• NAME
• SYNOPSIS
• DESCRIPTION
• Terms and List Operators (Leftward)
• The Arrow Operator
• Auto-increment and Auto-decrement
• Exponentiation
• Symbolic Unary Operators
• Binding Operators
• Multiplicative Operators
• Additive Operators
• Shift Operators
• Named Unary Operators
• Relational Operators
• Equality Operators
• Bitwise And
• Bitwise Or and Exclusive Or
• C-style Logical And
• C-style Logical Or
• Range Operators
• Conditional Operator
• Assignment Operators
• Comma Operator
• List Operators (Rightward)
• Logical Not
• Logical And
• Logical or and Exclusive Or
• C Operators Missing From Perl
• Quote and Quote-like Operators
• Regexp Quote-Like Operators
• Gory details of parsing quoted constructs
• I/O Operators
• Constant Folding
• Bitwise String Operators
• Integer Arithmetic
• Floating-point Arithmetic
• Bigger Numbers

NAME

perlop - Perl operators and precedence

SYNOPSIS

Perl operators have the following associativity and precedence, listed from highest precedence to lowest. Operators borrowed from C keep the same precedence relationship with each other, even where C's precedence is slightly screwy. (This makes learning Perl easier for C folks.) With very few exceptions, these all operate on scalar values only, not array values.

    left        terms and list operators (leftward)
    left        ->
    nonassoc    ++ --
    right       **
    right       ! ~ \ and unary + and -
    left        =~ !~
    left        * / % x
    left        + - .
    left        << >>
    nonassoc    named unary operators
    nonassoc    < > <= >= lt gt le ge
    nonassoc    == != <=> eq ne cmp
    left        &
    left        | ^
    left        &&
    left        ||
    nonassoc    ..  ...
    right       ?:
    right       = += -= *= etc.
    left        , =>
    nonassoc    list operators (rightward)
    right       not
    left        and
    left        or xor

In the following sections, these operators are covered in precedence order.

Many operators can be overloaded for objects. See the overload manpage.

DESCRIPTION

Terms and List Operators (Leftward)

A TERM has the highest precedence in Perl. They include variables, quote and quote-like operators, any expression in parentheses, and any function whose arguments are parenthesized. Actually, there aren't really functions in this sense, just list operators and unary operators behaving as functions because you put parentheses around the arguments. These are all documented in the perlfunc manpage.

If any list operator (print(), etc.) or any unary operator (chdir(), etc.) is followed by a left parenthesis as the next token, the operator and arguments within parentheses are taken to be of highest precedence, just like a normal function call.

In the absence of parentheses, the precedence of list operators such as print, sort, or chmod is either very high or very low depending on whether you are looking at the left side or the right side of the operator. For example, in

    @ary = (1, 3, sort 4, 2);
    print @ary;         # prints 1324

the commas on the right of the sort are evaluated before the sort, but the commas on the left are evaluated after. In other words, list operators tend to gobble up all arguments that follow, and then act like a simple TERM with regard to the preceding expression. Be careful with parentheses:

    # These evaluate exit before doing the print:
    print($foo, exit);  # Obviously not what you want.
    print $foo, exit;   # Nor is this.

    # These do the print before evaluating exit:
    (print $foo), exit; # This is what you want.
    print($foo), exit;  # Or this.
    print ($foo), exit; # Or even this.

Also note that

    print ($foo & 255) + 1, "\n";

probably doesn't do what you expect at first glance. See Named Unary Operators for more discussion of this.

Also parsed as terms are the do {} and eval {} constructs, as well as subroutine and method calls, and the anonymous constructors [] and {}.

See also Quote and Quote-like Operators toward the end of this section, as well as I/O Operators.

The Arrow Operator

``->'' is an infix dereference operator, just as it is in C and C++. If the right side is either a [...], {...}, or a (...) subscript, then the left side must be either a hard or symbolic reference to an array, a hash, or a subroutine respectively. (Or technically speaking, a location capable of holding a hard reference, if it's an array or hash reference being used for assignment.) See the perlreftut manpage and the perlref manpage.

