Crate perl_lexer 

Context-aware Perl lexer with mode-based tokenization

This crate provides a high-performance lexer for Perl that handles the inherently context-sensitive nature of the language. The lexer uses a mode-tracking system to correctly disambiguate ambiguous syntax like / (division vs. regex) and properly parse complex constructs like heredocs, quote-like operators, and nested delimiters.

Architecture

The lexer is organized around several key concepts:

• Mode Tracking: LexerMode tracks whether the parser expects a term or an operator, enabling correct disambiguation of context-sensitive tokens.
• Checkpointing: LexerCheckpoint and Checkpointable support incremental parsing by allowing the lexer state to be saved and restored.
• Budget Limits: Protection against pathological input with configurable size limits for regex patterns, heredoc bodies, and delimiter nesting depth.
• Position Tracking: Position maintains line/column information for error reporting and LSP integration.
• Unicode Support: Full Unicode identifier support following Perl 5.14+ semantics.

Usage

Basic Tokenization

use perl_lexer::{PerlLexer, TokenType};

let mut lexer = PerlLexer::new("my $x = 42;");
let tokens = lexer.collect_tokens();

// First token is the keyword `my`
assert!(matches!(&tokens[0].token_type, TokenType::Keyword(k) if &**k == "my"));
// Tokens include variables, operators, literals, and EOF
assert!(matches!(&tokens.last().map(|t| &t.token_type), Some(TokenType::EOF)));

Context-Aware Parsing

The lexer automatically tracks context to disambiguate operators:

use perl_lexer::{PerlLexer, TokenType};

// Division operator (after a term)
let mut lexer = PerlLexer::new("42 / 2");
// Regex operator (at start of expression)
let mut lexer2 = PerlLexer::new("/pattern/");

Checkpointing for Incremental Parsing

use perl_lexer::{PerlLexer, Checkpointable};

let mut lexer = PerlLexer::new("my $x = 1;");
let checkpoint = lexer.checkpoint();

// Parse some tokens
let _ = lexer.next_token();

// Restore to checkpoint
lexer.restore(&checkpoint);

Configuration Options

use perl_lexer::{PerlLexer, LexerConfig};

let config = LexerConfig {
    parse_interpolation: true,  // Parse string interpolation
    track_positions: true,      // Track line/column positions
    max_lookahead: 1024,        // Maximum lookahead for disambiguation
};

let mut lexer = PerlLexer::with_config("my $x = 1;", config);

Context Sensitivity Examples

Perl’s grammar is highly context-sensitive. The lexer handles these cases:

• Division vs. Regex: / is division after terms, regex at expression start
• Modulo vs. Hash Sigil: % is modulo after terms, hash sigil at expression start
• Glob vs. Exponent: ** can be exponentiation or glob pattern start
• Defined-or vs. Regex: // is defined-or after terms, regex at expression start
• Heredoc Markers: << can be left shift, here-doc, or numeric less-than-less-than

Budget Limits

To prevent hangs on pathological input, the lexer enforces these limits:

• MAX_REGEX_BYTES: 64KB maximum for regex patterns
• MAX_HEREDOC_BYTES: 256KB maximum for heredoc bodies
• MAX_DELIM_NEST: 128 levels maximum nesting depth for delimiters

When limits are exceeded, the lexer emits an UnknownRest token preserving all previously parsed symbols, allowing continued analysis.

Integration with perl-parser

The lexer is designed to work seamlessly with perl_parser::Parser:

use perl_parser::Parser;

let code = "sub hello { print qq{Hello, world!\\n}; }";
let mut parser = Parser::new(code);
let ast = parser.parse()?;

The parser automatically creates and manages a PerlLexer instance internally.

Re-exports

pub use checkpoint::CheckpointCache;

pub use checkpoint::Checkpointable;

pub use checkpoint::LexerCheckpoint;

pub use error::LexerError;

pub use error::Result;

pub use mode::LexerMode;

pub use token::StringPart;

pub use token::Token;

pub use token::TokenType;

Modules

checkpoint

Lexer checkpointing for incremental parsing

error

Error types for the Perl lexer

mode

Lexer modes for context-sensitive parsing

token

Token types and structures for the Perl lexer

Structs

LexerConfig

Configuration for the lexer

PerlLexer

Mode-aware Perl lexer

Position

A position in a source file with byte offset, line, and column