There is no switch to turn Advanced Functions on. They live in the Functions picker inside every formula editor. Open a validation rule to see them, because that context accepts REGEX while a formula field does not.
- Open a validation rule on the object
From Setup, open Object Manager and pick the object. Click Validation Rules in the left sidebar, then click New.
- Write the condition that blocks the save
The Error Condition Formula has to return a Boolean. The rule fires and the save fails when it evaluates to TRUE.
- Filter the Functions list to Advanced
Below the formula box sits a Functions list with a category dropdown above it. Set the dropdown to Advanced.
- Insert the function and fill in its arguments
Pick the function, click Insert Selected Function, and replace the placeholder arguments. The picker only offers what the current context supports, so the list is shorter in a formula field.
- Run Check Syntax before you save
Check Syntax reports a context problem the same way it reports a typo. If the formula reads correctly, the problem is where you are writing it.
Inserts a correctly spelled field reference, including from a related object.
Comparison and logic operators, so you are not guessing.
Set it to Advanced to see this category, minus anything the context blocks.
- REGEX returns TRUE on a match and a validation rule blocks the save on TRUE. Wrap it in NOT() unless you mean to reject the values that match.
- Salesforce parses REGEX patterns with Java Platform SE 6 syntax, and every backslash has to be doubled. An expression pasted from an online tester rarely compiles first time.