Otherwise, the right side is a method name or a simple scalar variable containing either the method name or a subroutine reference, and the left side must be either an object (a blessed reference) or a class name (that is, a package name). See the perlobj manpage.

Auto-increment and Auto-decrement

``++'' and ``--'' work as in C. That is, if placed before a variable, they increment or decrement the variable before returning the value, and if placed after, increment or decrement the variable after returning the value.

The auto-increment operator has a little extra builtin magic to it. If you increment a variable that is numeric, or that has ever been used in a numeric context, you get a normal increment. If, however, the variable has been used in only string contexts since it was set, and has a value that is not the empty string and matches the pattern /^[a-zA-Z]*[0-9]*$/, the increment is done as a string, preserving each character within its range, with carry:

    print ++($foo = '99');      # prints '100'
    print ++($foo = 'a0');      # prints 'a1'
    print ++($foo = 'Az');      # prints 'Ba'
    print ++($foo = 'zz');      # prints 'aaa'

The auto-decrement operator is not magical.

Exponentiation

Binary ``**'' is the exponentiation operator. It binds even more tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is implemented using C's pow(3) function, which actually works on doubles internally.)

Symbolic Unary Operators

Unary ``!'' performs logical negation, i.e., ``not''. See also not for a lower precedence version of this.

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".

Unary ``~'' performs bitwise negation, i.e., 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 use the & operator to mask off the excess bits.

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 the perlreftut manpage and the perlref manpage. 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.

Binding Operators

Binary ``=~'' binds a scalar expression to a pattern match. Certain operations search or modify the string $_ by default. This operator makes that kind of operation work on some other string. The right argument is a search pattern, substitution, or transliteration. The left argument is what is supposed to be searched, substituted, or transliterated instead of the default $_. When used in scalar context, the return value generally indicates the success of the operation. Behavior in list context depends on the particular operator. See Regexp Quote-Like Operators for details.

If the right argument is an expression rather than a search pattern, substitution, or transliteration, it is interpreted as a search pattern at run time. This can be less efficient than an explicit search, because the pattern must be compiled every time the expression is evaluated.

Binary ``!~'' is just like ``=~'' except the return value is negated in the logical sense.

Multiplicative Operators

Binary ``*'' multiplies two numbers.

Binary ``/'' divides two numbers.

Binary ``%'' computes the modulus of two numbers. Given integer operands $a and $b: If $b is positive, then $a % $b is $a minus the largest multiple of $b that is not greater than $a. If $b is negative, then $a % $b is $a minus the smallest multiple of $b that is not less than $a (i.e. the result will be less than or equal to zero). Note than when use integer is in scope, ``%'' give you direct access to the modulus operator as implemented by your C compiler. This operator is not as well defined for negative operands, but it will execute faster.

Binary ``x'' is the repetition operator. In scalar context or if the left operand is not enclosed in parentheses, it returns a string consisting of the left operand repeated the number of times specified by the right operand. In list context, if the left operand is enclosed in parentheses, it repeats the list.

    print '-' x 80;             # print row of dashes

    print "\t" x ($tab/8), ' ' x ($tab%8);      # tab over

    @ones = (1) x 80;           # a list of 80 1's
    @ones = (5) x @ones;        # set all elements to 5

Additive Operators

Binary ``+'' returns the sum of two numbers.

Binary ``-'' returns the difference of two numbers.

Binary ``.'' concatenates two strings.

Shift Operators

Binary ``<<'' returns the value of its left argument shifted left by the number of bits specified by the right argument. Arguments should be integers. (See also Integer Arithmetic.)

Binary ``>>'' returns the value of its left argument shifted right by the number of bits specified by the right argument. Arguments should be integers. (See also Integer Arithmetic.)

