Table of Contents

About

The railroad component will render a railroad diagram from a BNF grammar.

Example

Below is the BNF of the select sql statement of the H2 database. 

<railroad>
<code bnf display="none">
'SELECT' [ 'TOP' term ] [ 'DISTINCT' | 'ALL' ] selectExpression {',' selectExpression} \
'FROM' tableExpression {',' tableExpression} [ 'WHERE' expression ] \
[ 'GROUP BY' expression {',' expression} ] [ 'HAVING' expression ] \
[ ( 'UNION' [ 'ALL' ] | 'MINUS' | 'EXCEPT' | 'INTERSECT' ) select ] [ 'ORDER BY' order {',' order} ] \
[ 'LIMIT' expression [ 'OFFSET' expression ] [ 'SAMPLE_SIZE' rowCountInt ] ] \
[ 'FOR UPDATE' ];
</code>
</railroad>

This BNF grammar will render as the below railroad diagram. 

'SELECT' [ 'TOP' term ] [ 'DISTINCT' | 'ALL' ] selectExpression {',' selectExpression} \
'FROM' tableExpression {',' tableExpression} [ 'WHERE' expression ] \
[ 'GROUP BY' expression {',' expression} ] [ 'HAVING' expression ] \
[ ( 'UNION' [ 'ALL' ] | 'MINUS' | 'EXCEPT' | 'INTERSECT' ) select ] [ 'ORDER BY' order {',' order} ] \
[ 'LIMIT' expression [ 'OFFSET' expression ] [ 'SAMPLE_SIZE' rowCountInt ] ] \
[ 'FOR UPDATE' ];

Syntax

To render a railroad diagram, you just need to enclose a code block with a bnf grammar with the railroad component 

<railroad>
    <code bnf display="none">
    ... bnf code
    </code>
</railroad>

display=“none” is added to show you how to not print a code block but this is optional.

Bnf

The supported BNF is described below: 

- definition
    =
    :=
    ::=
- concatenation
    ,
    <whitespace>
- termination
    ;
- alternation
    |
- option
    [ ... ]
    ?
- repetition
    { ... } => 0..N
    expression* => 0..N
    expression+ => 1..N
    <digits> * expression => <digits>...<digits>
    <digits> * [expression] => <0>...<digits>
    <digits> * expression? => <0>...<digits>
- grouping
    ( ... )
- literal
    " ... " or ' ... '
- special characters
    (? ... ?)
- comments
    (* ... *)