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Character translation is done with translit:
Expands to string, with each character that occurs in chars translated into the character from replacement with the same index.
If replacement is shorter than chars, the excess characters are deleted from the expansion. If replacement is omitted, all characters in string that are present in chars are deleted from the expansion.
As a GNU extension, both chars and replacement can contain character-ranges, e.g., `a-z' (meaning all lowercase letters) or `0-9' (meaning all digits). To include a dash `-' in chars or replacement, place it first or last.
It is not an error for the last character in the range to be `larger' than the first. In that case, the range runs backwards, i.e., `9-0' means the string `9876543210'.
The macro
translitis recognized only with parameters.
translit(`GNUs not Unix', `A-Z')
=>s not nix
translit(`GNUs not Unix', `a-z', `A-Z')
=>GNUS NOT UNIX
translit(`GNUs not Unix', `A-Z', `z-a')
=>tmfs not fnix
The first example deletes all uppercase letters, the second converts lowercase to uppercase, and the third `mirrors' all uppercase letters, while converting them to lowercase. The two first cases are by far the most common.
Omitting chars evokes a warning, but still produces output.
translit(`abc')
error-->m4:stdin:1: Warning: too few arguments to builtin `translit'
=>abc