apple/pkl · error · RendererException

Values of type `Regex` cannot be rendered as YAML. Value: %s

Error message

Values of type `Regex` cannot be rendered as YAML. Value: %s

What it means

The YAML renderer does not support java.util.regex.Pattern values. Pkl Regex values are patterns, not renderable data, so visitRegex throws a RendererException rather than emitting a guessable representation. The error exists to fail fast instead of silently rendering a misleading string form.

Source

Thrown at pkl-core/src/main/java/org/pkl/core/YamlRenderer.java:223

              value.getSimpleName()));
    }

    @Override
    public void visitTypeAlias(TypeAlias value) {
      throw new RendererException(
          String.format(
              "Values of type `TypeAlias` cannot be rendered as YAML. Value: %s",
              value.getSimpleName()));
    }

    @Override
    public void visitNull() {
      emitter.emit(YamlUtils.plainScalar("null", Tag.NULL));
    }

    @Override
    public void visitRegex(Pattern value) {
      throw new RendererException(
          String.format("Values of type `Regex` cannot be rendered as YAML. Value: %s", value));
    }

    private void doVisitIterable(Iterable<?> iterable, @Nullable String tag) {
      emitter.emit(
          new SequenceStartEvent(
              Optional.empty(), Optional.ofNullable(tag), true, FlowStyle.BLOCK));
      for (var elem : iterable) visit(elem);
      emitter.emit(new SequenceEndEvent(Optional.empty(), Optional.empty()));
    }

    private void doVisitProperties(Map<String, ?> properties) {
      emitter.emit(
          new MappingStartEvent(Optional.empty(), Optional.empty(), true, FlowStyle.BLOCK));

      for (var entry : properties.entrySet()) {
        var value = entry.getValue();

View on GitHub (pinned to f3efcbfc9b)

Solutions

Convert the Regex to a string before rendering (render its pattern source instead of the Pattern object).
Exclude regex/validation properties from the rendered document (e.g. keep them hidden or in a separate non-output module).
If YAML must contain the pattern, wrap it in a String-typed property holding the pattern text.

Example fix

// before (Pkl)
output {
  ["pattern"] = myRegex
}
// after
output {
  ["pattern"] = myRegex.patternString
}

Defensive patterns

Strategy: validation

Validate before calling

// before rendering
if (anyValue is java.util.regex.Pattern || valueIsPklRegex(value)) {
  throw IllegalArgumentException("Convert Regex to its pattern string before YAML rendering")
}

Type guard

fun isYamlRenderable(v: Any?): Boolean =
  v !is java.util.regex.Pattern && (v == null || v is String || v is Number || v is Boolean || v is List<*> || v is Map<*, *> || v is org.pkl.core.runtime.VmObject)

Prevention

Convert Regex values to pattern strings before output rendering.
Keep validation-only regex properties out of output documents.
Declare output properties as String when they hold pattern text.

When it happens

Trigger: Rendering a property whose value is a Regex (e.g. a property declared as `regex` or produced by Regex(#"...")`) via YamlRenderer, or including a Regex-valued member in an object passed to YAML output.

Common situations: Validation-heavy configs where pattern properties live next to data properties and a tool renders the whole module to YAML; forgetting to convert a Regex to its .patternString before output.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

AI-assisted analysis of apple/pkl@f3efcbfc9b (2026-09-08). Data as JSON: /api/errors/6e8179bd8177519f. Report an issue: GitHub.