Why there is no sign character in the syntax of constants?

Question

Why doesn't the standard include a sign character in the syntax of constants?

It is mentioning only digits and sign character is only present in exponents.

Answer 1

The standard does not bother with the sign in front of numeric literals because it would be redundant.

The syntax already captures the sign as part of unary plus + and unary minus - operators. When you write

int a = -4;

the syntax of the right-hand side could be adequately described as a unary minus - expression with the operand of 4. This is the approach that the standard takes.

Answer 2

If - were a part of the constant -2 then 4-2 would be a syntax error (since a token is always the longest possible sequence of characters). Also, the semantics of -2147483648 and - 2147483648 would be different (the first one would be an int and the second one a long, assuming int is 32 bits and long is longer). Both of those things would be confusing.

If the - is always an operator, the semantics of -2147483648 are sometimes a little unexpected, but the more common x-1 works as expected. So that's how most programming languages, including C, work.