Regular is a toy regular expression engine.
I wanted to understand how regular expressions worked, so I read this and then started working on regular. ☀️ The approach is to use derivatives, as described in the article. When looking at how I could test correctness, this was helpful.
Instead of using normal syntax, I came up with something I liked:
abc : Standard string, matches "abc"
|[a,b,c]| : Alternation of regexes.
*(abc) : Matches the enclosed regex zero or more times
_ : Matches any character
For example, ap|[e,ple]| would match ape* and apple*. It's not super polished -- none of the special characters can be escaped to be matched, for example, but otherwise supports everything expected (including nested expressions like a|[b, |[c, d]|, e, f]|).
repl:
python3 regular.py
Example:
$ python3 regular.py
>> a*(b|[c,def]|de)f:af
True
>> a*(b|[c,def]|de)f:aff
False
Batch mode:
python3 regular.py filename
In both modes, the input format is
regex:test
These are sensitive to spaces, but I didn't give much thought to making it resilient to bad input.
Tests check each step of the implementation and can be found in tests.py. They can be run with make test.
MIT, see LICENSE