Explain BNF syntax for NID in RFC 2141
I am having trouble understanding some [BNF](http://en.wikipedia.org/wiki/Backus%E2%80%93Naur_Form) syntax from [RFC2141](http://tools.ietf.org/html/rfc2141).

The line is `<NID> ::= <let-num> [ 1,31<let-num-hyp> ]`.  I think it means that `<NID>` is a symbol for a string, with constrained by two rules:

1.  The string must be begin with a single occurence of any of the `<let-num>` characters.
2.  This character may be followed by 0-31 occurrences* of any of the `<let-num-hyp>` characters.

Am I reading this correctly?  Because, if I am, some of the implications are a bit confusing.

**equivalent to "optionally, 1-31 occurrences*

The complete BNF syntax for a `<NID>` (Namespace Identifier) in [RFC2141](http://tools.ietf.org/html/rfc2141) is:

    <NID>         ::= <let-num> [ 1,31<let-num-hyp> ]
    <let-num-hyp> ::= <upper> | <lower> | <number> | "-"
    <let-num>     ::= <upper> | <lower> | <number>
    <upper>       ::= "A" | "B" | "C" | "D" | "E" | "F" | "G" | "H" |
                      "I" | "J" | "K" | "L" | "M" | "N" | "O" | "P" |
                      "Q" | "R" | "S" | "T" | "U" | "V" | "W" | "X" |
                      "Y" | "Z"
    <lower>       ::= "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" |
                      "i" | "j" | "k" | "l" | "m" | "n" | "o" | "p" |
                      "q" | "r" | "s" | "t" | "u" | "v" | "w" | "x" |
                      "y" | "z"
    <number>      ::= "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" |
                      "8" | "9"
Explain BNF syntax for NID in RFC 2141
===

I am having trouble understanding some [BNF](http://en.wikipedia.org/wiki/Backus%E2%80%93Naur_Form) syntax from [RFC2141](http://tools.ietf.org/html/rfc2141).

The line is `<NID> ::= <let-num> [ 1,31<let-num-hyp> ]`.  I think it means that `<NID>` is a symbol for a string, with constrained by two rules:

1.  The string must be begin with a single occurence of any of the `<let-num>` characters.
2.  This character may be followed by 0-31 occurrences* of any of the `<let-num-hyp>` characters.

Am I reading this correctly?  Because, if I am, some of the implications are a bit confusing.

**equivalent to "optionally, 1-31 occurrences*

The complete BNF syntax for a `<NID>` (Namespace Identifier) in [RFC2141](http://tools.ietf.org/html/rfc2141) is:

    <NID>         ::= <let-num> [ 1,31<let-num-hyp> ]
    <let-num-hyp> ::= <upper> | <lower> | <number> | "-"
    <let-num>     ::= <upper> | <lower> | <number>
    <upper>       ::= "A" | "B" | "C" | "D" | "E" | "F" | "G" | "H" |
                      "I" | "J" | "K" | "L" | "M" | "N" | "O" | "P" |
                      "Q" | "R" | "S" | "T" | "U" | "V" | "W" | "X" |
                      "Y" | "Z"
    <lower>       ::= "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" |
                      "i" | "j" | "k" | "l" | "m" | "n" | "o" | "p" |
                      "q" | "r" | "s" | "t" | "u" | "v" | "w" | "x" |
                      "y" | "z"
    <number>      ::= "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" |
                      "8" | "9"