Symbolic Unary Operators

Symbolic Unary Operators

Unary "!" performs logical negation, that is, "not". See also not for a lower precedence version of this.

Unary "-" performs arithmetic negation if the operand is numeric, including any string that looks like a number. 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 the string "-bareword" . If, however, the string begins with a non-alphabetic 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 (-) at ....

Unary "~" performs bitwise negation, that is, 1's complement. For example, 0666 & ~027 is 0640. (See also Integer Arithmetic and Bitwise String Operators.) Note that the width of the result is platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64 bits wide on a 64-bit platform, so if you are expecting a certain bit width, remember to use the "&" operator to mask off the excess bits.

When complementing strings, if all characters have ordinal values under 256, then their complements will, also. But if they do not, all characters will be in either 32- or 64-bit complements, depending on your architecture. So for example, ~"\x{3B1}" is "\x{FFFF_FC4E}" on 32-bit machines and "\x{FFFF_FFFF_FFFF_FC4E}" on 64-bit machines.

If the experimental "bitwise" feature is enabled via use feature 'bitwise' , then unary "~" always treats its argument as a number, and an alternate form of the operator, "~." , always treats its argument as a string. So ~0 and ~"0" will both give 2**32-1 on 32-bit platforms, whereas ~.0 and ~."0" will both yield "\xff" . This feature produces a warning unless you use no warnings 'experimental::bitwise' .

Unary "+" has no effect whatsoever, even on strings. It is useful syntactically for separating a function name from a parenthesized expression that would otherwise be interpreted as the complete list of function arguments. (See examples above under Terms and List Operators (Leftward).)

Unary "\" creates a reference to whatever follows it. See perlreftut and perlref. Do not confuse this behavior with the behavior of backslash within a string, although both forms do convey the notion of protecting the next thing from interpolation.