Need help - value must be 11 character

I am quite new with Erpnext and want to limit one of my custom field so that only 11 character will be accepted as valid.

For example: 12345678901 is a valid value. if person enters 1234567890 the field will warn that the numbers entered not consist of 11 character.

Anyone know such a script ?

Check: https://manual.erpnext.com/contents/customize-erpnext/custom-scripts/custom-script-examples