Hi PAU_OUHK ,
If i understood your problem statement correctly, you want respondent to enter numeric responses which would be in 8-20 characters in length.
In this case, yes, the regular expression which you have written is absolutely correct.
Also, the input which you have provided in the screenshot (111) i believe it failed because it has character length less than 8 and you are validating the responses which are between 8-20.
Let me know if my understanding is correct here?

https://www.qualtrics.com/community/discussion/comment/27525#Comment_27525Yes, you're right! But even 8 characters is filled, the validation can't pass and my personal error message can't show. Also, I changed the colors of "Error" & "Highlights" under old Look & Feel editor but it doesn't work. Please advise. Thanks much.

https://www.qualtrics.com/community/discussion/comment/27564#Comment_27564Hello, I'm grateful to have you help me solve the above problems. Thanks a million.

The error message you are getting is because you didn't answer the first question. Looks like you must have force response turned on.

Thanks for your reply. The second question is answered wrongly but the error message don't show. Also, please help solve the problem of old Look & Feel editor. Thanks a lot!