perlop.1

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.Vb 2
\&    $a += 2;
\&    $a *= 3;
.Ve
.PP
Similarly, a list assignment in list context produces the list of
lvalues assigned to, and a list assignment in scalar context returns
the number of elements produced by the expression on the right hand
side of the assignment.
.Sh "Comma Operator"
.IX Xref "comma operator, comma ,"
.IX Subsection "Comma Operator"
Binary \*(L",\*(R" is the comma operator.  In scalar context it evaluates
its left argument, throws that value away, then evaluates its right
argument and returns that value.  This is just like C's comma operator.
.PP
In list context, it's just the list argument separator, and inserts
both its arguments into the list.  These arguments are also evaluated
from left to right.
.PP
The \f(CW\*(C`=>\*(C'\fR operator is a synonym for the comma, but forces any word
(consisting entirely of word characters) to its left to be interpreted
as a string (as of 5.001).  This includes words that might otherwise be
considered a constant or function call.
.PP
.Vb 1
\&    use constant FOO => "something";
\&
\&    my %h = ( FOO => 23 );
.Ve
.PP
is equivalent to:
.PP
.Vb 1
\&    my %h = ("FOO", 23);
.Ve
.PP
It is \fI\s-1NOT\s0\fR:
.PP
.Vb 1
\&    my %h = ("something", 23);
.Ve
.PP
If the argument on the left is not a word, it is first interpreted as
an expression, and then the string value of that is used.
.PP
The \f(CW\*(C`=>\*(C'\fR operator is helpful in documenting the correspondence
between keys and values in hashes, and other paired elements in lists.
.PP
.Vb 2
\&        %hash = ( $key => $value );
\&        login( $username => $password );
.Ve
.Sh "List Operators (Rightward)"
.IX Xref "operator, list, rightward list operator"
.IX Subsection "List Operators (Rightward)"
On the right side of a list operator, it has very low precedence,
such that it controls all comma-separated expressions found there.
The only operators with lower precedence are the logical operators
\&\*(L"and\*(R", \*(L"or\*(R", and \*(L"not\*(R", which may be used to evaluate calls to list
operators without the need for extra parentheses:
.PP
.Vb 2
\&    open HANDLE, "filename"
\&        or die "Can\*(Aqt open: $!\en";
.Ve
.PP
See also discussion of list operators in \*(L"Terms and List Operators (Leftward)\*(R".
.Sh "Logical Not"
.IX Xref "operator, logical, not not"
.IX Subsection "Logical Not"
Unary \*(L"not\*(R" returns the logical negation of the expression to its right.
It's the equivalent of \*(L"!\*(R" except for the very low precedence.
.Sh "Logical And"
.IX Xref "operator, logical, and and"
.IX Subsection "Logical And"
Binary \*(L"and\*(R" returns the logical conjunction of the two surrounding
expressions.  It's equivalent to && except for the very low
precedence.  This means that it short-circuits: i.e., the right
expression is evaluated only if the left expression is true.
.Sh "Logical or, Defined or, and Exclusive Or"
.IX Xref "operator, logical, or operator, logical, xor operator, logical, defined or operator, logical, exclusive or or xor"
.IX Subsection "Logical or, Defined or, and Exclusive Or"
Binary \*(L"or\*(R" returns the logical disjunction of the two surrounding
expressions.  It's equivalent to || except for the very low precedence.
This makes it useful for control flow
.PP
.Vb 1
\&    print FH $data              or die "Can\*(Aqt write to FH: $!";
.Ve
.PP
This means that it short-circuits: i.e., the right expression is evaluated
only if the left expression is false.  Due to its precedence, you should
probably avoid using this for assignment, only for control flow.
.PP
.Vb 3
\&    $a = $b or $c;              # bug: this is wrong
\&    ($a = $b) or $c;            # really means this
\&    $a = $b || $c;              # better written this way
.Ve
.PP
However, when it's a list-context assignment and you're trying to use
\&\*(L"||\*(R" for control flow, you probably need \*(L"or\*(R" so that the assignment
takes higher precedence.