Named Unary Operators

The various named unary operators are treated as functions with one argument, with optional parentheses. These include the filetest operators, like -f, -M, etc. See the perlfunc manpage.

If any list operator (print(), etc.) or any unary operator (chdir(), etc.) is followed by a left parenthesis as the next token, the operator and arguments within parentheses are taken to be of highest precedence, just like a normal function call. Examples:

    chdir $foo    || die;       # (chdir $foo) || die
    chdir($foo)   || die;       # (chdir $foo) || die
    chdir ($foo)  || die;       # (chdir $foo) || die
    chdir +($foo) || die;       # (chdir $foo) || die

but, because * is higher precedence than ||:

    chdir $foo * 20;    # chdir ($foo * 20)
    chdir($foo) * 20;   # (chdir $foo) * 20
    chdir ($foo) * 20;  # (chdir $foo) * 20
    chdir +($foo) * 20; # chdir ($foo * 20)

    rand 10 * 20;       # rand (10 * 20)
    rand(10) * 20;      # (rand 10) * 20
    rand (10) * 20;     # (rand 10) * 20
    rand +(10) * 20;    # rand (10 * 20)

See also Terms and List Operators (Leftward).

Relational Operators

Binary ``<'' returns true if the left argument is numerically less than the right argument.

Binary ``>'' returns true if the left argument is numerically greater than the right argument.

Binary ``<='' returns true if the left argument is numerically less than or equal to the right argument.

Binary ``>='' returns true if the left argument is numerically greater than or equal to the right argument.

Binary ``lt'' returns true if the left argument is stringwise less than the right argument.

Binary ``gt'' returns true if the left argument is stringwise greater than the right argument.

Binary ``le'' returns true if the left argument is stringwise less than or equal to the right argument.

Binary ``ge'' returns true if the left argument is stringwise greater than or equal to the right argument.

Equality Operators

Binary ``=='' returns true if the left argument is numerically equal to the right argument.

Binary ``!='' returns true if the left argument is numerically not equal to the right argument.

Binary ``<=>'' returns -1, 0, or 1 depending on whether the left argument is numerically less than, equal to, or greater than the right argument.

Binary ``eq'' returns true if the left argument is stringwise equal to the right argument.

Binary ``ne'' returns true if the left argument is stringwise not equal to the right argument.

Binary ``cmp'' returns -1, 0, or 1 depending on whether the left argument is stringwise less than, equal to, or greater than the right argument.

``lt'', ``le'', ``ge'', ``gt'' and ``cmp'' use the collation (sort) order specified by the current locale if use locale is in effect. See the perllocale manpage.

Bitwise And

Binary ``&'' returns its operators ANDed together bit by bit. (See also Integer Arithmetic and Bitwise String Operators.)

Bitwise Or and Exclusive Or

Binary ``|'' returns its operators ORed together bit by bit. (See also Integer Arithmetic and Bitwise String Operators.)

Binary ``^'' returns its operators XORed together bit by bit. (See also Integer Arithmetic and Bitwise String Operators.)

C-style Logical And

Binary ``&&'' performs a short-circuit logical AND operation. That is, if the left operand is false, the right operand is not even evaluated. Scalar or list context propagates down to the right operand if it is evaluated.

C-style Logical Or

Binary ``||'' performs a short-circuit logical OR operation. That is, if the left operand is true, the right operand is not even evaluated. Scalar or list context propagates down to the right operand if it is evaluated.

