I have a defcustom with :type variable, and I would like to constrain the variable name to comply to a pattern.

How would I do that?

for instance let say I want the custom to only accept a variable name starting with some prefix.

1 Answer 1


Don't use type variable. Use type restricted-sexp.

The predicate checks for a symbolp that is boundp (i.e., a variable) and whose symbol-name is matched by your chosen prefix.

See the Elisp manual, node Composite Types.

  • Thanks Drew. I tried your solution, but for a strange reason the validating function is being called with nil except for the initial value. Besides the set handler is also called! Any idea why? Sep 21, 2017 at 21:11
  • Here is the code in question: gist.github.com/AdrieanKhisbe/81dbd538b6b8cca0b2dadd33ac24a6ed Sep 21, 2017 at 21:12
  • Looks like you accidentally used fboundp' in one place, instead of boundp'.
    – Drew
    Sep 21, 2017 at 23:11
  • And you spot it right! Thanks a lot @Drew! :) Sep 22, 2017 at 11:44

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