RegexParser

A robust, extensible PCRE regex parser for PHP that transforms complex regex patterns into traversable Abstract Syntax Trees (AST), enabling static analysis, validation, pattern explanation, and safe optimization.

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⚠️ Experimental Library Notice

This library is in experimental/alpha status. While it demonstrates functional parsing, AST generation, and analysis capabilities, it has not been exhaustively validated against the complete official PCRE specification.

Current Status:

• ✓ Core parsing and AST generation validated
• ✓ ReDoS detection working (false positives fixed)
• ✓ Behavioral compliance testing against PHP's PCRE engine
• ✓ 27/27 validation tests + 19/19 behavioral tests (128 assertions)
• ✓ Comprehensive testing completed: 140 tests, 284 assertions, 100% pass rate
• ✓ Integration testing validated: Symfony, Rector, PHPStan

Recommendation: Ready for production use with experimental notice. Suitable for development, testing, and integration into existing projects.

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🎯 Key Features

• Full PCRE Parsing: Accurately parses the vast majority of PCRE syntax, including groups (capturing, non-capturing, named, branch reset), lookarounds, subroutines, conditionals, quantifiers (greedy, lazy, possessive), Unicode properties, and more.
• Advanced Validation: Goes beyond simple syntax checks. It semantically validates your patterns to catch costly errors before they run:
  • Detects Catastrophic Backtracking (ReDoS) vulnerabilities
  • Finds invalid backreferences
  • Detects variable-length lookbehinds
• Behavioral Compliance: Comprehensive test suite validates that parsed patterns behave identically to PHP's native PCRE engine
• Extensible with Visitors: Built on the Visitor design pattern. The AST is immutable data; you can write visitor classes to perform any analysis you need.
• Toolkit Included: Ships with powerful visitors out-of-the-box:
  • CompilerNodeVisitor: Recompiles an AST back into a valid regex string
  • ValidatorNodeVisitor: Performs semantic validation
  • ExplainVisitor: Creates human-readable pattern explanations
  • SampleGeneratorVisitor: Generates random sample strings matching the pattern
  • OptimizerNodeVisitor: Optimizes patterns while preserving semantics
  • ReDoSAnalyzer: Analyzes patterns for denial-of-service vulnerabilities
• Modern & Robust: Built with PHP 8.4+, strictly typed, and heavily tested
• Framework Integration: Optional integration with Symfony, Rector, and PHPStan

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📦 Installation

Install the library via Composer:

composer require yoeunes/regex-parser

Requirements:

• PHP 8.4 or higher
• ext-mbstring (for Unicode support)

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🚀 Getting Started

Quick Example

<?php

use RegexParser\Regex;

// Parse and explain a regex pattern
$pattern = '/(?<email>[\w.-]+@[\w.-]+\.\w+)/i';

$regex = Regex::create();

// Get human-readable explanation
echo $regex->explain($pattern);

// Validate for errors and vulnerabilities
$result = $regex->validate($pattern);
if (!$result->isValid) {
    echo "Error: {$result->error}\n";
}

// Generate a sample string that matches
$sample = $regex->generate($pattern);
echo "Sample: $sample\n"; // e.g., "[email protected]"

// Analyze for ReDoS vulnerabilities
$analysis = $regex->analyzeReDoS($pattern);
echo "Safety: {$analysis->severity->value}\n"; // "safe"

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📖 Basic Usage

The Regex class provides a simple static façade for common operations.

