boost/libs/spirit/repository/doc/qi/seek.qbk

[/==============================================================================
    Copyright (C) 2001-2011 Joel de Guzman
    Copyright (C) 2001-2011 Hartmut Kaiser
    Copyright (C) 2011 Jamboree 

    Distributed under the Boost Software License, Version 1.0. (See accompanying
    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
===============================================================================/]

[section:seek Qi Seek Parser Directive ]

[heading Description]

The `seek[]` parser-directive skips all input until the subject parser matches.

[heading Header]

    // forwards to <boost/spirit/repository/home/qi/directive/seek.hpp>
    #include <boost/spirit/repository/include/qi_seek.hpp>

Also, see __include_structure__.

[heading Namespace]

[table
    [[Name]]
    [[`boost::spirit::repository::qi::seek`]]
]

[heading Model of]

[:__unary_parser_concept__]

[variablelist Notation
    [[`a`]          [A __parser_concept__.]]
]

[heading Expression Semantics]

Semantics of an expression is defined only where it differs from, or is
not defined in __unary_parser_concept__.

[table
    [[Expression]               [Semantics]]
    [[`seek[a]`]                [Advances until the parser `a` matches.]]
]

[heading Attributes]

See __qi_comp_attr_notation__.

[table
    [[Expression]       [Attribute]]
    [[`seek[a]`]
[``a: A --> seek[a]: A
a: Unused --> seek[a]: Unused``]]
]

[heading Complexity]

[:The overall complexity is defined by the complexity of its subject
parser. The complexity of `seek` itself is O(N), where N is the number
of unsuccessful matches.]

[note *seeking sequence with skipping*

Using `seek[a >> b]` with skipping is inefficient, because when sequence fails, the backtracked position is non-skipped.
The solution is to ensure the input will always be pre-skipped, for example:
``
    seek[lexeme[skip[a >> b]]]
``	
does the trick.]

[heading Example]

[import ../../example/qi/seek.cpp]

The following example shows a simple use case of the `seek[]` directive, parsing C-style comment.
(For the full source of the example, see [@../../example/qi/seek.cpp seek.cpp])

Some namespace aliases:

[reference_qi_seek_namespace]

The input string and its iterators:

[reference_qi_seek_vars]

Parsing and showing the result:

[reference_qi_seek_parse]

[endsect]