Programming Perl, 4th Edition

Ideographic Unary Operators

Most unary operators just have names (see Named Unary and File Test Operators later in this chapter), but some operators are deemed important enough to merit their own special symbolic representation. All of these operators seem to have something to do with negation. Blame the mathematicians.

Unary ! performs logical negation; that is, “not”. See not for a lower precedence version of logical negation. The value of a negated operand is true (1) if the operand is false (numeric 0, string "0", the null string, or undefined), and false ("") if the operand is true.

Unary – performs arithmetic negation if the operand is numeric. If the operand is an identifier, a string consisting of a minus sign concatenated with the identifier is returned. Otherwise, if the string starts with a plus or minus, a string starting with the opposite sign is returned. One effect of these rules is that –bareword is equivalent to "–bareword".^[61] If, however, the string begins with a nonalphabetic character (excluding “+” or “-”), Perl will attempt to convert the string to a numeric and the arithmetic negation is performed. If the string cannot be cleanly converted to a numeric, Perl will give the warning “Argument "the string" isn't numeric in negation (–)”.

Unary ~ performs bitwise negation; that is, 1’s complement. For example, 0666 & ~027 is 0640. By definition, this is somewhat nonportable when limited by the word size of your machine. For example, on a 32-bit machine, ~123 is ...