Help with optional values in grammar

[dirodriguezm]

I'm trying to parse a SQL like language and I can't find a way to define the grammar for a "column reference".

The strings I'm trying to parse look like "catalog.schema.table.column" or could be just "column" or "table.column" or even "schema.table.column".

So for that kind of input, the grammar is defined like this in BNF:

<column_reference> ::= [ <qualifier> <period> ] <identifier>
<qualifier> ::= <table_name> | <identifier>
<table_name> ::= [ <schema_name> <period> ] <identifier>
<schema_name> ::= [ <identifier> <period> ] <identifier>

What I've tried so far is this:

type ColumnReference struct {
	Qualifier *Qualifier `parser:"(@@ '.')?"`
	ColumnName string `parser:"@Ident"`
}

type Qualifier struct {
	TableName *TableName `parser:"@@"`
	CorrelationName *string `| @Ident`
}

type TableName struct {
	SchemaName *SchemaName `parser:"(@@ '.')?"`
	Identifier string `parser:"@Ident"`
}

type SchemaName struct {
	CatalogName string `parser:"(@Ident '.')?"`
	UnqualifiedSchemaName string `parser:"@Ident"`
}

But with that, I can only successfully parse "a" or "a.b.c.d".

What could I do to fix this ? The only alternate solution I've come with is to put each item (catalog, schema, table and column) in an array, but that kind of breaks the original grammar.

Any help would be appreciated.