We were looking at scanners in the class on 12_1.
We started with why we need scanners? what are some of the advantages of having scanning as a separate stage?
What inputs do scanners expect and what output do they produce?
What are the major tasks of scanners? place dividers and classify substrings.
While doing this, what are the things that scanners must do: scan characters and do look-ahead. 
We inferred the last two while looking at examples of different PLs.
We did not look at lower-level implementation details of scanning and look-ahead.

Then we got started with how we specify different classes / loosely called tokens? using regular expressions.
Following this, we briefly looked at regular expressions. 
Regular expressions are a way to define regular languages and a language is a set of strings. A language is always defined over a set of characters called alphabet or vocabulary. 
The strings belonging to the language are a sequence of characters drawn from this set. 
So, regular expressions define a set of strings.
We then used regular expressions to define the classes of strings. 
Immediately after this, I Introduced you to scanner generators.

A brief summary of regular expressions follows:
regular expressions can be thought of as a syntax to specify a set of strings.
Standard definition of regular expressions has 5 kinds. 2 base types and 3 compound types. Later on, some short-hand notations were introduced.
The base types are: 1) single character set indicated by 'c' where c is part of the alphabet and 2) epsilon, the set of empty string. Note that this is not a language with no strings (or empty set). This is a language with one string that is empty.
The compound types are: Union, Concatenation, and Kleene Closure. 
Union is denoted by the expression A + B. This expression denotes the union of the set of strings belonging to Language expressed by A and the set of strings belonging to Language expressed by B.
Concatenation is denoted by the expression AB. This expression denotes the concatenation of the set of strings belonging to Language expressed by A and the set of strings belonging to Language expressed by B. 
The concatenation is done by taking the cross product i.e. any string from language A can be concatenated with any string from Language B.
Kleene Closure is denoted by A*. This is equivalent to the set {epsilon, A, AA, AAA, ... A....A i times, where i>=0} 
Later introduction to the list of expressions include:
One or more instances denoted by A+
Zero or one instance denoted by A?
Character classes are a shorthand notation denoted by [a1-an] which is equivalent to a1+a2+...+an, where a1 to an are from the set of alphabets. 


Today and tomorrow:
Connect the dots between regular expressions, scanner generator, and scanner.
Demo of a scanner generator
What is 'regular' about regular languages? Notion of Formal Languages.