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std::regex_...orm_primary(3)  C++ Standard Libary std::regex_...orm_primary(3)

NAME
       std::regex_traits::transform_primary	-    std::regex_traits::trans-
       form_primary

Synopsis
	  template< class ForwardIt >
	  string_type transform_primary( ForwardIt  first,  ForwardIt  last  )
       const;

	  For  the  character sequence [first, last), obtains the primary sort
       key in the imbued
	  locale's collating order, that is, the sort key that is based	on the
       positions of
	  the letters and collation units in the national  alphabet,  ignoring
       case, diacritics,
	  variants, etc. If a primary sort key compares	less than another pri-
       mary sort key
	  with	operator<, then	the character sequence that produced the first
       sort key	comes
	  before the character sequence	that produced the second sort key,  in
       the currently
	  imbued locale's primary collation order.

	  The regex library uses this trait to match characters	against	equiv-
       alence classes.
	  For example, the regex [[=a=]] is equivalent to the character	c1 if
	  traits.transform_primary(c1)	is equivalent to traits.transform_pri-
       mary("a") (which
	  is true for any c1 from "Aa "	in the U.S. English locale). Note that
	  transform_primary() takes  a	character  sequence  argument  because
       equivalence classes
	  may  be  multicharacter,  such  as [[=ch=]] in Czech or [[=dzs=]] in
       Hungarian.

	  There	is no portable way to define primary  sort  key	 in  terms  of
       std::locale since
	  the	conversion  from  the  collation  key  returned	 by  std::col-
       late::transform() to the
	  primary equivalence key is locale-specific, and if the user replaces
       the
	  std::collate facet, that conversion is no longer known to the	 stan-
       dard library's
	  std::regex_traits.	 Standard     library	 specializations    of
       std::regex_traits return	an
	  empty	string unless the std::collate facet of	 the  currently-imbued
       locale was not
	  replaced   by	 the  user,  and  still	 matches  the  system-supplied
       std::collate facet), in
	  which	case std::collate_byname<CharT>::transform(first, last)	is ex-
       ecuted and the
	  sort key it produces is converted to the expected primary  sort  key
       using a
	  locale-specific conversion.

Parameters
	  first,  last	- a pair of iterators which determines the sequence of
       characters to
			compare

Type requirements
	  -
	  ForwardIt must meet the requirements of LegacyForwardIterator.

Return value
	  The primary sort key for the character sequence [first, last)	in the
       currently
	  imbued locale, ignoring case,	variant, diacritics, etc.

Example
	  Demonstrates the regex feature  that	works  through	transform_pri-
       mary().

       // Run this code

	#include <iostream>
	#include <regex>

	int main()
	{
	    std::locale::global(std::locale("en_US.UTF-8"));
	    std::wstring str = L"Aa ";
	    std::wregex	re(L"[[=a=]]*",	std::regex::basic);
	    std::cout << std::boolalpha	<< std::regex_match(str, re) <<	'\n';
	}

Possible output:
	true

	   This	section	is incomplete
	   Reason: could use an	example	with user-defined regex_traits supply-
       ing user-defined
	   transform_primary

Category:
	    * Todo with	reason

http://cppreference.com		  2024.06.10	  std::regex_...orm_primary(3)

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