The difference between “require digit”, “requires a digit” and “requires digits”

Question

For example, we have few addresses like:

• "Box 111"
• "Some Rd Suite 1"

And we also have addresses that without any number:

• "Some Rd"

So to communicate that we prefer the address with numbers, what should we say?

"A valid mailing address requires":

1. digit
2. a digit
3. digits

Update:

Will there be any difference if we change "digit" to "number"?

"A valid mailing address requires":

1. number
2. a number
3. numbers

Update:

"contain digit/number" is a literal translate from Chinese. In Chinese, "digit/number" is used as an abstract concept here so no need to specify the quantity.

Are there the same thing for English, or we have to specify the quantity and modifiers to concrete it?

Answer 1

The most familiar way of saying this for many English speakers is to use "number", as in Awe's suggestion

A valid mailing address requires a number.

Or, change the wording to say what the person must do

You must include a number in the mailing address.

similar to this real-world set of address guidelines for street problem reporting. These instructions happen to also show Will Martin's suggestion to show examples of valid input.

To address the other possible ways of saying this phrase:

• Both Options 1 ("address requires digit"/"address requires number") are ungrammatical in English as MT_Head explains.
• Both Options 3 ("address requires digits"/"address requires numbers") will confuse some readers who wonder if least two digits are required.
• Option 2 ("address requires a digit") will confuse some readers because "digit" is rarely used except with a quantity, as in, "Enter a 5-digit ZIP Code."
• Option 2 ("address requires a number") is the best option. Most readers are likely to interpret this as "Enter one integer, like 1 or 111, that contains one or more digits."

To summarize, in English, there is no obvious way to express "digit/number" as an abstract concept. For a technical audience, you may need to include more modifiers to be clear, but for most audiences, a phrase like "a number" is probably the most abstract or general way of referring to a numerical value.

Answer 2

Option #1 is definitely NOT the best; you must put an article before a singular count noun.

Options 2 and 3 are technically correct, but sound strange - I would rephrase this sentence as "A valid mailing address must contain at least one digit."

My apologies for the quality of that link - it's hard to find a freely linkable style manual (they're all behind paywalls.)

Answer 3

Answer to your update question:
Number and digit has a slightly different meaning. A number can contain one or more digits. The number 1 consists of one digit. The number 11 consists of two digits.

Based on this, the answer to your original question could be:

A valid mailing address requires one or more digits.

This is technically correct form, but in this contexts sounds a bit strange. The logical requirement should include that the digits should also form a single number. This phrase would allow for having single digits spread out with other words in between etc...

It would be better to use "a number":

A valid mailing address requires a number.