keep-unique

Prevents duplicate entries in a list — allowlists, registered IDs, route names.

Syntax

Attribute	Value	Default
keep-unique	empty, or a regex whose (?P<value>…) group is compared	whole line

Unlike keep-sorted, the regex goes directly in the attribute value — there is no separate keep-unique-pattern.

Example

# <block keep-unique>
"user_1",
"user_2",
"user_3",
# </block>

Uniqueness by regex

Compare only part of each line:

ids = [
    # <block keep-unique="ID:(?P<value>\d+)">
    "ID:1 Alice",
    "ID:2 Bob",
    "ID:1 Carol",  # Violation: ID:1 is already used
    # </block>
]

Without a value group the whole match is compared. Lines that do not match are skipped, as are lines whose match is empty. So is a line that matches a branch without the value group: ^(?:none|(?P<value>\w+))$ skips the line none.

Notes

• Blank lines are ignored.
• Comparison is on the trimmed line, so indentation does not create false uniqueness. A regex is applied to the trimmed line too, so ^ and $ anchor to the entry rather than to the indentation.
• Commonly combined with keep-sorted on the same block.

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