For the standard regex_traits, this function simply returns c unchanged.
This function is called when a regular expression needs to match a character and the regex object has collate as a syntax option. A custom traits class can define a different internal translation: Two characters will be considered equivalent if they translate to the same value using this function.
Parameters
c
A character. char_type is a member type, alias of the character type. In regex_traits it is an alias of its template parameter (charT).
Return value
The character translation of c. char_type is a member type, alias of the character type. In regex_traits it is an alias of its template parameter (charT).