The || and && operators differ from C's in that, rather than returning 0 or 1, they return the last value evaluated. Thus, a reasonably portable way to find out the home directory (assuming it's not ``0'') might be:

    $home = $ENV{'HOME'} || $ENV{'LOGDIR'} ||
        (getpwuid($<))[7] || die "You're homeless!\n";

In particular, this means that you shouldn't use this for selecting between two aggregates for assignment:

    @a = @b || @c;              # this is wrong
    @a = scalar(@b) || @c;      # really meant this
    @a = @b ? @b : @c;          # this works fine, though

As more readable alternatives to && and || when used for control flow, Perl provides and and or operators (see below). The short-circuit behavior is identical. The precedence of ``and'' and ``or'' is much lower, however, so that you can safely use them after a list operator without the need for parentheses:

    unlink "alpha", "beta", "gamma"
            or gripe(), next LINE;

With the C-style operators that would have been written like this:

    unlink("alpha", "beta", "gamma")
            || (gripe(), next LINE);

Using ``or'' for assignment is unlikely to do what you want; see below.

Range Operators

Binary ``..'' is the range operator, which is really two different operators depending on the context. In list context, it returns an array of values counting (up by ones) from the left value to the right value. If the left value is greater than the right value then it returns the empty array. The range operator is useful for writing foreach (1..10) loops and for doing slice operations on arrays. In the current implementation, no temporary array is created when the range operator is used as the expression in foreach loops, but older versions of Perl might burn a lot of memory when you write something like this:

    for (1 .. 1_000_000) {
        # code
    }

In scalar context, ``..'' returns a boolean value. The operator is bistable, like a flip-flop, and emulates the line-range (comma) operator of sed, awk, and various editors. Each ``..'' operator maintains its own boolean state. It is false as long as its left operand is false. Once the left operand is true, the range operator stays true until the right operand is true, AFTER which the range operator becomes false again. It doesn't become false till the next time the range operator is evaluated. It can test the right operand and become false on the same evaluation it became true (as in awk), but it still returns true once. If you don't want it to test the right operand till the next evaluation, as in sed, just use three dots (``...'') instead of two. In all other regards, ``...'' behaves just like ``..'' does.

The right operand is not evaluated while the operator is in the ``false'' state, and the left operand is not evaluated while the operator is in the ``true'' state. The precedence is a little lower than || and &&. The value returned is either the empty string for false, or a sequence number (beginning with 1) for true. The sequence number is reset for each range encountered. The final sequence number in a range has the string ``E0'' appended to it, which doesn't affect its numeric value, but gives you something to search for if you want to exclude the endpoint. You can exclude the beginning point by waiting for the sequence number to be greater than 1. If either operand of scalar ``..'' is a constant expression, that operand is implicitly compared to the $. variable, the current line number. Examples:

As a scalar operator:

    if (101 .. 200) { print; }  # print 2nd hundred lines
    next line if (1 .. /^$/);   # skip header lines
    s/^/> / if (/^$/ .. eof()); # quote body

    # parse mail messages
    while (<>) {
        $in_header =   1  .. /^$/;
        $in_body   = /^$/ .. eof();
        # do something based on those
    } continue {
        close ARGV if eof;              # reset $. each file
    }

As a list operator:

    for (101 .. 200) { print; } # print $_ 100 times
    @foo = @foo[0 .. $#foo];    # an expensive no-op
    @foo = @foo[$#foo-4 .. $#foo];      # slice last 5 items

The range operator (in list context) makes use of the magical auto-increment algorithm if the operands are strings. You can say

    @alphabet = ('A' .. 'Z');

to get all normal letters of the alphabet, or

    $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];

to get a hexadecimal digit, or

    @z2 = ('01' .. '31');  print $z2[$mday];

to get dates with leading zeros. If the final value specified is not in the sequence that the magical increment would produce, the sequence goes until the next value would be longer than the final value specified.

Conditional Operator

Ternary ``?:'' is the conditional operator, just as in C. It works much like an if-then-else. If the argument before the ? is true, the argument before the : is returned, otherwise the argument after the : is returned. For example:

    printf "I have %d dog%s.\n", $n,
            ($n == 1) ? '' : "s";

Scalar or list context propagates downward into the 2nd or 3rd argument, whichever is selected.

    $a = $ok ? $b : $c;  # get a scalar
    @a = $ok ? @b : @c;  # get an array
    $a = $ok ? @b : @c;  # oops, that's just a count!

