fc EXPR
    fc      Returns the casefolded version of EXPR. This is the internal
            function implementing the "\F" escape in double-quoted strings.

            Casefolding is the process of mapping strings to a form where
            case differences are erased; comparing two strings in their
            casefolded form is effectively a way of asking if two strings
            are equal, regardless of case.

            Roughly, if you ever found yourself writing this

                lc($this) eq lc($that)    # Wrong!
                    # or
                uc($this) eq uc($that)    # Also wrong!
                    # or
                $this =~ /^\Q$that\E\z/i  # Right!

            Now you can write

                fc($this) eq fc($that)

            And get the correct results.

            Perl only implements the full form of casefolding, but you can
            access the simple folds using "casefold()" in Unicode::UCD and
            "prop_invmap()" in Unicode::UCD. For further information on
            casefolding, refer to the Unicode Standard, specifically
            sections 3.13 "Default Case Operations", 4.2 "Case-Normative",
            and 5.18 "Case Mappings", available at
            <http://www.unicode.org/versions/latest/>, as well as the Case
            Charts available at <http://www.unicode.org/charts/case/>.

            If EXPR is omitted, uses $_.

            This function behaves the same way under various pragma, such as
            within "use feature 'unicode_strings", as "lc" does, with the
            single exception of "fc" of LATIN CAPITAL LETTER SHARP S
            (U+1E9E) within the scope of "use locale". The foldcase of this
            character would normally be "ss", but as explained in the "lc"
            section, case changes that cross the 255/256 boundary are
            problematic under locales, and are hence prohibited. Therefore,
            this function under locale returns instead the string
            "\x{17F}\x{17F}", which is the LATIN SMALL LETTER LONG S. Since
            that character itself folds to "s", the string of two of them
            together should be equivalent to a single U+1E9E when foldcased.

            While the Unicode Standard defines two additional forms of
            casefolding, one for Turkic languages and one that never maps
            one character into multiple characters, these are not provided
            by the Perl core; However, the CPAN module "Unicode::Casing" may
            be used to provide an implementation.

            This keyword is available only when the "fc" feature is enabled,
            or when prefixed with "CORE::"; See feature. Alternately,
            include a "use v5.16" or later to the current scope.