Explain identifier with example rules for constructing identifier name.

 In computer programming, an identifier is a name given to a variable, function, or any other entity in the program. An identifier is used to refer to the entity it represents throughout the program. Here are the rules for constructing identifier names:

1. An identifier can only be composed of letters, digits, and underscores (_).
2. The first character of an identifier must be a letter or an underscore. It cannot be a digit.
3. An identifier should not contain any spaces or special characters such as #, $, %, &, *, etc.
4. An identifier name must be unique within its scope.
5. Identifiers are case-sensitive, meaning that uppercase and lowercase letters are treated as distinct characters. For example, "myVar" and "myvar" are two different identifiers.

Here are some examples of valid identifiers:

• myVar
• my_var
• variable2
• _underscore

And here are some examples of invalid identifiers:

• 123var (because the first character is a digit)
• my-var (because it contains a hyphen)
• my var (because it contains a space)
• if (because it's a reserved keyword in many programming languages)