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#### Qi Keyword Parser Directive

###### Description

The `kwd[]`, `dkwd[]` and `ikwd[]`, `idkwd[]` provide a powerful and flexible mechanism for parsing keyword based input. It works in conjuction with the / operator to create an effective keyword parsing loop. The keyword parsing loop doesn't require the keywords to appear in a defined order in the input but also provides the possibility to check how many times a keyword appears in the input.

The kwd directive will parse the keywords respecting case sensitivity whereas the ikwd direcive is case insensitive. You can mix the kwd and ikwd directives inside a set of keywords, but be aware that this has a small overhead. It should be prefered not to mix the kwd and ikwd directives.

The dkwd and idkwd provide a mechanism to pase distinct keywords. These directives require that the skipper successeds parsing input right after the keyword part.

dkwd("keyword1")='>>int_

is equivalent to:

lit("keyword1") >> skipper+ >> '=' >> int_

All the keyword directives can be mixed inside a keyword list.

The kwd directive is very similar to the repeat directive in that it enables to enforce keyword occurrence constraints but also provides very interesting speed improvement over the pure EBNF syntax or the Nabialek-Trick.

###### Header

    // forwards to <boost/spirit/repository/home/qi/directive/kwd.hpp>
    #include <boost/spirit/repository/include/qi_kwd.hpp>

###### Synopsis

Expression | Semantics
--- | ---
`kwd(keyword)[subject]` | Parse ( `"keyword"` > `subject`) zero or more times.
`kwd(keyword,n)[subject]` | Parse ( `"keyword"` > `subject`) exactly `n` times.
`kwd(keyword,min, max)[subject]` | Parse ( `"keyword"` > `subject`) at least `min` times and at most `max` times.
`kwd(keyword,min, inf)[subject]` | Parse ( `"keyword"` > `subject`) at least `min` or more.

For non case sensitive keywords use the ikwd directive. If distinct keyword parsing is required, use the dkwd and idkwd directive instead.

###### Parameters

Parameter | Description
--- | ---
`keyword` | The parser for the opening (the prefix).
`subject` | The parser for the input sequence following the keyword part.
`n` | Int representing the exact number of times the keyword must be repeated.
`min` | Int representing the minimum number of times the keyword must be repeated.
`max` | Int representing the maximum number of times the keyword must be repeated.

The keyword as well as the subject parameters can be any valid spirit parser. The parameter n, min and max are integer constants.

###### Attributes

Expression | Attribute
--- | ---
`kwd(k1)[a]` |
    a: A --> kwd(k1)[a]: optional<A> or vector<A>
    a: Unused --> kwd(k1)[a]: Unused
`kwd(k1,n)[a]` |
    a: A --> kwd(k1,n)[a]: optional<A> or vector<A>
    a: Unused --> kwd(k1,n)[a]: Unused
`kwd(k1,min, max)[a]` |
    a: A --> kwd(k1,min, max)[a]: optional<A> or vector<A>
    a: Unused --> kwd(k1,min, max)[a]: Unused
`kwd(k1,min, inf)[a]` |
    a: A --> kwd(k1,min, inf)[a]: optional<A> or vector<A>
    a: Unused --> kwd(k1,min, inf)[a]: Unused

###### Complexity

> The overall complexity is defined by the complexity of its subject parser. The complexity of the keyword list construct `kwd` itself is O(N), where N is the number of repetitions executed.

> In the case where all the keywords are strings, the complexity of the keyword list itself determined by the complexity of the internal TST contents :

> O(log n+k)

> Where k is the length of the string to be searched in a TST with n strings.

> When the keywords used are complex parsers, then the complexity is the sum of the sub parser complexities.

###### Example

Please refer to keyword_list.