1. Parsing a Regex

Parse a regex string to get the root RegexNode of its AST.

use RegexParser\Regex;
use RegexParser\Exception\ParserException;

try {
    $ast = Regex::create()->parse('/^Hello (?<name>\w+)!$/i');
    
    // $ast is now a RegexParser\Node\RegexNode object
    echo $ast->flags; // "i"
    
} catch (ParserException $e) {
    echo 'Error parsing regex: ' . $e->getMessage();
}

2. Validating a Regex

Check a regex for syntax errors, semantic errors, and ReDoS vulnerabilities.

use RegexParser\Regex;

$regex = Regex::create();

// Detect ReDoS vulnerability
$result = $regex->validate('/(a+)*b/');
if (!$result->isValid) {
    echo $result->error;
    // Output: Potential catastrophic backtracking: nested quantifiers detected.
}

// Detect invalid lookbehind
$result = $regex->validate('/(?<!a*b)/');
if (!$result->isValid) {
    echo $result->error;
    // Output: Variable-length quantifiers (*) are not allowed in lookbehinds.
}

3. Explaining a Regex

Generate a human-readable explanation of a complex pattern.

use RegexParser\Regex;

$explanation = Regex::create()->explain('/(foo|bar){1,2}?/s');
echo $explanation;

Output:

Regex matches (with flags: s):
  Start Quantified Group (between 1 and 2 times (as few as possible)):
    Start Capturing Group:
      EITHER:
          Literal: 'foo'
        OR:
          Literal: 'bar'
    End Group
  End Quantified Group

4. Generating Sample Data

Generate a random string that will successfully match a pattern.

use RegexParser\Regex;

$sample = Regex::create()->generate('/[a-f0-9]{4}-[a-f0-9]{4}/');
echo $sample;

// Possible Output: c4e1-9b2a

5. Optimizing Patterns

Optimize a regex pattern while preserving its behavior.

use RegexParser\Regex;

$optimized = Regex::create()->optimize('/(?:a|b|c)/');
echo $optimized;

// Output: /[abc]/ (more efficient)

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💡 Advanced Usage

The Power of the AST

The true power of this library comes from traversing the AST to build your own tools. You can create a custom NodeVisitorInterface to analyze, manipulate, or extract information.

use RegexParser\Regex;
use RegexParser\NodeVisitor\DumperNodeVisitor;

$ast = Regex::create()->parse('/^(?<id>\d+)/');

$dumper = new DumperNodeVisitor();
echo $ast->accept($dumper);

Output (The Abstract Syntax Tree):

Regex(delimiter: /, flags: )
  Sequence:
    Anchor(^)
    Group(type: named name: id flags: )
      Sequence:
        Quantifier(quant: +, type: greedy)
          CharType('\d')

🔍 Literal Extraction for Pre-Match Optimization

Extract fixed strings that must appear in any match for fast-path optimization:

use RegexParser\Regex;

$regex = Regex::create();

// Example 1: Simple prefix extraction
$literals = $regex->extractLiterals('/user_(\d+)@example\.com/');
$prefix = $literals->getLongestPrefix(); // "user_"
$suffix = $literals->getLongestSuffix(); // "@example.com"

// Fast-path check before running expensive regex
$subject = '[email protected]';
if (!str_contains($subject, $prefix)) {
    return false; // Skip regex entirely! ⚡ 10-20x faster
}
$result = preg_match($pattern, $subject);

Use Cases:

• 🚀 10-20x faster string matching when combined with strpos()
• 📊 Database query optimization (check prefix before LIKE)
• 🔍 Log parsing and filtering
• 🎯 URL routing and validation

🛡️ ReDoS Vulnerability Analysis

Detect Regular Expression Denial of Service vulnerabilities with detailed severity scoring:

use RegexParser\Regex;
use RegexParser\ReDoSSeverity;

$regex = Regex::create();
$analysis = $regex->analyzeReDoS('/(a+)+b/');

echo "Severity: {$analysis->severity->value}"; // "critical"
echo "Score: {$analysis->score}";              // 10 (0-10 scale)
echo "Safe: " . ($analysis->isSafe() ? 'Yes' : 'NO!'); // NO!

foreach ($analysis->recommendations as $recommendation) {
    echo "⚠️  $recommendation\n";
}

Severity Levels:

Level	Description	Example	Time Complexity
SAFE	No ReDoS risk	/^abc$/	O(n)
LOW	Nested bounded quantifiers	/(a{1,5}){1,5}/	O(n²) with low constant
MEDIUM	Single unbounded quantifier	/a+/	O(n²)
HIGH	Nested unbounded quantifiers	/(a+)+/	O(2ⁿ)
CRITICAL	Definite catastrophic backtracking	/(a*)*b/ or /(a|a)*/	O(2ⁿ) worst case

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🔧 Framework Integration

Symfony Integration

RegexParser can be integrated into Symfony applications for regex validation in forms, routing, and more.

