regex level3 what a string mean?  Go to the Training: Regex challenge

my regex: CENSORED
And it told me that I captured a string which is unwanted .
I want to know if it means "we just need filename, we don't need a string your regex may match" or "there is are unexpected chars and you shouldn't capture them "
And i have tested my regex somewhere, it works. Now I have no idea and run out of my brains

Please try to not spoil the challenge by including possible/partial solutions.

Perhaps learn about what "capture" means in this context. The error you get actually tells you how to resolve it.

I am sorry about spoiling, and thank you for reminding me that what is "capture " actually means. I should have learnt more before I solve this problem