BNF for comma separated sequence?

Question

Can't come up with a BNF grammar for the sequence of characters (possibly empty), separated by comma, but not starting or ending with a comma,

So this is OK:

  <--- Empty sequence is ok!
A
A,B
A,B,C

This is NOT ok:

A,
,A
A,,B
AB

The empty case throws me off. What I got so far is:

<char-seq> ::= <empty> | <char> , <char-seq> | <char>

but this produces strings like A, :-(

score 13 · Accepted Answer · answered Sep 19 '12 at 13:47

13

The empty char sequence is what gives you the trouble. You need a rule that matches a non-empty sequence to be separate from the rule that matches both an empty and a non-empty one, like this:

<char-seq> ::= <empty> | <non-empty-char-seq>
<non-empty-char-seq> ::= <char> | <char> , <non-empty-char-seq>

answered Sep 19 '12 at 13:47

Sergey Kalinichenko

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thanks for your solution but I think this BNF is infinite recursive ? it should be like that ::= /*empty*/| | , am i right ? – Melih Altıntaş Nov 07 '13 at 16:52
1

@MelihAltıntaş The `` is allowed by ``, but not by ``, which must start and end in a character. – Sergey Kalinichenko Nov 07 '13 at 16:55

score 4 · Answer 2 · answered Sep 19 '12 at 13:46

4

<char-seq> ::= <empty> | <chars>
<chars> ::= <char> | <char> , <chars>

answered Sep 19 '12 at 13:46

Istvan Neuwirth

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2
4

BNF for comma separated sequence?

2 Answers2