How to determine if a C language identifier is valid?

In the C language, identifiers are used to name entities such as variables, functions, and structures. Legal C identifiers must adhere to the following rules:

1. Identifiers can only consist of letters (both uppercase and lowercase), numbers (0-9), and underscores (_).
2. The first character of an identifier must be a letter or underscore.
3. There is no limit to the length of identifiers, but for most compilers, only the first 31 characters of an identifier are taken into account.
4. Keywords such as if, for, int, etc. cannot be used as identifiers.
5. Identifiers are case-sensitive, for example “Variable” and “variable” are two different identifiers.

To determine if an identifier is valid, you can evaluate each character according to the rules above. If there is any character that does not follow the rules or is in the wrong position, then the identifier can be considered invalid.