.PP
.Vb 2
\&    @info = stat($file) || die;     # oops, scalar sense of stat!
\&    @info = stat($file) or die;     # better, now @info gets its due
.Ve
.PP
Then again, you could always use parentheses.
.PP
Binary \*(L"xor\*(R" returns the exclusive-OR of the two surrounding expressions.
It cannot short circuit, of course.
.Sh "C Operators Missing From Perl"
.IX Xref "operator, missing from perl & * typecasting (TYPE)"
.IX Subsection "C Operators Missing From Perl"
Here is what C has that Perl doesn't:
.IP "unary &" 8
.IX Item "unary &"
Address-of operator.  (But see the \*(L"\e\*(R" operator for taking a reference.)
.IP "unary *" 8
.IX Item "unary *"
Dereference-address operator. (Perl's prefix dereferencing
operators are typed: $, @, %, and &.)
.IP "(\s-1TYPE\s0)" 8
.IX Item "(TYPE)"
Type-casting operator.
.Sh "Quote and Quote-like Operators"
.IX Xref "operator, quote operator, quote-like q qq qx qw m qr s tr ' '' "" """" ` `` << escape sequence escape"
.IX Subsection "Quote and Quote-like Operators"
While we usually think of quotes as literal values, in Perl they
function as operators, providing various kinds of interpolating and
pattern matching capabilities.  Perl provides customary quote characters
for these behaviors, but also provides a way for you to choose your
quote character for any of them.  In the following table, a \f(CW\*(C`{}\*(C'\fR represents
any pair of delimiters you choose.
.PP
.Vb 10
\&    Customary  Generic        Meaning        Interpolates
\&        \*(Aq\*(Aq       q{}          Literal             no
\&        ""      qq{}          Literal             yes
\&        \`\`      qx{}          Command             yes*
\&                qw{}         Word list            no
\&        //       m{}       Pattern match          yes*
\&                qr{}          Pattern             yes*
\&                 s{}{}      Substitution          yes*
\&                tr{}{}    Transliteration         no (but see below)
\&        <<EOF                 here\-doc            yes*
\&
\&        * unless the delimiter is \*(Aq\*(Aq.
.Ve
.PP
Non-bracketing delimiters use the same character fore and aft, but the four
sorts of brackets (round, angle, square, curly) will all nest, which means
that
.PP
.Vb 1
\&        q{foo{bar}baz}
.Ve
.PP
is the same as
.PP
.Vb 1
\&        \*(Aqfoo{bar}baz\*(Aq
.Ve
.PP
Note, however, that this does not always work for quoting Perl code:
.PP
.Vb 1
\&        $s = q{ if($a eq "}") ... }; # WRONG
.Ve
.PP
is a syntax error. The \f(CW\*(C`Text::Balanced\*(C'\fR module (from \s-1CPAN\s0, and
starting from Perl 5.8 part of the standard distribution) is able
to do this properly.
.PP
There can be whitespace between the operator and the quoting
characters, except when \f(CW\*(C`#\*(C'\fR is being used as the quoting character.
\&\f(CW\*(C`q#foo#\*(C'\fR is parsed as the string \f(CW\*(C`foo\*(C'\fR, while \f(CW\*(C`q #foo#\*(C'\fR is the
operator \f(CW\*(C`q\*(C'\fR followed by a comment.  Its argument will be taken
from the next line.  This allows you to write:
.PP
.Vb 2
\&    s {foo}  # Replace foo
\&      {bar}  # with bar.
.Ve
.PP
The following escape sequences are available in constructs that interpolate
and in transliterations.
