External References

This curated list of external resources provides authoritative information about regex syntax, engine behavior, and performance. These are the sources PHPRegex uses when verifying diagnostics and documenting edge cases.

Table of Contents

PCRE2 Specification

The authoritative source for PCRE2 syntax and behavior.

Key topics covered:

• Pattern modifiers (imsxuADJSUX)
• Escape sequences
• Assertions and zero-width matches
• Atomic grouping and possessive quantifiers
• Backtracking control verbs

PHP-Specific References

PHP’s implementation of PCRE and language-specific features.

PHP-specific notes:

• PHP uses PCRE2 (since PHP 7.3)
• \g{0} is invalid - use \g<0> or (?R)
• Backtrack limits via pcre.backtrack_limit
• Recursion limits via pcre.recursion_limit

Security and ReDoS

Understanding and preventing Regular Expression Denial of Service attacks.

Key concepts:

• Exponential vs polynomial backtracking
• Common risky patterns
• Mitigation strategies
• Detection techniques

Engine Internals and Theory

How regex engines actually work under the hood.

Why this matters:

• Backtracking engines (PCRE) can be exponential
• Non-backtracking engines (RE2) are linear
• Understanding tradeoffs helps write better patterns

Tutorials and Practical Guidance

Learn regex from beginner to advanced.

Recommended learning path:

1. Start with Regular-Expressions.info basics
2. Practice on RegexOne
3. Explore advanced topics on RexEgg
4. Study PCRE2 manual for PHP-specifics

Testing and Visualization

Tools to test, debug, and visualize regex patterns.

regex101 tips:

1. Set flavor to “PCRE (PHP)”
2. Use the structure panel to see groups
3. Check the match information for backtracking
4. Use the pattern generator for test cases

Unicode References

Unicode support in regex patterns.

Common Unicode patterns:

// Match any letter (any language)
'/\p{L}/u'

// Match Chinese characters
'/\p{Han}/u'

// Match emoji
'/\p{Emoji}/u'

// Match numbers in any script
'/\p{N}/u'

Engine Compatibility

How other regex engines compare to PCRE2.

RE2 comparison (useful for understanding PCRE tradeoffs):

// PCRE2 - can be exponential
preg_match('/(a+)+b/', $input);

// RE2 - always linear
// $re2->match('(a+)+b', $input);  // No exponential behavior

Quick Reference Cards

Citation Style

When referencing these sources in documentation or code comments:

// See: PCRE2 Pattern Syntax
// https://www.pcre.org/current/doc/html/pcre2pattern.html

// Based on: OWASP ReDoS Prevention
// https://community.owasp.org/attacks/Regular_expression_Denial_of_Service_-_ReDoS

Related Documentation

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