deepset-ai/haystack · error · ValueError

Pattern '{pattern}' contains multiple capture groups. Please

Error message

Pattern '{pattern}' contains multiple capture groups. Please specify a pattern with at most one capture group.

What it means

AnswerBuilder._check_num_groups_in_regex raises this when the extraction pattern has more than one regex capture group. AnswerBuilder uses exactly one capture group to extract the answer substring, so patterns with 2+ groups are ambiguous and rejected.

Source

Thrown at haystack/components/builders/answer_builder.py:312

                        start, end = int(start_str), int(end_str)
                        if start > end:
                            continue
                        # Clamp the range end to the number of documents to avoid materializing a huge
                        # set from an out-of-range citation like `[1-999999999]` in the Generator output.
                        if num_documents is not None:
                            end = min(end, num_documents)
                        idxs.update(range(start - 1, end))
                    else:
                        idxs.add(int(part) - 1)
            else:
                idxs.add(int(match) - 1)
        return idxs

    @staticmethod
    def _check_num_groups_in_regex(pattern: str) -> None:
        num_groups = re.compile(pattern).groups
        if num_groups > 1:
            raise ValueError(
                f"Pattern '{pattern}' contains multiple capture groups. "
                f"Please specify a pattern with at most one capture group."
            )

View on GitHub (pinned to e318778c9b)

Solutions

Rewrite the pattern to have exactly one capture group around the content you want extracted.
Convert helper parentheses to non-capturing groups with (?:...).
Preprocess/strip fixed prefixes with string operations instead of capturing them.

Example fix

// before
pattern = r"(Answer: )(\d+)"
// after
pattern = r"Answer: (\d+)"

Defensive patterns

Strategy: validation

Validate before calling

import re
num_groups = re.compile(pattern).groups
assert num_groups <= 1, f"pattern '{pattern}' has {num_groups} capture groups"

Try / catch

try:
    result = answer_builder.run(replies=replies, pattern=pattern)
except ValueError as e:
    if "multiple capture groups" in str(e):
        logging.error("Bad extraction pattern: %s", e)
    raise

Prevention

Count capture groups with re.compile(p).groups before passing patterns.
Use non-capturing (?:...) for structural grouping.
Keep extraction patterns to exactly one group around the answer text.

When it happens

Trigger: Passing a pattern with multiple capturing parentheses to AnswerBuilder.run(pattern=...) or the AnswerBuilder(pattern=...) constructor — e.g. r"(Answer: )(\d+)".

Common situations: Writing an extraction regex with a non-capturing delimiter accidentally wrapped in parentheses; copying a validation regex into the extraction pattern; converting from another regex-based extractor that supported multiple groups.

Related errors

AI-assisted analysis of deepset-ai/haystack@e318778c9b (2026-08-30). Data as JSON: /api/errors/2e77f8cbbf0efc40. Report an issue: GitHub.