How to understand this EBNF Pascal Definition

Question

I'm implementing a Pascal parser from this EBNF defintion. There is something I don't understand in the following specifications:

variable
   entire-variable | component-variable | referenced-variable 

entire-variable
   variable-identifier | field-identifier

component-variable
   indexed-variable | field-designator | file-buffer

field-designator
   record-variable "." field-identifier

Assume we want to apply the variable production on a.b[0]. Since a conforms to the entire-variable production, this will prevent component-variable from detecting the field-designator a.b and therefore the . following a will stop the parser.

I don't have the Pascal spec in front of me but I can make a few observations. When the parser sees 'a', it should already know what 'a' is defined as. Also the parser can look-ahead and see the '.'. — Stuart, Jan 15 '19 at 03:06
Readers of this question might also want to take a look at [How to define Pascal variables in PetitParser](https://stackoverflow.com/q/54207918/4081336) — Leandro Caniglia, Jan 19 '19 at 11:32

score 3 · Accepted Answer · edited Jan 15 '19 at 13:33

3

Since EBNF doesn't have ordered choices, the longest match is often used to determine which rules apply.

edited Jan 15 '19 at 13:33

Ken White

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answered Jan 15 '19 at 06:22

Uran

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Thanks Uran. This makes sense. I didn't know that the choices have no order. – Leandro Caniglia Jan 15 '19 at 09:46

How to understand this EBNF Pascal Definition

1 Answers1