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What is Case-Folding?
In non-Unicode contexts, a common idiom to compare two strings
case-insensitively is lc($this) eq lc($that). Before comparing two strings
we normalize them to an all-lowercase version. "Hello", "HELLO", and
"HeLlO" all have the same lowercase form ("hello"), so it doesn't matter
which one we start with; they are all equal to one another after lc.
In Unicode, things aren't so simple. A Unicode character might have
mappings for uppercase, lowercase, and titlecase, and the lowercase mapping
of the uppercase mapping of a given character might not be the character
that you started with! For example lc(uc("\N{LATIN SMALL LETTER SHARP S"))
is "ss", not the eszett we started off with! Case-folding is a part of the
Unicode standard that allows any two strings that differ from one another
only by case to map to the same "case-folded" form, even when those strings
include characters with complex case-mappings.
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