The operator may be assigned to if both the 2nd and 3rd arguments are legal lvalues (meaning that you can assign to them):

    ($a_or_b ? $a : $b) = $c;

Because this operator produces an assignable result, using assignments without parentheses will get you in trouble. For example, this:

    $a % 2 ? $a += 10 : $a += 2

Really means this:

    (($a % 2) ? ($a += 10) : $a) += 2

Rather than this:

    ($a % 2) ? ($a += 10) : ($a += 2)

That should probably be written more simply as:

    $a += ($a % 2) ? 10 : 2;

Assignment Operators

``='' is the ordinary assignment operator.

Assignment operators work as in C. That is,

    $a += 2;

is equivalent to

    $a = $a + 2;

although without duplicating any side effects that dereferencing the lvalue might trigger, such as from tie(). Other assignment operators work similarly. The following are recognized:

    **=    +=    *=    &=    <<=    &&=
           -=    /=    |=    >>=    ||=
           .=    %=    ^=
                 x=

Although these are grouped by family, they all have the precedence of assignment.

Unlike in C, the scalar assignment operator produces a valid lvalue. Modifying an assignment is equivalent to doing the assignment and then modifying the variable that was assigned to. This is useful for modifying a copy of something, like this:

    ($tmp = $global) =~ tr [A-Z] [a-z];

Likewise,

    ($a += 2) *= 3;

is equivalent to

    $a += 2;
    $a *= 3;

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.

Comma Operator

Binary ``,'' 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.

In list context, it's just the list argument separator, and inserts both its arguments into the list.

The => digraph is mostly just a synonym for the comma operator. It's useful for documenting arguments that come in pairs. As of release 5.001, it also forces any word to the left of it to be interpreted as a string.

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 ``and'', ``or'', and ``not'', which may be used to evaluate calls to list operators without the need for extra parentheses:

    open HANDLE, "filename"
        or die "Can't open: $!\n";

See also discussion of list operators in Terms and List Operators (Leftward).

Logical Not

Unary ``not'' returns the logical negation of the expression to its right. It's the equivalent of ``!'' except for the very low precedence.

Logical And

Binary ``and'' 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.

Logical or and Exclusive Or

Binary ``or'' 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

    print FH $data              or die "Can't write to FH: $!";

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.

    $a = $b or $c;              # bug: this is wrong
    ($a = $b) or $c;            # really means this
    $a = $b || $c;              # better written this way

However, when it's a list-context assignment and you're trying to use ``||'' for control flow, you probably need ``or'' so that the assignment takes higher precedence.

    @info = stat($file) || die;     # oops, scalar sense of stat!
    @info = stat($file) or die;     # better, now @info gets its due

Then again, you could always use parentheses.

Binary ``xor'' returns the exclusive-OR of the two surrounding expressions. It cannot short circuit, of course.

C Operators Missing From Perl

Here is what C has that Perl doesn't:

unary &

Address-of operator. (But see the ``\'' operator for taking a reference.)

unary *

Dereference-address operator. (Perl's prefix dereferencing operators are typed: $, @, %, and &.)

(TYPE)

Type-casting operator.

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 {} represents any pair of delimiters you choose.

    Customary  Generic        Meaning        Interpolates
        ''       q{}          Literal             no
        ""      qq{}          Literal             yes
        ``      qx{}          Command             yes (unless '' is delimiter)
                qw{}         Word list            no
        //       m{}       Pattern match          yes (unless '' is delimiter)
                qr{}          Pattern             yes (unless '' is delimiter)
                 s{}{}      Substitution          yes (unless '' is delimiter)
                tr{}{}    Transliteration         no (but see below)

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

        q{foo{bar}baz}

is the same as

        'foo{bar}baz'

Note, however, that this does not always work for quoting Perl code:

        $s = q{ if($a eq "}") ... }; # WRONG

is a syntax error. The Text::Balanced module on CPAN is able to do this properly.

