crowdsecurity/crowdsec · error

while compiling grok's expression: %w

Error message

while compiling grok's expression: %w

What it means

A grok node may carry an expr-lang expression (ExpValue) evaluated at runtime. Compile invokes expr.Compile with the standard event options; a syntactically or semantically invalid expression is wrapped as 'while compiling grok's expression'.

Source

Thrown at pkg/parser/grok.go:70

	} else if g.RegexpValue != "" {
		if strings.HasSuffix(g.RegexpValue, "\n") {
			logger.Debugf("Beware, pattern ends with \\n: %q", g.RegexpValue)
		}

		rg.RunTimeRegexp, err = pctx.Grok.Compile(g.RegexpValue)
		if err != nil {
			return nil, fmt.Errorf("failed to compile grok %q: %v", g.RegexpValue, err)
		}

		logger.Tracef("%s regexp: %s", g.RegexpValue, rg.RunTimeRegexp.String())
	}

	// if grok source is an expression
	if g.ExpValue != "" {
		rg.RunTimeValue, err = expr.Compile(g.ExpValue,
			exprhelpers.GetExprOptions(map[string]any{"evt": &pipeline.Event{}})...)
		if err != nil {
			return nil, fmt.Errorf("while compiling grok's expression: %w", err)
		}
	}

	/* load grok statics */
	// compile expr statics if present
	for _, static := range g.Statics {
		compiled, err := static.Compile()
		if err != nil {
			return nil, err
		}

		rg.RuntimeStatics = append(rg.RuntimeStatics, *compiled)
	}

	return rg, nil
}

func (g *GrokPattern) Validate() error {

View on GitHub (pinned to 909b515798)

Solutions

1. Fix the expression syntax in the 'expression' field of the grok node; ensure it only references known helpers (evt.*, functions from pkg/exprhelpers).
2. Test the expression with `cscli hub test` against a sample line to see the compile error in context.
3. Check the expr-lang documentation and exprhelpers package for available functions and valid field paths.
4. Verify collection version compatibility if the expression came from a hub collection.

Example fix

// before
grok:
  expression: evt.Parsed.some_field ==
// after
grok:
  expression: evt.Parsed.some_field == 'bar'

opts := exprhelpers.GetExprOptions(map[string]any{"evt": &pipeline.Event{}})
_, err := expr.Compile(expValue, opts...)
if err != nil {
    return fmt.Errorf("grok expression %q invalid: %w", expValue, err)
}

if _, err := node.Compile(pctx); err != nil {
    if strings.Contains(err.Error(), "compiling grok's expression") {
        logger.Errorf("bad expr in grok expression; check syntax and helper names: %v", err)
    }
    return err
}

When it happens

Trigger: Compile() calls expr.Compile(g.ExpValue, ...) and the expression fails to parse/type-check — unknown variables, wrong syntax, invalid function calls.

Common situations: Custom parser config with a bad expression (typo in evt field, missing quotes around a string, using a function that doesn't exist in exprhelpers); copying an expression from a newer/older crowdsec version using removed helpers.

Related errors

• unable to find grok %q: %v
• empty grok %q
• failed to compile grok %q: %v
• non-empty filter %q was not compiled
• failed to compile node in '%s' : %s

AI-assisted analysis of crowdsecurity/crowdsec@909b515798 (2026-09-06). Data as JSON: /api/errors/a4efc82503dd60fd. Report an issue: GitHub.