Domain pattern ' ' contains empty parameter name at position

Error message

Domain pattern '%s' contains empty parameter name at position %d

What it means

Inside parseDomainPattern, labels starting with ':' are treated as domain parameters (e.g. :tenant.example.com). A bare ':' (length 1) means the parameter name is missing, which would make the captured value unusable, so it panics with the label position.

Solutions

Always give parameter labels a name: ":tenant.example.com" not ":.example.com".
Validate parameter labels before registering: a label starting with ':' must have len > 1.
If the parameter is optional, omit the label entirely instead of emitting an empty parameter.

Example fix

// before
app.Domain(":.example.com")

// after
app.Domain(":tenant.example.com")

Defensive patterns

Strategy: validation

Validate before calling

// Ensure every parameter label has a name
func validateParamLabels(p string) error {
    for i, label := range strings.Split(p, ".") {
        if label == ":" {
            return fmt.Errorf("empty parameter name at position %d", i)
        }
    }
    return nil
}

Prevention

Always name parameter labels (:tenant, not :).
Validate templated patterns before registering them.
Document the parameter-label syntax for route authors.

When it happens

Trigger: Passing a pattern like ":.example.com" or "example.:" where a parameter label has no name.

Common situations: Typing a colon without the name; templating that emits ':' when a variable is empty; refactor that introduced a bare parameter marker.

Related errors

AI-assisted analysis of gofiber/fiber@a105acad6c (2026-08-11). Data as JSON: /api/errors/eea7447685490db8. Report an issue: GitHub.

Appendix: source

Thrown at domain.go:94

		panic(fmt.Sprintf("Domain pattern '%s' has %d parts, which exceeds the maximum of %d",
			pattern, len(parts), maxDomainParts))
	}

	m := domainMatcher{
		parts:    make([]string, len(parts)),
		numParts: len(parts),
	}

	for i, part := range parts {
		// Validate no empty labels (e.g., "example..com" is invalid)
		if part == "" {
			panic(fmt.Sprintf("Domain pattern '%s' contains empty label at position %d", pattern, i))
		}

		if part[0] == ':' {
			// Validate parameter name is not empty
			if len(part) == 1 {
				panic(fmt.Sprintf("Domain pattern '%s' contains empty parameter name at position %d", pattern, i))
			}
			paramName := part[1:]
			// Validate parameter name contains only ASCII-safe characters (a-z, A-Z, 0-9, underscore, hyphen).
			// Using explicit ASCII ranges rather than unicode.IsLetter/IsDigit to reject non-ASCII
			// characters that are invalid in DNS names.
			for _, ch := range paramName {
				if !isASCIIAlphanumeric(ch) && ch != '_' && ch != '-' {
					panic(fmt.Sprintf("Domain pattern '%s' contains invalid parameter name '%s' with character '%c'", pattern, paramName, ch))
				}
			}
			m.paramIdx = append(m.paramIdx, i)
			m.paramNames = append(m.paramNames, paramName) // preserve original case
			m.parts[i] = part                              // keep ":param" marker for matching
		} else {
			// Only lowercase constant labels (RFC 4343)
			// Enforce RFC 1035 per-label length limit (63 characters)
			if len(part) > 63 {
				panic(fmt.Sprintf("Domain pattern '%s' has label '%s' exceeding RFC 1035 limit of 63 characters (%d chars)",

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