deepset-ai/haystack · error · ValueError

Invalid regex pattern '{mime_type}'.

Error message

Invalid regex pattern '{mime_type}'.

What it means

DocumentTypeRouter.__init__ compiles each entry in mime_types as a regular expression with re.compile; if a MIME type string is not a valid regex, the re.error is wrapped in this ValueError.

Source

Thrown at haystack/components/routers/document_type_router.py:105

        if mime_type_meta_field is None and file_path_meta_field is None:
            raise ValueError(
                "At least one of 'mime_type_meta_field' or 'file_path_meta_field' must be provided to determine MIME "
                "types."
            )
        self.mime_type_meta_field = mime_type_meta_field
        self.file_path_meta_field = file_path_meta_field

        if additional_mimetypes:
            for mime, ext in additional_mimetypes.items():
                mimetypes.add_type(mime, ext)

        self._mime_type_patterns = []
        for mime_type in mime_types:
            try:
                pattern = re.compile(mime_type)
            except re.error as e:
                raise ValueError(f"Invalid regex pattern '{mime_type}'.") from e
            self._mime_type_patterns.append(pattern)

        component.set_output_types(self, unclassified=list[Document], **dict.fromkeys(mime_types, list[Document]))
        self.mime_types = mime_types
        self.additional_mimetypes = additional_mimetypes

    def run(self, documents: list[Document]) -> dict[str, list[Document]]:
        """
        Categorize input documents into groups based on their MIME type.

        MIME types can either be directly available in document metadata or derived from file paths using the
        standard Python `mimetypes` module and custom mappings.

        :param documents:
            A list of documents to be categorized.

        :returns:
            A dictionary where the keys are MIME types (or `"unclassified"`) and the values are lists of documents.

View on GitHub (pinned to e318778c9b)

Solutions

1. Fix the MIME type string so it is a valid regex (escape special characters)
2. Use plain MIME types like 'application/pdf' which are valid regexes
3. Test your pattern with re.compile(pattern) in a REPL to see the regex error

Example fix

# before
router = DocumentTypeRouter(mime_types=["text/["])
# after
router = DocumentTypeRouter(mime_types=["text/html"])

import re
for mt in mime_types:
    re.compile(mt)  # raises re.error before the router does

def is_valid_mime_regex(mime_type: str) -> bool:
    import re
    try:
        re.compile(mime_type)
        return True
    except re.error:
        return False

try:
    router = DocumentTypeRouter(mime_types=mime_types, mime_type_meta_field="meta.mimetype")
except ValueError as e:
    if "Invalid regex pattern" in str(e):
        mime_types = [mt for mt in mime_types if is_valid_mime_regex(mt)]
        router = DocumentTypeRouter(mime_types=mime_types, mime_type_meta_field="meta.mimetype")
    else:
        raise

When it happens

Trigger: mime_types entries containing regex-special characters that form invalid patterns, e.g. 'text/[' or a stray '(' in a MIME type string.

Common situations: Hand-editing MIME type lists; copying patterns with unbalanced brackets; confusing glob-style patterns (e.g. 'image/*' is fine, but broken brackets are not).

Related errors

• Invalid MIME type or regex pattern '{mime_type}'.
• The list of mime types cannot be empty.
• At least one of 'mime_type_meta_field' or 'file_path_meta_fi
• The list of mime types cannot be empty.
• Tool execution requires at least one tool.

AI-assisted analysis of deepset-ai/haystack@e318778c9b (2026-08-30). Data as JSON: /api/errors/c164ab5e6896af13. Report an issue: GitHub.