# Class Template `basic_regex_filter`

Description
Headers
Installation
Reference

## Description

The class template `basic_regex_filter` performs text substitutions using regular expressions from the Boost Regular Expression Library ([Maddock]).

Each `basic_regex_filter` stores a regular expression and a formatting function. As unfiltered data is scanned for matches, portions of data which fall between regular expression matches are forwarded unchanged. Each time a match is found, the corresponding `match_results` object is passed to the formatting function and the return value is forwarded in place of the matched text.

Users may construct instances of `basic_regex_filter` either from formatting functions or from format strings.

Since a regular expression may need to look arbitrarily far ahead in a character sequence, `basic_regex_filter` derives from `aggregate_filter` and processes an entire sequence of data at once.

## Headers

`<boost/iostreams/filter/regex.hpp>`

## Installation

The template `basic_regex_filter` uses the Boost Regular Expression Library, which must be built separately. See here for details.

## Reference

#### Synopsis

    namespace boost { namespace iostreams {

    #include <boost/function.hpp>
    #include <boost/regex.hpp>

    template< typename Ch,
              typename Tr = regex_traits<Ch>,
              typename Alloc = std::allocator<Ch> >
    class basic_regex_filter
        : public aggregate_filter<Ch, std::char_traits<Ch>, Alloc> // Exposition only
    {
    public:
        typedef boost::function1<
                    std::basic_string<Ch>,
                    const match_results<const Ch*>&
                >
                formatter;
        basic_regex_filter( const basic_regex<Ch, Tr, Alloc>& pattern,
                            const formatter& replace,
                            regex_constants::match_flag_type flags =
                                regex_constants::match_default );
        basic_regex_filter( const basic_regex<Ch, Tr, Alloc>& pattern,
                            const std::basic_string<Ch>& fmt,
                            regex_constants::match_flag_type flags =
                                regex_constants::match_default,
                            regex_constants::match_flag_type fmt_flags =
                                regex_constants::format_default );
        basic_regex_filter( const basic_regex<Ch, Tr, Alloc>& pattern,
                            const Ch* fmt,
                            regex_constants::match_flag_type flags =
                                regex_constants::match_default,
                            regex_constants::match_flag_type fmt_flags =
                                regex_constants::format_default );
    };

    typedef basic_regex_filter<char>     regex_filter;
    typedef basic_regex_filter<wchar_t>  wregex_filter;

    } } // End namespace boost::io

#### Template parameters

Ch | - | The character type
Tr | - | The regular expression traits type
Alloc | - | A standard library allocator type ([ISO], 20.1.5), used to allocate character buffers

#### `basic_regex_filter::formatter`

        typedef boost::function1<
                    std::basic_string<Ch>,
                    const match_results<const Ch*>&
                >
                formatter;

The type of object which a `basic_regex_filter` uses to determine the replacement text for a given regular expression match. Each time a match is found, a corresponding `match_results` object is passed to a `formatter`, which returns the appropriate replacement text. Since Boost.Function objects are implictly constructible from function objects with the correct signature, users of regular expression Filters may define their own function objects with the correct signature and pass them to the `basic_regex_filter` constructor which takes a `formatter`. E.g.,

        struct my_formatter {
            std::string operator()(const match_results<const char*>& match)
            {
                // Examine match and return the appropriate replacement text
            }
        };
        int main()
        {
            regex         pattern( ... );
            regex_filter  filter(pattern, my_formatter());
            ...
        }

#### `basic_regex_filter::basic_regex_filter`

        basic_regex_filter( const basic_regex<Ch, Tr, Alloc>& pattern,
                            const formatter& replace,
                            regex_constants::match_flag_type flags =
                                regex_constants::match_default );

Constructs a `basic_regex_filter` from the given regular expression, formatting function and match flags. The parameters have the following interpretations:

pattern | - | The regular expression to be matched against the stream of unfiltered data
replace | - | A function which will be passed each `match_results` object in succession and whose return values will be be used as replacement text
flags | - | Used to construct a regex_iterator

        basic_regex_filter( const basic_regex<Ch, Tr, Alloc>& pattern,
                            const std::basic_string<Ch>& fmt,
                            regex_constants::match_flag_type flags =
                                regex_constants::match_default,
                            regex_constants::match_flag_type fmt_flags =
                                regex_constants::format_default );

Constructs a `basic_regex_filter` from the given regular expression, format string and pair of match flags. The parameters have the following interpretations:

pattern | - | The regular expression to be matched against the stream of unfiltered data
fmt | - | A format string which specifies the replacement text for each regular expression match, using match_results::format
flags | - | Used to construct a regex_iterator
fmt_flags | - | The flags argument to match_results::format

        basic_regex_filter( const basic_regex<Ch, Tr, Alloc>& pattern,
                            const Ch* fmt,
                            regex_constants::match_flag_type flags =
                                regex_constants::match_default,
                            regex_constants::match_flag_type fmt_flags =
                                regex_constants::format_default );

Constructs a `basic_regex_filter` from the given regular expression, format string and pair of match flags. The parameters have the following interpretations:

pattern | - | The regular expression to be matched against the stream of unfiltered data
fmt | - | A format string which specifies the replacement text for each regular expression match, using match_results::format
flags | - | Used to construct a regex_iterator
fmt_flags | - | The flags argument to match_results::format

Revised 20 May, 2004

© Copyright Jonathan Turkanis, 2004

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)