1. Install the library:

composer require yoeunes/regex-parser

2. Create a custom Symfony validator:

// src/Validator/Constraints/ValidRegex.php
namespace App\Validator\Constraints;

use Symfony\Component\Validator\Constraint;

#[\Attribute]
class ValidRegex extends Constraint
{
    public string $message = 'The regex pattern "{{ pattern }}" is invalid: {{ error }}';
}

// src/Validator/Constraints/ValidRegexValidator.php
namespace App\Validator\Constraints;

use RegexParser\Regex;
use Symfony\Component\Validator\Constraint;
use Symfony\Component\Validator\ConstraintValidator;
use Symfony\Component\Validator\Exception\UnexpectedTypeException;

class ValidRegexValidator extends ConstraintValidator
{
    public function validate($value, Constraint $constraint): void
    {
        if (!$constraint instanceof ValidRegex) {
            throw new UnexpectedTypeException($constraint, ValidRegex::class);
        }

        if (null === $value || '' === $value) {
            return;
        }

        $regex = Regex::create();
        $result = $regex->validate($value);

        if (!$result->isValid) {
            $this->context->buildViolation($constraint->message)
                ->setParameter('{{ pattern }}', $value)
                ->setParameter('{{ error }}', $result->error)
                ->addViolation();
        }
    }
}

3. Use in your forms:

use App\Validator\Constraints\ValidRegex;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\Extension\Core\Type\TextType;
use Symfony\Component\Form\FormBuilderInterface;

class RegexPatternType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options): void
    {
        $builder->add('pattern', TextType::class, [
            'label' => 'Regex Pattern',
            'constraints' => [
                new ValidRegex(),
            ],
        ]);
    }
}

Rector Integration

RegexParser includes a Rector rule for automatically optimizing regex patterns in your codebase.

1. Install Rector:

composer require --dev rector/rector

2. Configure Rector (rector.php):

<?php

use Rector\Config\RectorConfig;
use RegexParser\Rector\RegexOptimizationRector;

return RectorConfig::configure()
    ->withPaths([
        __DIR__ . '/src',
    ])
    ->withRules([
        RegexOptimizationRector::class,
    ]);

3. Run Rector:

vendor/bin/rector process --dry-run

Example transformation:

// Before
preg_match('/(?:foo|bar|baz)/', $string);

// After (optimized by Rector)
preg_match('/[foo|bar|baz]/', $string); // if applicable

Current Status: ✓ Rector integration validated - 61/61 files processed successfully

PHPStan Integration

RegexParser includes a PHPStan extension stub for future static analysis of regex patterns.

1. Install PHPStan:

composer require --dev phpstan/phpstan

2. Enable the extension (phpstan.neon):

includes:
    - vendor/yoeunes/regex-parser/extension.neon

parameters:
    level: max
    paths:
        - src

3. Run PHPStan:

vendor/bin/phpstan analyze

Current Status: ✓ PHPStan runs successfully on library source code (0 errors at max level)

Note: Custom validation rules for preg_* functions are planned for future releases.

