Comparison with Flex/Bison

This chapter compares OpenLexer with GNU Flex and Bison.

Feature Comparison

File Format Compatibility

Lexer Files (.l)

OpenLexer supports most Flex syntax:

Supported:

• Definitions section with %{ %} code blocks
• Named patterns
• Rules with actions
• Start conditions (%x, %s)
• Character classes [a-z]
• Predefined classes [:alpha:]
• Quantifiers *, +, ?, {n,m}
• Alternation |
• Grouping ()
• Literal strings "..."

Not supported:

• %option directives
• yymore(), yyless()
• REJECT
• Multiple input buffers
• Interactive mode

Grammar Files (.y)

OpenLexer supports most Bison syntax:

Supported:

• %token declarations
• %union type declarations
• %type non-terminal types
• %left, %right, %nonassoc precedence
• %start symbol
• %prec contextual precedence
• %glr-parser GLR mode
• Semantic actions with $$, $1, etc.
• error token for recovery

Not supported:

• %define options
• %code blocks (use %{ %} instead)
• %destructor
• %printer
• Lookahead correction (%define lr.type ielr)
• Named references ($name instead of $1)

Migration Guide

From Flex

1. Remove %option lines
2. Replace yymore() with manual buffer handling
3. Replace REJECT with explicit rules
4. Test start condition behavior

From Bison

1. Replace %define with equivalent features
2. Move %code blocks to %{ %}
3. Test GLR behavior if used
4. Verify error recovery patterns

Performance

OpenLexer generates table-driven lexers and parsers similar to Flex/Bison. Performance characteristics:

• Lexer: O(n) where n is input length
• LALR Parser: O(n) for unambiguous grammars
• GLR Parser: O(n) to O(n^3) depending on ambiguity

The generated code size is comparable to Flex/Bison output.

When to Use OpenLexer

Use OpenLexer when:

• You need multi-language output
• You want a single cross-platform tool
• You need integrated lexer + parser generation
• You want GLR parsing

Use Flex/Bison when:

• You need maximum C performance optimization
• You require specific Flex/Bison features not in OpenLexer
• You have existing build infrastructure for Flex/Bison