Question

Give regular expressions for

(c) C indentifiers

(d) Binary strings consisting of either an odd number of 1s or an odd number of 0s

Answer 1

Identifers are the names given to variables, functions, arrays, unions, structures, strings and user-defined data.

Identifiers are the names which are given to the programming elements used for identification purpose.

There are some rules for naming identifiers in C programming language.

• The Valid identifiers can be combination of uppercase alphabets, lowercase alphabiths, digits, special charactes, and underscore. (a-z, A-Z, 0-9, _, etc., )
• The first letter of identifier should be either a letter or underscore(_), but not a digit.
• The maximum length of the identifier is <=31 in C programming language.
• We can not use Keywords( INT, CHAR, FLOAT, etc.,) as identifiers.
• Blank spaces are not allowed in names of identifiers.

Example:

int xyz;

int class1;

here xyz, class1 are identifiers.

c) The regular expression for C identifiers is

[A-Za-z_][A-Za-z0-9_]*{0,30}

The first charecter is a letter or underscore followed by zero or more charecters.

In above regular expression,

[A-Za-z_] specifies the first charecter of the identifier starts with the alphabet or underscore(_).

A-Z: Uppercase alphabets A, B, C, ... , Z

a-z: Lowercase alphabets a, b, c, ... , z

_ : Underscore

[A-Za-z0-9_]* specifies the character of identifiers with the alphabets or numbers or _

* specifies occurance zero or more times of (A_Za-z0-9_].

{ } Used to limit the length .

d) Regular expression for binary strings consisting of either an odd number of 1s or an odd number of 0s

The alphabet = {0,1}.