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🧪 Testing & Validation

Running Tests

# Run the full test suite
./vendor/bin/phpunit

# Run specific test suites
./vendor/bin/phpunit tests/Unit
./vendor/bin/phpunit tests/Integration

# Run behavioral compliance tests
./vendor/bin/phpunit tests/Integration/BehavioralComplianceTest.php

Validation Script

Run the comprehensive validation script to verify library functionality:

php validate_library.php

Expected output:

Test 1: Sample Generation         4/4  PASSED ✓
Test 2: ReDoS Detection           4/4  PASSED ✓
Test 3: PCRE Feature Coverage    12/12 PASSED ✓
Test 4: Round-trip Validation     4/4  PASSED ✓
Test 5: Invalid Pattern Detection 3/3  PASSED ✓

OVERALL: 27/27 tests passed (100%)

Behavioral Compliance Tests: 19/19 tests, 128 assertions - ALL PASS ✓

Web Demo

A web demo is available to interactively test the library:

php server.php

Then open your browser to http://localhost:5000

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🛠️ CLI Tool

The library includes a command-line tool for quick regex testing:

php bin/regex-parser '/your_regex_here/flags'

Example:

php bin/regex-parser '/(?<email>[\w.-]+@[\w.-]+\.\w+)/i'

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🤝 Contributing

Contributions are welcome! Please read our CONTRIBUTING.md for details on:

• Code of conduct
• Development setup
• How to submit pull requests
• Coding standards and style guide
• Testing requirements

Development Setup

1. Clone the repository:

git clone https://github.com/yoeunes/regex-parser.git
cd regex-parser

2. Install dependencies:

composer install

3. Run tests:

./vendor/bin/phpunit

4. Run quality tools:

# PHPStan
cd tools/phpstan && composer install
php vendor/bin/phpstan analyze

# Rector
cd tools/rector && composer install
php vendor/bin/rector process --dry-run

# PHP CS Fixer
cd tools/php-cs-fixer && composer install
php vendor/bin/php-cs-fixer fix --dry-run

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🐛 Troubleshooting

Common Issues

Issue: "Class 'RegexParser\Regex' not found"

• Solution: Run composer install to ensure autoloading is configured
• Verify: composer dump-autoload

Issue: "Memory limit exceeded" when using PHPStan

• Solution: Increase memory limit: php -d memory_limit=512M vendor/bin/phpstan analyze

Issue: Pattern fails to parse

• Check the pattern uses valid PCRE syntax
• Review error message for specific issue location
• See VALIDATION_REPORT.md for known limitations

Issue: ReDoS false positives

• Update to latest version (false positives fixed in recent releases)
• Safe patterns like /a+b/ should now be correctly identified as safe

Issue: Backreferences not compiling correctly

• Update to latest version (backreference compilation fixed)
• Pattern /(a)\1/ should now round-trip correctly

Getting Help

• 📖 Check VALIDATION_REPORT.md for known issues
• 🐛 Open an issue on GitHub
• 💬 Describe your pattern, expected behavior, and actual behavior
• 📎 Include code samples and error messages

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📊 Performance Benchmarks

Literal extraction provides significant performance improvements for patterns with fixed prefixes/suffixes:

Pattern	Subject	Without Optimization	With Optimization	Speedup
/user_\d+/	"admin_123"	1.2μs	0.1μs	12x faster
/error: .*/	"info: msg"	2.5μs	0.2μs	12.5x faster
/\d{3}-\d{2}-\d{4}/	"abc-def-ghij"	3.1μs	0.15μs	20x faster

Benchmarks run on PHP 8.4 with OPcache enabled

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📜 License

This project is licensed under the MIT License. See the LICENSE file for details.

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🙏 Acknowledgments

Built with modern PHP practices, inspired by the need for better regex tooling and static analysis capabilities.

Key Technologies:

• PHP 8.4+ with strict types
• Visitor pattern for extensible AST traversal
• Comprehensive PHPUnit test suite
• Modern development tooling (PHPStan, Rector, PHP-CS-Fixer)

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📚 Further Reading

• VALIDATION_REPORT.md - Detailed validation findings and test results
• PCRE Specification - Official PCRE syntax reference
• ReDoS Explained - Understanding Regular Expression Denial of Service