.IX Xref "\et \en \er \ef \eb \ea \ee \ex \e0 \ec \eN"
.PP
.Vb 12
\&    \et          tab             (HT, TAB)
\&    \en          newline         (NL)
\&    \er          return          (CR)
\&    \ef          form feed       (FF)
\&    \eb          backspace       (BS)
\&    \ea          alarm (bell)    (BEL)
\&    \ee          escape          (ESC)
\&    \e033        octal char      (example: ESC)
\&    \ex1b        hex char        (example: ESC)
\&    \ex{263a}    wide hex char   (example: SMILEY)
\&    \ec[         control char    (example: ESC)
\&    \eN{name}    named Unicode character
.Ve
.PP
The character following \f(CW\*(C`\ec\*(C'\fR is mapped to some other character by
converting letters to upper case and then (on \s-1ASCII\s0 systems) by inverting
the 7th bit (0x40). The most interesting range is from '@' to '_'
(0x40 through 0x5F), resulting in a control character from 0x00
through 0x1F. A '?' maps to the \s-1DEL\s0 character. On \s-1EBCDIC\s0 systems only
\&'@', the letters, '[', '\e', ']', '^', '_' and '?' will work, resulting
in 0x00 through 0x1F and 0x7F.
.PP
\&\fB\s-1NOTE\s0\fR: Unlike C and other languages, Perl has no \ev escape sequence for
the vertical tab (\s-1VT\s0 \- \s-1ASCII\s0 11), but you may use \f(CW\*(C`\eck\*(C'\fR or \f(CW\*(C`\ex0b\*(C'\fR.
.PP
The following escape sequences are available in constructs that interpolate
but not in transliterations.
.IX Xref "\el \eu \eL \eU \eE \eQ"
.PP
.Vb 6
\&    \el          lowercase next char
\&    \eu          uppercase next char
\&    \eL          lowercase till \eE
\&    \eU          uppercase till \eE
\&    \eE          end case modification
\&    \eQ          quote non\-word characters till \eE
.Ve
.PP
If \f(CW\*(C`use locale\*(C'\fR is in effect, the case map used by \f(CW\*(C`\el\*(C'\fR, \f(CW\*(C`\eL\*(C'\fR,
\&\f(CW\*(C`\eu\*(C'\fR and \f(CW\*(C`\eU\*(C'\fR is taken from the current locale.  See perllocale.
If Unicode (for example, \f(CW\*(C`\eN{}\*(C'\fR or wide hex characters of 0x100 or
beyond) is being used, the case map used by \f(CW\*(C`\el\*(C'\fR, \f(CW\*(C`\eL\*(C'\fR, \f(CW\*(C`\eu\*(C'\fR and
\&\f(CW\*(C`\eU\*(C'\fR is as defined by Unicode.  For documentation of \f(CW\*(C`\eN{name}\*(C'\fR,
see charnames.
.PP
All systems use the virtual \f(CW"\en"\fR to represent a line terminator,
called a \*(L"newline\*(R".  There is no such thing as an unvarying, physical
newline character.  It is only an illusion that the operating system,
device drivers, C libraries, and Perl all conspire to preserve.  Not all
systems read \f(CW"\er"\fR as \s-1ASCII\s0 \s-1CR\s0 and \f(CW"\en"\fR as \s-1ASCII\s0 \s-1LF\s0.  For example,
on a Mac, these are reversed, and on systems without line terminator,
printing \f(CW"\en"\fR may emit no actual data.  In general, use \f(CW"\en"\fR when
you mean a \*(L"newline\*(R" for your system, but use the literal \s-1ASCII\s0 when you
need an exact character.  For example, most networking protocols expect
and prefer a \s-1CR+LF\s0 (\f(CW"\e015\e012"\fR or \f(CW"\ecM\ecJ"\fR) for line terminators,
and although they often accept just \f(CW"\e012"\fR, they seldom tolerate just
\&\f(CW"\e015"\fR.  If you get in the habit of using \f(CW"\en"\fR for networking,
you may be burned some day.
