in place of " You're welcome." For example, if I thank someone in email, ...
Apr 20, 2007 ... I'd normally say "it's no problem", but now that I've heard some chap saying "it's not a problem" I'm no longer sure whether "it's no problem" is ...
Jan 12, 2015 ... [It] is not [a problem][to do that]. (we added the pronoun "it". A pronoun, speaking simply, is a word that is used as a stand
in for a noun.

Do what you want, no worries...
If it's your job, no worries.
It's OK, no worries.
(hurried) Okay, no worries, love.
But no worries, Shane thinks he can reattach it.
I do know, and, again, no worries.
And if you are looking for a Multilingual WordPress theme, no worries thanks to the support of the plugin WPML.
But no worries, we can repurpose the wheel.