Python Regular Expression Parser

Utility to convert Python Regular Expression to formal definition and finite automata

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What?

With this code you can convert regular expression given in the Python format to a (close) equivalent formal definition.

For a quick guide on Python regex, see the official documentation.

Formal Definition

This is the formal definition we convert Python regex to:

• a, b, c, ...: members of the alphabet used in the expression
• ab or a&b: the concatenation of a and b (i.e., a must be directly followed by b)
• a|b: the choice of a or b (i.e., either a or b is accepted); other literature also uses +
• a*: the Kleene Star operator (i.e., a can be repeated 0 or more times)

Hint: use \e for an empty string in inputs.

Usage

Run the script with

python main.py <destination> -e <expression> 

where destination must be one of REGEX, SYA, NFA or DFA.

• REGEX simply converts the given expression into a formal defintion regular expression
• SYA converts the given expression into a post-fix notation using the shunting-yard algorithm
• NFA creates a nondeterministic finite automaton
• DFA creates a deterministic finite automaton

Other optional arguments include -s for strict mode. Here, an error is thrown instead of making any assumptions when converting an expression (e.g., for lookahead assertion). Also, -d enables debug mode, which prints conversion steps (i.e., all previous steps in the list above).

For a complete explanation of command-line arguments see

python main.py -h

Examples

REGEX

python main.py REGEX -e '(ab|c)a*'

Output:

(a&b|c)&a*

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SYA

python main.py SYA -e '(ab|c)a*'

Output:

ab&c|a*&

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NFA

python main.py NFA -e '(ab|c)a*'

Output:

States
        Q: {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}

Initial State(s)
        q0:  1

Final State(s)
        F: [10]

Transition Function
        δ:
                 1 -> 2 with '##e##'
                 1 -> 5 with '##e##'
                 4 -> 7 with '##e##'
                 6 -> 7 with '##e##'
                 2 -> 3 with 'a'
                 3 -> 4 with 'b'
                 5 -> 6 with 'c'
                 7 -> 8 with '##e##'
                 7 -> 10 with '##e##'
                 9 -> 10 with '##e##'
                 9 -> 8 with '##e##'
                 8 -> 9 with 'a'

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DFA

python main.py DFA -e '(ab|c)a*'

Output:

States
        Q: {1, 2, 3, 4, 5}

Initial State(s)
        q0:  1

Final State(s)
        F: [2, 4, 5]

Transition Function
        δ:
                 1 -> 2 with 'c'
                 1 -> 3 with 'a'
                 3 -> 4 with 'b'
                 4 -> 5 with 'a'
                 5 -> 5 with 'a'
                 2 -> 5 with 'a'

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SYA - DEBUG

python main.py SYA -e '(ab|c)a*' -d

Output:

Escaped string:  (ab|c)a*
Operator-filled string:  (a&b|c)&a*
Shunting-yard string:  ab&c|a*& 

ab&c|a*&

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License

MIT License

Copyright (c) 2022 Tobias Kalmbach

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

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