We have an input field, into which the user must type a value but only certain characters are allowed. The allowed characters are:

  • A-Z (any case)
  • the numbers 0-9
  • the $ character
  • the minus character
  • the underscore character

If a user enters invalid characters a tooltip appears which currently states:

The ID may only contain the characters: A-Z, a-z, 0-9, $, -, _

Feedback from our tester states that he finds the commas confusing as he thinks that the error message states that a comma should be allowed! The developer thinks that the lack of commas in the error message makes it confusing.

Is there a standard way to display this sort of information? Can anyone advise?

Note: This is for an English website.

  • If it is an ID field, I am surprised that you allow the $ character Dec 10, 2015 at 13:26

3 Answers 3


Suggested wording: You can only use upper case letters (A to Z), lower case letters (a to z), numbers (0 to 9), and the following characters: $ - _

My suggestion is a bit more long-winded than the one above, but I have avoided using passive voice.

Also, given that there are some people who don't know what upper case and lower case mean, I've added examples, and used the word 'to' in order to avoid ambiguity.screenshot of the number of times this question has been asked on Yahoo Answers

I added a space between the other characters as I think that looks nicer. I kind of agree with your test guy that the commas are ambivalent, but I think it would be difficult to get objective evidence for that. You could do a quick hallway usability test by showing people the list and asking them what characters they think are allowed.

You might also want to read the security considerations around passwords and usernames, e.g. if someone's login fails, don't tell them whether it was their username or password that was incorrect; if they forget their password, just reset it and send them a link to reset their password.

  • Thank you for the detailed answer. I was going to write "what about spaces" (because in my example a space character is not allowed), but this could of course be written explicitly.
    – Phil
    Dec 10, 2015 at 13:57
  • If they do try to enter any spaces, you could just replace them with an underscore. Dec 10, 2015 at 14:03

Instead of just listing the allowed characters be a bit more descriptive like this:

Only uppercase letters, lowercase letters, numbers or the symbols $-_ are allowed.

  • So you would recommend putting the characters together with no spaces between them? Is this standard ?
    – Phil
    Dec 10, 2015 at 10:50

You could use same method that's used when users for long product keys or date of birth into a single input field where the a space or dash automatically appears after a certain amount of characters before the cursor moves to the next

  • In this situation, the field is one long string, so your suggestion is not applicable
    – Phil
    Dec 10, 2015 at 10:59

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