There can be whitespace between the operator and the quoting characters, except when # is being used as the quoting character. q#foo# is parsed as the string foo, while q #foo# is the operator q followed by a comment. Its argument will be taken from the next line. This allows you to write:

    s {foo}  # Replace foo
      {bar}  # with bar.

For constructs that do interpolate, variables beginning with ``$'' or ``@'' are interpolated, as are the following escape sequences. Within a transliteration, the first eleven of these sequences may be used.

    \t          tab             (HT, TAB)
    \n          newline         (NL)
    \r          return          (CR)
    \f          form feed       (FF)
    \b          backspace       (BS)
    \a          alarm (bell)    (BEL)
    \e          escape          (ESC)
    \033        octal char      (ESC)
    \x1b        hex char        (ESC)
    \x{263a}    wide hex char   (SMILEY)
    \c[         control char    (ESC)
    \N{name}    named char

    \l          lowercase next char
    \u          uppercase next char
    \L          lowercase till \E
    \U          uppercase till \E
    \E          end case modification
    \Q          quote non-word characters till \E

If use locale is in effect, the case map used by \l, \L, \u and \U is taken from the current locale. See the perllocale manpage. For documentation of \N{name}, see the charnames manpage.

All systems use the virtual "\n" to represent a line terminator, called a ``newline''. 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 "\r" as ASCII CR and "\n" as ASCII LF. For example, on a Mac, these are reversed, and on systems without line terminator, printing "\n" may emit no actual data. In general, use "\n" when you mean a ``newline'' for your system, but use the literal ASCII when you need an exact character. For example, most networking protocols expect and prefer a CR+LF ("\012\015" or "\cJ\cM") for line terminators, and although they often accept just "\012", they seldom tolerate just "\015". If you get in the habit of using "\n" for networking, you may be burned some day.

You cannot include a literal $ or @ within a \Q sequence. An unescaped $ or @ interpolates the corresponding variable, while escaping will cause the literal string \$ to be inserted. You'll need to write something like m/\Quser\E\@\Qhost/.

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 \Q to interpolate a variable literally.

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 NOT interpolate within double quotes, nor do single quotes impede evaluation of variables when used within double quotes.

Regexp Quote-Like Operators

Here are the quote-like operators that apply to pattern matching and related activities.

?PATTERN?

This is just like the /pattern/ search, except that it matches only once between calls to the reset() operator. This is a useful optimization when you want to see only the first occurrence of something in each file of a set of files, for instance. Only ?? patterns local to the current package are reset.

    while (<>) {
        if (?^$?) {
                            # blank line between header and body
        }
    } continue {
        reset if eof;       # clear ?? status for next file
    }

This usage is vaguely depreciated, which means it just might possibly be removed in some distant future version of Perl, perhaps somewhere around the year 2168.

m/PATTERN/cgimosx

/PATTERN/cgimosx

Searches a string for a pattern match, and in scalar context returns true if it succeeds, false if it fails. If no string is specified via the =~ or !~ operator, the $_ string is searched. (The string specified with =~ need not be an lvalue--it may be the result of an expression evaluation, but remember the =~ binds rather tightly.) See also the perlre manpage. See the perllocale manpage for discussion of additional considerations that apply when use locale is in effect.

Options are:

    c   Do not reset search position on a failed match when /g is in effect.
    g   Match globally, i.e., find all occurrences.
    i   Do case-insensitive pattern matching.
    m   Treat string as multiple lines.
    o   Compile pattern only once.
    s   Treat string as single line.
    x   Use extended regular expressions.

If ``/'' is the delimiter then the initial m is optional. With the m you can use any pair of non-alphanumeric, non-whitespace characters as delimiters. This is particularly useful for matching path names that contain ``/'', to avoid LTS (leaning toothpick syndrome). If ``?'' is the delimiter, then the match-only-once rule of ?PATTERN? applies. If ``''' is the delimiter, no interpolation is performed on the PATTERN.