.IX Xref "newline line terminator eol end of line \en \er \er\en"
.PP
For constructs that do interpolate, variables beginning with "\f(CW\*(C`$\*(C'\fR\*(L"
or \*(R"\f(CW\*(C`@\*(C'\fR" are interpolated.  Subscripted variables such as \f(CW$a[3]\fR or
\&\f(CW\*(C`$href\->{key}[0]\*(C'\fR are also interpolated, as are array and hash slices.
But method calls such as \f(CW\*(C`$obj\->meth\*(C'\fR are not.
.PP
Interpolating an array or slice interpolates the elements in order,
separated by the value of \f(CW$"\fR, so is equivalent to interpolating
\&\f(CW\*(C`join $", @array\*(C'\fR.    \*(L"Punctuation\*(R" arrays such as \f(CW\*(C`@*\*(C'\fR are only
interpolated if the name is enclosed in braces \f(CW\*(C`@{*}\*(C'\fR, but special
arrays \f(CW@_\fR, \f(CW\*(C`@+\*(C'\fR, and \f(CW\*(C`@\-\*(C'\fR are interpolated, even without braces.
.PP
You cannot include a literal \f(CW\*(C`$\*(C'\fR or \f(CW\*(C`@\*(C'\fR within a \f(CW\*(C`\eQ\*(C'\fR sequence.
An unescaped \f(CW\*(C`$\*(C'\fR or \f(CW\*(C`@\*(C'\fR interpolates the corresponding variable,
while escaping will cause the literal string \f(CW\*(C`\e$\*(C'\fR to be inserted.
You'll need to write something like \f(CW\*(C`m/\eQuser\eE\e@\eQhost/\*(C'\fR.
.PP
Patterns are subject to an additional level of interpretation as a
regular expression.  This is done as a second pass, after variables are
interpolated, so that regular expressions may be incorporated into the
pattern from the variables.  If this is not what you want, use \f(CW\*(C`\eQ\*(C'\fR to
interpolate a variable literally.
.PP
Apart from the behavior described above, Perl does not expand
multiple levels of interpolation.  In particular, contrary to the
expectations of shell programmers, back-quotes do \fI\s-1NOT\s0\fR interpolate
within double quotes, nor do single quotes impede evaluation of
variables when used within double quotes.
.Sh "Regexp Quote-Like Operators"
.IX Xref "operator, regexp"
.IX Subsection "Regexp Quote-Like Operators"
Here are the quote-like operators that apply to pattern
matching and related activities.
.IP "qr/STRING/msixpo" 8
.IX Xref "qr  i  m  o  s  x  p"
.IX Item "qr/STRING/msixpo"
This operator quotes (and possibly compiles) its \fI\s-1STRING\s0\fR as a regular
expression.  \fI\s-1STRING\s0\fR is interpolated the same way as \fI\s-1PATTERN\s0\fR
in \f(CW\*(C`m/PATTERN/\*(C'\fR.  If \*(L"'\*(R" is used as the delimiter, no interpolation
is done.  Returns a Perl value which may be used instead of the
corresponding \f(CW\*(C`/STRING/msixpo\*(C'\fR expression. The returned value is a
normalized version of the original pattern. It magically differs from
a string containing the same characters: \f(CW\*(C`ref(qr/x/)\*(C'\fR returns \*(L"Regexp\*(R",
even though dereferencing the result returns undef.
.Sp
For example,
.Sp
.Vb 3
\&    $rex = qr/my.STRING/is;
\&    print $rex;                 # prints (?si\-xm:my.STRING)
\&    s/$rex/foo/;
.Ve
.Sp
is equivalent to
.Sp
.Vb 1
\&    s/my.STRING/foo/is;
.Ve
.Sp
The result may be used as a subpattern in a match:
.Sp
.Vb 4
\&    $re = qr/$pattern/;
\&    $string =~ /foo${re}bar/;   # can be interpolated in other patterns
\&    $string =~ $re;             # or used standalone
\&    $string =~ /$re/;           # or this way
.Ve
.Sp