1) Regular Expressions and Languages

Regular expressions are patterns used to specify a set of strings. They are used for; - Finding particular words in a stream - Finding combinations of characters in strings

S can be defined by a regular expression ab* S = {a, ab, abb, abbb} | - Separates alternatives * - Indicates that there are 0 or more of the preceding element. + - Indicated there is 1 or more of the preceding element. ? - Indicates that there are 0 or 1 of the preceding element. ( ) - used to identify groupings

A set is a well-defined collection of objects, objects in s set are called elements. A set is unordered and each element can only occur once. Common Sets: N = Natural Number {0, 1, 2,…} Z+ = Positive Integer {1, 2, 3,…} Z = all integers {…, -2, -1, 0, 1, 2, …} Q = rational numbers { number expressed as a fraction} R = real numbers { Numbers that contain a decimal place}