PATTERN may contain variables, which will be interpolated (and the pattern recompiled) every time the pattern search is evaluated, except for when the delimiter is a single quote. (Note that $) and $| might not be interpolated because they look like end-of-string tests.) If you want such a pattern to be compiled only once, add a /o after the trailing delimiter. This avoids expensive run-time recompilations, and is useful when the value you are interpolating won't change over the life of the script. However, mentioning /o constitutes a promise that you won't change the variables in the pattern. If you change them, Perl won't even notice. See also qr//.

If the PATTERN evaluates to the empty string, the last successfully matched regular expression is used instead.

If the /g option is not used, m// in list context returns a list consisting of the subexpressions matched by the parentheses in the pattern, i.e., ($1, $2, $3...). (Note that here $1 etc. are also set, and that this differs from Perl 4's behavior.) When there are no parentheses in the pattern, the return value is the list (1) for success. With or without parentheses, an empty list is returned upon failure.

Examples:

    open(TTY, '/dev/tty');
    <TTY> =~ /^y/i && foo();    # do foo if desired

    if (/Version: *([0-9.]*)/) { $version = $1; }

    next if m#^/usr/spool/uucp#;

    # poor man's grep
    $arg = shift;
    while (<>) {
        print if /$arg/o;       # compile only once
    }

    if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))

This last example splits $foo into the first two words and the remainder of the line, and assigns those three fields to $F1, $F2, and $Etc. The conditional is true if any variables were assigned, i.e., if the pattern matched.

The /g modifier specifies global pattern matching--that is, matching as many times as possible within the string. How it behaves depends on the context. In list context, it returns a list of the substrings matched by any capturing parentheses in the regular expression. If there are no parentheses, it returns a list of all the matched strings, as if there were parentheses around the whole pattern.

In scalar context, each execution of m//g finds the next match, returning true if it matches, and false if there is no further match. The position after the last match can be read or set using the pos() function; see pos in the perlfunc manpage. A failed match normally resets the search position to the beginning of the string, but you can avoid that by adding the /c modifier (e.g. m//gc). Modifying the target string also resets the search position.

You can intermix m//g matches with m/\G.../g, where \G is a zero-width assertion that matches the exact position where the previous m//g, if any, left off. The \G assertion is not supported without the /g modifier. (Currently, without /g, \G behaves just like \A, but that's accidental and may change in the future.)

Examples:

    # list context
    ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);

    # scalar context
    $/ = ""; $* = 1;  # $* deprecated in modern perls
    while (defined($paragraph = <>)) {
        while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
            $sentences++;
        }
    }
    print "$sentences\n";

    # using m//gc with \G
    $_ = "ppooqppqq";
    while ($i++ < 2) {
        print "1: '";
        print $1 while /(o)/gc; print "', pos=", pos, "\n";
        print "2: '";
        print $1 if /\G(q)/gc;  print "', pos=", pos, "\n";
        print "3: '";
        print $1 while /(p)/gc; print "', pos=", pos, "\n";
    }

The last example should print:

    1: 'oo', pos=4
    2: 'q', pos=5
    3: 'pp', pos=7
    1: '', pos=7
    2: 'q', pos=8
    3: '', pos=8

A useful idiom for lex-like scanners is /\G.../gc. You can combine several regexps like this to process a string part-by-part, doing different actions depending on which regexp matched. Each regexp tries to match where the previous one leaves off.

