error-pattern-safety

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:

1. JavaScript's regex.exec() with the g flag uses lastIndex to track position
2. When a pattern matches zero-width, lastIndex doesn't advance
3. 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

1. Always require at least one character match

   • Use .+ instead of .* when you need "something"
   • Ensure pattern has required prefix/suffix

2. Never use bare .* as the entire pattern

   • Always combine with required text: error.*
   • Never just .* or .*?

3. Test patterns against empty string

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

4. 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!")
    }
}