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lectes

Name the rules. Get the tokens. lectes is a small Python scanner (lexer) generator. Give it named regexes, and it yields tokens.

• No dependencies. It uses only the standard library and needs Python 3.13+.
• Predictable matching. The longest match wins, and a tie goes to the earlier rule.
• Grammar files. Write one NAME regex rule per line, in Python or in a .lectes file.
• A lectes command. Scan files or stdin from the shell, as aligned text or JSON Lines.
• Locations included. Every token carries its offset, line and column.
• Unmatched text is reported. By default it raises an error, but you can skip it or handle it yourself.

Try it in the playground: lectes.dev lets you write a grammar and see the tokens as you type, right in your browser. Hover a token to see which rules fought for it.

Installation

As a library:

pip install lectes   # or: uv add lectes

As a command-line tool:

uv tool install lectes                # or: pipx install lectes
uvx lectes grammar.lectes input.txt   # run once without installing

Quick start

from lectes import Configuration, Scanner

config = Configuration.from_text(r"""
# Longest match wins; on a tie, the earlier rule wins.
FOR     for
IN      in
ID      [a-zA-Z_][a-zA-Z0-9_]*
INT     [0-9]+
WS      [ ]+
""")

scanner = Scanner(config)

for token in scanner.scan("for item in items format 42"):
    if token.rule.name != "WS":
        print(token.location.column, token.rule.name, repr(token.literal))

1 FOR 'for'
5 ID 'item'
10 IN 'in'
13 ID 'items'
19 ID 'format'
26 INT '42'

for matches both FOR and ID with the same length, so the earlier rule, FOR, wins. format is an ID, because the longer match always wins.

You can also build the rules in Python:

from lectes import Configuration, Regex, Rule

config = Configuration(
    [
        Rule(name="FOR", regex=Regex("for")),
        Rule(name="ID", regex=Regex("[a-zA-Z_][a-zA-Z0-9_]*")),
    ]
)

The usage guide covers grammar errors and warnings, custom handlers, unmatched text and debugging.

Command line

The lectes command scans a file, or stdin, with a grammar file:

$ cat arithmetic.lectes
INT     [0-9]+
PLUS    \+
WS      \s+

$ echo '12 + 3' | lectes arithmetic.lectes
1:1     INT   '12'
1:3     WS    ' '
1:4     PLUS  '+'
1:5     WS    ' '
1:6     INT   '3'
1:7     WS    '\n'

Use --format json for JSON Lines output, and --check to validate a grammar without scanning. The exit codes are 0 on success, 1 for unmatched text, and 2 for a usage, grammar or I/O error. See the command-line docs for every option.

License

MIT