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## Function template join_if

boost::algorithm::join_if — Conditional join algorithm.

## Synopsis

    // In header: <boost/algorithm/string/regex.hpp>

    template<typename SequenceSequenceT, typename Range1T, typename CharT,
             typename RegexTraitsT>
      range_value< SequenceSequenceT >::type
      join_if(const SequenceSequenceT & Input, const Range1T & Separator,
              const basic_regex< CharT, RegexTraitsT > & Rx,
              match_flag_type Flags = match_default);

## Description

This algorithm joins all strings in a 'list' into one long string. Segments are concatenated by given separator. Only segments that match the given regular expression will be added to the result

This is a specialization of join_if algorithm.

Parameters: |
`Flags` | Regex options
`Input` | A container that holds the input strings. It must be a container-of-containers.
`Rx` | A regular expression
`Separator` | A string that will separate the joined segments.
Returns: | Concatenated string.
Notes: | This function provides the strong exception-safety guarantee