Error Pattern Safety Agent Skill

Error Pattern Safety Guidelines

Use these regex safety rules in agentic engines to prevent JavaScript infinite loops.

The Problem

With the JavaScript global flag (/pattern/g), zero-width matches can cause infinite loops because:

• JavaScript's regex.exec() with the g flag uses lastIndex to track position
• When a pattern matches zero-width, lastIndex doesn't advance
• The same position is matched repeatedly, causing an infinite loop

Dangerous Pattern Examples

❌ NEVER USE THESE PATTERNS:

// Pure .* - matches everything including empty string at end
/.*/g

// Single character with * - matches zero or more (including zero)
/a*/g

// Patterns that can match empty string
/(x|y)*/g

Safe Pattern Examples

✅ ALWAYS USE PATTERNS LIKE THESE:

// Required prefix before .*
/error.*/gi
/error.*permission.*denied/gi

// Specific structure with required content
/\[(\d{4}-\d{2}-\d{2})\]\s+(ERROR):\s+(.+)/g

// Required characters throughout
/access denied.*user.*not authorized/gi

Pattern Safety Rules

• Always require at least one character match
• Use .+ instead of .* when you need "something"
• Ensure pattern has required prefix/suffix

• Never use bare .* as the entire pattern
• Always combine with required text: error.*
• Never just .* or .*?

• Test patterns against empty string

   const regex = /your-pattern/g;
   if (regex.test("")) {
     throw new Error("Pattern matches empty string - DANGEROUS!");
   }

• Use specific anchors when possible
• Start: ^error.*
• End: .*error$
• Word boundaries: \berror\b

Validation Tests

All error patterns must pass these tests:

Go Tests (pkg/workflow/engine_error_patterns_infinite_loop_test.go)

// Test that pattern doesn't match empty string
func TestPatternSafety(t *testing.T) {
    pattern := "your-pattern"
    regex := regexp.MustCompile(pattern)
    
    if regex.MatchString("") {
        t.Error("Pattern matches empty string!")
    }
}

JavaScript Tests (pkg/workflow/js/validate_errors.test.cjs)

test("should not match empty string", () => {
  const regex = new RegExp("your-pattern", "g");
  expect(regex.test("")).toBe(false);
});

Safety Mechanisms in validate_errors.cjs

The validate_errors.cjs script has built-in protections:

• Zero-width detection: Checks if regex.lastIndex stops advancing
• Iteration warning: Warns at 1000 iterations
• Hard limit: Stops at 10,000 iterations to prevent hang

// Safety check in validate_errors.cjs
if (regex.lastIndex === lastIndex) {
  core.error(`Infinite loop detected! Pattern: ${pattern.pattern}`);
  break;
}

Adding New Error Patterns

When adding new error patterns to engines:

• Write the pattern with required content

   {
       Pattern:      `(?i)error.*permission.*denied`,
       LevelGroup:   0,
       MessageGroup: 0,
       Description:  "Permission denied error",
   }

• Test against empty string
• Run: make test-unit
• Checks: TestAllEnginePatternsSafe

• Test with actual log samples
• Ensure it matches real errors
• Ensure it doesn't match informational text

• Document the pattern
• Add clear description
• Note what it's designed to catch

Pattern Conversion: Go to JavaScript

Patterns are converted from Go to JavaScript:

// Go pattern (case-insensitive flag)
Pattern: `(?i)error.*permission.*denied`

// Converted to JavaScript
new RegExp("error.*permission.*denied", "gi")

The (?i) prefix is removed because JavaScript uses the i flag instead.

Examples from Current Codebase

✅ Safe Patterns

// Requires "error" prefix
Pattern: `(?i)error.*permission.*denied`

// Requires specific timestamp format
Pattern: `(\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}\.\d{3}Z)\s+\[(ERROR)\]\s+(.+)`

// Requires "access denied" prefix
Pattern: `(?i)access denied.*user.*not authorized`

How to Fix Unsafe Patterns

If you find a pattern that matches empty string:

Before (unsafe):

Pattern: `.*error.*`  // Can match empty at start/end

After (safe):

Pattern: `error.*`     // Requires "error" at start
// OR
Pattern: `.*error.+`   // Requires "error" and at least one char after
// OR
Pattern: `\berror\b.*` // Requires word "error"

Testing Checklist

Before committing pattern changes:

• [ ] Run make test-unit
• [ ] Check TestAllEnginePatternsSafe passes
• [ ] Check TestErrorPatternsNoInfiniteLoopPotential passes
• [ ] Run JavaScript tests: cd pkg/workflow/js && npm test
• [ ] Verify pattern matches intended error messages
• [ ] Verify pattern doesn't match informational text

References

• Go regex syntax: https://pkg.go.dev/regexp/syntax
• JavaScript regex: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions
• Test files:
• pkg/workflow/engine_error_patterns_infinite_loop_test.go
• pkg/workflow/js/validate_errors.test.cjs
• pkg/workflow/error_pattern_tuning_test.go