regecks 0.1.0

A from-scratch regular expression engine with TUI

Regecks

A regex engine built from scratch using python, and a TUI using Textual!

Ever part of this including stuff like the lexer, parser, the ast, and the backtracking were written by hand :)

Installation

Requires uv

git clone https://github.com/kashsuks/regecks
cd regecks
uv sync

Running

uv run regecks

Keybindings

Key	Action
Tab	Move between pattern and test string fields
Ctrl+R	Run match manually
Ctrl+C	Quit

Matches get updated live

Supported Syntax

Literals

Pattern	Match
a	The character a
abc	The string abc exactly

Wildcard

Pattern	Match
.	Any single character except newline

Quantifiers

All quantifiers are greedy and therefore consume as much as possible.

Pattern	Match
a*	Zero or more a
a+	One or more a
a?	Zero or one a
a{3}	Exactly 3 a
a{2,}	2 or more a
a{2,4}	Between 2 and 4 a (inclusive)

Anchors

Pattern	Match
^abc	abc only at the start of the string
abc$	abc only at the end of the string
\b	Word boundary (between \w and \W)
\B	Non-word boundary

Character classes

Pattern	Match
[abc]	Any one of a, b, or c
[a-z]	Any lowercase letter
[A-Z0-9]	Any uppercase letter or digit
[^abc]	Any character except a, b, c
[^0-9]	Any non-digit

Escape sequences

Pattern	Match
\d	Any digit [0-9]
\D	Any non-digit
\w	Any word character [a-zA-Z0-9_]
\W	Any non-word character
\s	Any whitespace (space, tab, newline, etc)
\S	Any non-whitespace
\n	Newline
\t	Tab
\n	Literal dot (escaping special chars)

Alternation

Pattern	Match
cat|dog	Either cat or dog
foo|bar|baz	Any of the three

Groups

Pattern	Match
(abc)	abc, captured as group 1
(a)(b)	ab, with a as group 1 and b as group 2
(?:abc)	abc, not captured
(?Pabc)	abc, captured as named group name

Flags

Pattern	Match
(?iabc)	Match abc case-insensitively and matches ABC, abc, etc.

The flags wrap everything that follows it in a pattern

Lookahead and lookbehind

These are zero-width assertions meaning they check for a condition without consuming characters.