1. AN IMPLEMENTATION OF A REGULAR EXPRESSION PARSER
SANDESH KANGONDI
BASSEM ABUEIN

2. INTRODUCTION
A regular expression parser basically parses the regular expression in the following steps.
Takes as input the regular expression.
Converts the input regular expression to an NFA.
Converts the NFA obtained to a DFA.
Finally converted to a minimum DFA.
Goal:implementation of a Regular Expression parser .

3. Specification Has a GUI to understand the states and transitions
Use of ^ and $ tokens to specify match at the beginning and ending of the pattern respectively.
A C# implementation – object oriented.
Has a feature allowing for the control the greediness of the parser - allowing you to experience the different behavior of greediness.
Eg: When Greediness is set to false. An expression "a_*p" in string "appleandpotato"- will match "ap" and not "appleandp".

4. FEATURES

5. APPLICATIONS
Applications of parsing include everything from simple phrase finding, for proper name recognition to full semantic analysis of text, e.g. for information extraction or machine translation.

6. DESIGN

7. Class diagram for the parser

8. DESIGN
The Set class is a simple­ representation of a Set in mathematics.
The Map class is a map between a key and one or more objects.
The State class holds the data structure of the automata.
RegEx - main class that actually uses other classes.
The RegExValidator class is used to validate a pattern string.
Validation done using Recursive Descent Parsing.
Besides validating the pattern, it does two other tasks: insertion of implicit tokens making it explicit and expanding character classes.

9. RECURSIVE DESCENT PARSER
A recursive descent parser is a top-down parser built from a set of mutually-recursive procedures (or a non-recursive equivalent) where each such procedure usually implements one of the production rules of the grammar.
structure of the resulting program closely mirrors that of the grammar it recognizes.

10. RECURSIVE DESCENT PARSER
One easy way to do recursive descent parsing is to have each parse method take the tokens it needs, build a parse tree, and put the parse tree on a global stack
Write a parse method for each nonterminal in the grammar
Each parse method should get the tokens it needs, and only those tokens
Those tokens (usually) go on the stack
Each parse method may call other parse methods, and expect those methods to leave their results on the stack
Each (successful) parse method should leave one result on the stack

11. Running and TESTING
The following slide show a sansnapshot of running program.
The input is regular expression : a_*p
The output as you see.
Note that at the left side of this snapshot we can search string for specific regular expression pattern.

12. Running and TESTING

13. References
Michael Sipser. Introduction to the Theory of Computation, Second
Edition Cambridge, Massachusetts.
"Discrete Mathematics and Its Applications" - Kenneth H. Rosen (Fourth Edition)
"Compilers - Principles, Techniques and Tools" - Aho, Sethi and Ullman
Introduction to Automata Theory, Languages and Computation, John E. Hopcroft, Rajeev Motwani and Jeffrey D. Ullman, 2nd edition,