A0LREe: 0-reversible Automaton (A₀) Learner and Regular Expression extractor. Input of the program is a list of strings, output is a regular expression.

A0LREe is a pipeline of three algorithms:

• 0-Reversible Learner: constructs a Finite-State Automaton (FSA) from the list of strings
• State-Elimination: extracts a regular expression from the FSA
• Nested Regular Expressions (work in progress): simplifies the regular expression during the extraction process.

This implementation only serves as a proof-of-concept and has some serious drawbacks. For a detailed description see the accompanying paper. For an illustration of the workflow, see below.

• automaton.py - Automaton and Node classes, used by other modules
• a0_learner.py - Constructs an Automaton instance from input
• re_parser.py - Extracts a Regular Expression with the State Elimination Algorithm
• nested_re.py - NestedRE class, used to efficiently merge expressions during extraction (partially completed)
• a0lree.py - Provides user interface

• Python 3

The following Python packages a required for drawing graphs:

• graphviz
• PyQt5

both can be installed with pip.

Mandatory argument:

• One of:
  • -c: read examples from stdin
    • seperator is space
  • <filepath> read examples from file (expected as last argument), file format:
    • each string on a seperate path
    • no seperator symbols
    • empty line means empty string should be accepted by the regular expression

Optional arguments:

• -h, --help: print help
• -v: draw graphs and print more info to STDOUT

Example:

$ ./a0lree.py -v examples.txt

or

$ ./a0lree.py -c b ab aab aaaab
>>> a*b 

The file example.txt contains the strings [b, ab, aab, aaaab]. A0LREe does its work in the following steps.

• Constructs a Prefix Tree, an FSA which accepts exactly these four strings
  

• Merges the accepting nodes of the Prefix Tree
  

• Merges nodes with non-deterministic transitions. The resulting graph is a 0-reversible FSA (see paper for the definition):
  

• The FSA is converted into a uniform FSA, which is required by the State-Elimination algorithm
  

• Iteratively eliminates nodes of the FSA, until only the initial final nodes are left
  

• The resulting graph has only one edge, and the label of this edge is the output of the program