What causes "Invalid regex {e.pattern!r}: {e}" in pytest-dev/pytest?

Fix the regex syntax in the message or module field

pytest-dev/pytest · error · UsageError

Invalid regex

Error message

Invalid regex {e.pattern!r}: {e}

What it means

Raised by parse_warning_filter() when the message or module field of a warning filter is not a valid regular expression. After parsing the fields, pytest compiles both the message and module patterns with re.compile(); if either fails, a re.error is caught and re-raised as UsageError with the pattern and error.

Solutions

Fix the regex syntax in the message or module field
For config-provided filters, pytest auto-escapes the message and module (escape=True), so literal special chars are usually safe
For CLI -W flags, escape regex metacharacters manually or simplify to a plain substring

Example fix

# before (CLI flag, not auto-escaped)
pytest -W "ignore:(unbalanced:DeprecationWarning"

# after
pytest -W "ignore:\(balanced\):DeprecationWarning"

Defensive patterns

Strategy: validation

Validate before calling

import re
def validate_warning_filter_regex(arg: str) -> bool:
    parts = arg.split(':')
    # Pad to 5 fields
    while len(parts) < 5:
        parts.append('')
    message, module = parts[1], parts[3]
    try:
        re.compile(message)
        re.compile(module)
        return True
    except re.error:
        return False

Try / catch

from pytest import UsageError
try:
    parse_warning_filter(arg, escape=True)
except UsageError as e:
    print(f"Invalid regex in filter: {e}")

Prevention

For config-provided filters, pytest auto-escapes message/module so literal text is safe
For CLI -W flags, escape regex metacharacters or use simple substrings
Test regex patterns with re.compile before adding to config

When it happens

Trigger: Providing a warning filter where the message or module segment contains an invalid regex. Example: filterwarnings = ["ignore:'(::DeprecationWarning"] — the message pattern has an unbalanced parenthesis. When escape=True (the default for config-provided filters), special chars are escaped, but raw CLI flags (-W) are not escaped.

Common situations: Using unescaped special regex characters in a CLI -W flag's message or module field. Malformed regex from a typo. Copying a filter string that was intended for a different regex engine.

Related errors

AI-assisted analysis of pytest-dev/pytest@0d6fbdeffa (2026-08-11). Data as JSON: /api/errors/fc62040c9ce5398a. Report an issue: GitHub.

Appendix: source

Thrown at src/_pytest/config/__init__.py:2320

        message = re.escape(message)
    if module and escape:
        module = re.escape(module) + r"\Z"
    if lineno_:
        try:
            lineno = int(lineno_)
            if lineno < 0:
                raise ValueError("number is negative")
        except ValueError as e:
            raise UsageError(
                error_template.format(error=f"invalid lineno {lineno_!r}: {e}")
            ) from None
    else:
        lineno = 0
    try:
        re.compile(message)
        re.compile(module)
    except re.error as e:
        raise UsageError(
            error_template.format(error=f"Invalid regex {e.pattern!r}: {e}")
        ) from None
    return action, message, category, module, lineno


def _resolve_warning_category(category: str) -> type[Warning]:
    """
    Copied from warnings._getcategory, but changed so it lets exceptions (specially ImportErrors)
    propagate so we can get access to their tracebacks (#9218).
    """
    __tracebackhide__ = True
    if not category:
        return Warning

    if "." not in category:
        import builtins as m

        klass = category

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