What causes "Regular expression failed to compile: %v" in hashicorp/nomad?

Test the pattern with Go's regexp (RE2) syntax, e.g. go run with regexp.Compile, before submitting.

Regular expression failed to compile: %v

Error message

Regular expression failed to compile: %v

What it means

For operand = "regexp" the constraint's RTarget must be a valid Go regular expression. regexp.Compile is run at validation time and any compile error is surfaced as 'Regular expression failed to compile: %v'.

Source

Thrown at nomad/structs/structs.go:10074

	if c.Operand == "" {
		mErr.Errors = append(mErr.Errors, errors.New("Missing constraint operand"))
	}

	// requireLtarget specifies whether the constraint requires an LTarget to be
	// provided.
	requireLtarget := true

	// Perform additional validation based on operand
	switch c.Operand {
	case ConstraintDistinctHosts:
		requireLtarget = false
	case ConstraintSetContainsAll, ConstraintSetContainsAny, ConstraintSetContains:
		if c.RTarget == "" {
			mErr.Errors = append(mErr.Errors, fmt.Errorf("Set contains constraint requires an RTarget"))
		}
	case ConstraintRegex:
		if _, err := regexp.Compile(c.RTarget); err != nil {
			mErr.Errors = append(mErr.Errors, fmt.Errorf("Regular expression failed to compile: %v", err))
		}
	case ConstraintVersion:
		if _, err := version.NewConstraint(c.RTarget); err != nil {
			mErr.Errors = append(mErr.Errors, fmt.Errorf("Version constraint is invalid: %v", err))
		}
	case ConstraintSemver:
		if _, err := semver.NewConstraint(c.RTarget); err != nil {
			mErr.Errors = append(mErr.Errors, fmt.Errorf("Semver constraint is invalid: %v", err))
		}
	case ConstraintDistinctProperty:
		// If a count is set, make sure it is convertible to a uint64
		if c.RTarget != "" {
			count, err := strconv.ParseUint(c.RTarget, 10, 64)
			if err != nil {
				mErr.Errors = append(mErr.Errors, fmt.Errorf("Failed to convert RTarget %q to uint64: %v", c.RTarget, err))
			} else if count < 1 {
				mErr.Errors = append(mErr.Errors, fmt.Errorf("Distinct Property must have an allowed count of 1 or greater: %d < 1", count))
			}

View on GitHub (pinned to 482b49bf1a)

Solutions

Test the pattern with Go's regexp (RE2) syntax, e.g. go run with regexp.Compile, before submitting.
Fix common syntax errors: balance [], (), escape backslashes, avoid lookahead/lookbehind (unsupported in RE2).
Simplify the pattern or use a version/semver constraint instead of regex where possible.

Example fix

// before
constraint {
  operand = "regexp"
  attribute = "${node.datacenter}"
  value     = "dc[1-2("
}
// after
constraint {
  operand = "regexp"
  attribute = "${node.datacenter}"
  value     = "dc[1-2]"
}

Defensive patterns

Strategy: validation

Validate before calling

if _, err := regexp.Compile(value); err != nil {
    return fmt.Errorf("constraint regex invalid: %v", err)
}

Prevention

Precompile patterns with Go regexp (RE2) before shipping jobs
Avoid PCRE-only features (lookaheads, backrefs)
Run nomad job validate in CI to catch regex errors early

When it happens

Trigger: constraint { operand = "regexp", value = "web[0-9" } — unbalanced brackets, trailing backslash, invalid repetition like '{2,1}', or unsupported syntax in the rtarget/value.

Common situations: Hand-written patterns with escaping mistakes (Windows-style backslashes); patterns copied from grep/PCRE with Go-unsupported constructs; interpolation producing malformed patterns.

Related errors

AI-assisted analysis of hashicorp/nomad@482b49bf1a (2026-09-04). Data as JSON: /api/errors/7fc47c3242197eec. Report an issue: GitHub.