You can use:

^(?!.{17})[a-z\d]+\/[a-z\d]+$

See an online demo

• ^ - Start-line anchor;
• (?!.{17}) - Negative lookahead to prevent 17 characters (other than newline). This can be more precise and you may want to change into 18 depending if you count the forward slash on top of the 16 alphanumeric characters.
• [a-z\d]+\/[a-z\d]+ - 1+ Alphanumeric chars, a single literal forward slash and again 1+ alphanumeric chars.
• $ - End-line anchor.

Note that I used case-insensitive matches, meaning your JS-pattern would look like:

/^(?!.{17})[a-z\d]+\/[a-z\d]+$/gi

const regex = /^(?!.{17})[a-z\d]+\/[a-z\d]+$/i;
[
  "aas/as12",
  "12/saad",
  "/"
].forEach(s =>
  console.log(`${s} --> ${regex.test(s)}`)
)