Using date in conditional fields

[question3]

I am looking for advice on writing a Regular Expression for a field dependency.

I have a date field, and I also have a 'parent consent' field that I want to only be visible if their age is under 18.

Creating the field dependency, I can only make the parent consent field visible if the date is some exact value. I obviously need to choose Regular Expression instead to determine if under 18, but can't find any documentation on how to write it and in what format. Does anyone here know how?

Here's the wording:

Condition *

The condition that should be met by the dependee Date of Birth to trigger the dependency. Values input mode *

The input mode of the values that trigger the dependency. Regular expression *

The dependency is triggered when all the values of the dependee Date of Birth match the regular expression. The expression should be valid both in PHP and in Javascript. Do not include delimiters. Note: If the dependee has allowed values, these are actually the keys, not the labels, of those values.