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Identifiers in C programming

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Identifiers are names that are given to various program elements, such as variables, functions and arrays. Identifiers consisted of letters and digits, in any order, except that first character must be a letter. Both upper and lower case letters are permitted, though common usage favors the use of lowercase letters for most type of identifiers. Upper and lowercase letters are not interchangeable (i.e. an uppercase letter is not equivalent to the corresponding lowercase letters). The underscore (  _ ) can also be included, and considered to be a letter. An underscore is often used in middle of an identifier. An identifier may also begin with an underscore.

Rules for Identifier:
1. First character must be an alphabet (or Underscore).
2. Must consist of only letters, digits or underscore.
3. Only first 31 characters are significant.
4. Cannot use a keyword.
5. Must not contain white space.

The following names are valid identifiers
x         a12         sum_1           _temp       name      area     tax_rate      TABLE
The following names are not valid identifier for the reason stated
4th                                           The first character must be letter
“x”                               Illegal characters (“)
Order-no                     Illegal character (-)

Error-flag                    Illegal character (blank space)
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