List (find) files with repeated pattern in their name

My question is so simple, but I could not find any solution yet.
I need to find files with the date (string) repeated in their names. For example:

20190101_fl_20190101.nc

20190101_fl_20190104.nc
20190102_fl_20190102.nc
20190102_fl_20190104.nc

I need to find 20190101_fl_20190101.nc and 20190102_fl_20190102.nc.

I have tried

ls 20190[0-9][0-9][0-9]_fl_20190[0-9][0-9][0-9].nc

But, as expected, it finds all possible combinations.

Any help would be highly appreciated.

>Solution :

You can use

find . -type f -regextype posix-extended -regex '.*/(20190[0-9]{3})_fl_\1\.nc$'

The regex matches

• .*/ – any chars up to the rightmost / (necessary because the pattern used with find requires a full string match)
• (20190[0-9]{3}) – Group 1: 2019 and any three digits
• _fl_ – a fixed substring
• \1 – backreference to Group 1 value
• \.nc – .nc string
• $ – end of input.

The -regextype posix-extended option is necessary since the pattern above is POSIX ERE compliant.