Regex for EC2 instance ID

I need a regex that will be able to match AWS EC2 instance IDs. Instance IDs have the following criteria:

• Can be either 8 or 17 characters
• Should start with i-
• Followed by either a-f or 0-9

Valid instance IDs are: i-ed3a2f7a or i-096e0bec99b504f82 or i-0cad9e810fbd12f4f

Invalid instance IDs are e123g12 or i-1fz5645m

I was able to create the following regex i-[a-f0-9](?:.{7}|.{16})$ but it is also accepting i-abcdeffh. h is not between a-f

Grateful if someone could help me out

>Solution :

You can make a regex to match the 8-character ID value and add an optional 9 characters after it:

^i-[a-f0-9]{8}(?:[a-f0-9]{9})?$

Demo on regex101

Note we use start and end of line anchors to prevent matching additional characters before or after the ID value. This regex will match only 8 or 17 character ID values, not 12 or 11 or 5 etc.