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Square brackets marking an optional part.
Also known as: Optional notation grammar, Zero or one notation
The Extended Backus-Naur Form notation marking a part of a rule that may be present or absent.
Marks a part of a grammar rule that may be present or absent.
Contexts: Technical drawing, Software
Uses the same square brackets that command-line usage strings use for an optional argument, which is where most people meet the convention without ever meeting the grammar.
Contexts: Technical drawing, Software
A pair of square brackets enclosing the optional part of a rule.
A fuller structured visual breakdown is not yet documented for this symbol.