Revision as of 03:33, 14 March 2008

Lexical analysis, the first step in the compilation process, splits the input data into segments and classifies them. Each segment of the input (a lexeme) will be assigned a label (the token).

In this case, we will be using regular expressions for recognizing portions of the input text.

Regular Expressions

Regular expressions are defined considering a finite alphabet Σ = { a, b, ..., c } and the empty string ε:

The languages (sets of strings) for each of these entities are:

• {ε}, for ε
• {a}, for an entry a in Σ

The following primitive constructors are defined:

• concatenation
• alternative
• Kleene-star (*)

Extensions (derived from the above):

• Transitive closure (+) - a+ ("one or more 'a'")
• Optionality (?) - a? ("zero or one 'a'")
• Character classes - [a-z] ("all chars in the 'a-z' range" - only one character is matched)

Recognizing/Matching Regular Expressions

Building the NFA: Thompson's Algorithm

Building DFAs from NFAs

DFA Minimization

Input Processing

Recognizing Multiple Expressions

Example 1: Ambiguous Expressions

Example 2: Backtracking