georgesittas / regex-interpreter

Simple regular expression interpreter

Regex Interpreter

The interpreter consists of the following phases:

1. Parse a regular expression
2. Create an NFA for it by walking the produced parse tree
3. Convert the NFA to an equivalent DFA
4. Compute the DFA's result by feeding it the input string

The following algorithms have been implemented:

• NFA creation: Thompson's construction algorithm
• NFA to DFA conversion: Powerset construction

Syntax

The interpreter expects regular expressions to conform to the following syntax:

• x, a single character.
• "_", the empty symbol.
• ".", any character.
• (r), r is a regular expression.
• r1 | r2, r1 and r2 are regular expressions (Union).
• r1r2, r1 and r2 are regular expressions (Concatenation).
• r*, r is a regular expression (Kleene star).

Functions

The following functions can be used after loading RegInterpreter.hs:

• makeNfa receives a regular expression ([Char]) & returns an NFA (Fsa).
• nfaToDfa receives an NFA (Fsa) & returns a DFA (Fsa).
• regexFullMatch receives a tuple (regex, string) & returns True if string is accepted by regex.
• regexPartMatch receives a tuple (regex, string) & returns all prefixes in string accepted by regex.

Testing

➜  Regex-Interpreter git:(main) ghci testSuite.hs
GHCi, version 9.2.4: https://www.haskell.org/ghc/  :? for help
[1 of 3] Compiling RegParser        ( RegParser.hs, interpreted )
[2 of 3] Compiling RegInterpreter   ( RegInterpreter.hs, interpreted )
[3 of 3] Compiling Main             ( testSuite.hs, interpreted )
Ok, three modules loaded.
ghci> testAll
True

Contributors

• George Sittas
• Jim Rontogiannis