cpp/regex/regex traits/translate nocase

Obtains the comparison key for the character, such that all characters that are equivalent to this character in the imbued locale, ignoring the case differences, if any, produce the same key.

When the regex library needs to match two characters and  and the flag  std is, it executes.

Standard library specializations of std return, that is, convert to lowercase, using the currently imbued locale.

Return value
The case-insensitive comparison key for in the currently imbued locale.