pandaman64/lean-regex

Correctly fail invalid range operators

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#152 opened on Dec 30, 2025

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Description

We have extended the bracketed character class parsers with new capabilities like #150 while breaking error handling of range operators. Namely,

  • [z-a] should be an error (as the range is valid only if 'z' ≦ 'a'), but is treated as a union of z, -, and a.
  • Also, [\d-a] should be an error, but accepted as a union

We want to restore the original behavior that returns .error .invalidRange in such cases.

I'd expect that we need a rework of the bracketed class parsers to conform to a grammar like this:

escapedChar   ::= '\' c
singleChar    ::= c (except for '[', ']', '\', ...)
atom          ::= escapedChar | singleChar
rangeExpr     ::= atom ('-' atom)?
setOp         ::= '&&' | '||' | '--' | '~~'
setExpr       ::= bracketed (setOp bracketed)*
singleExpr    ::= rangeExpr | setExpr
bracketed     ::= '[' ('^')? singleExpr* ']'

The idea is to make a range expression a proper level in the hierarchy and insert .commit at appropriate levels.

Contributor guide

Research direction
Study the current bracketed character class parser in the codebase, especially how it parses components like `z a` and `\d a`. Then implement the proposed grammar to distinguish range expressions from unions, and add error handling to return `.error .invalidRange` for invalid ranges like `[z a]` and `[\d a]`. Review issue #150 for related changes.
Tech stack
None
Domain
compilers
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
LeanGitParser combinators
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70