 $_ = <<'EOL';
      $url = new URI::URL "http://www/";;   die if $url eq "xXx";
 EOL
 LOOP:
    {
      print(" digits"),         redo LOOP if /\G\d+\b[,.;]?\s*/gc;
      print(" lowercase"),      redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
      print(" UPPERCASE"),      redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
      print(" Capitalized"),    redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
      print(" MiXeD"),          redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
      print(" alphanumeric"),   redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
      print(" line-noise"),     redo LOOP if /\G[^A-Za-z0-9]+/gc;
      print ". That's all!\n";
    }

Here is the output (split into several lines):

 line-noise lowercase line-noise lowercase UPPERCASE line-noise
 UPPERCASE line-noise lowercase line-noise lowercase line-noise
 lowercase lowercase line-noise lowercase lowercase line-noise
 MiXeD line-noise. That's all!

q/STRING/

'STRING'

A single-quoted, literal string. A backslash represents a backslash unless followed by the delimiter or another backslash, in which case the delimiter or backslash is interpolated.

    $foo = q!I said, "You said, 'She said it.'"!;
    $bar = q('This is it.');
    $baz = '\n';                # a two-character string

qq/STRING/

``STRING''

A double-quoted, interpolated string.

    $_ .= qq
     (*** The previous line contains the naughty word "$1".\n)
                if /\b(tcl|java|python)\b/i;      # :-)
    $baz = "\n";                # a one-character string

qr/STRING/imosx

This operators quotes--and compiles--its STRING as a regular expression. STRING is interpolated the same way as PATTERN in m/PATTERN/. If ``''' is used as the delimiter, no interpolation is done. Returns a Perl value which may be used instead of the corresponding /STRING/imosx expression.

For example,

    $rex = qr/my.STRING/is;
    s/$rex/foo/;

is equivalent to

    s/my.STRING/foo/is;

The result may be used as a subpattern in a match:

    $re = qr/$pattern/;
    $string =~ /foo${re}bar/;   # can be interpolated in other patterns
    $string =~ $re;             # or used standalone
    $string =~ /$re/;           # or this way

Since Perl may compile the pattern at the moment of execution of qr() operator, using qr() may have speed advantages in some situations, notably if the result of qr() is used standalone:

    sub match {
        my $patterns = shift;
        my @compiled = map qr/$_/i, @$patterns;
        grep {
            my $success = 0;
            foreach my $pat (@compiled) {
                $success = 1, last if /$pat/;
            }
            $success;
        } @_;
    }

Precompilation of the pattern into an internal representation at the moment of qr() avoids a need to recompile the pattern every time a match /$pat/ is attempted. (Perl has many other internal optimizations, but none would be triggered in the above example if we did not use qr() operator.)

Options are:

    i   Do case-insensitive pattern matching.
    m   Treat string as multiple lines.
    o   Compile pattern only once.
    s   Treat string as single line.
    x   Use extended regular expressions.

See the perlre manpage for additional information on valid syntax for STRING, and for a detailed look at the semantics of regular expressions.

qx/STRING/

`STRING`

A string which is (possibly) interpolated and then executed as a system command with /bin/sh or its equivalent. Shell wildcards, pipes, and redirections will be honored. The collected standard output of the command is returned; standard error is unaffected. In scalar context, it comes back as a single (potentially multi-line) string. In list context, returns a list of lines (however you've defined lines with $/ or $INPUT_RECORD_SEPARATOR).

Because backticks do not affect standard error, use shell file descriptor syntax (assuming the shell supports this) if you care to address this. To capture a command's STDERR and STDOUT together:

    $output = `cmd 2>&1`;

To capture a command's STDOUT but discard its STDERR:

    $output = `cmd 2>/dev/null`;

To capture a command's STDERR but discard its STDOUT (ordering is important here):

    $output = `cmd 2>&1 1>/dev/null`;

To exchange a command's STDOUT and STDERR in order to capture the STDERR but leave its STDOUT to come out the old STDERR:

    $output = `cmd 3>&1 1>&2 2>&3 3>&-`;

To read both a command's STDOUT and its STDERR separately, it's easiest and safest to redirect them separately to files, and then read from those files when the program is done:

    system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");

Using single-quote as a delimiter protects the command from Perl's double-quote interpolation, passing it on to the shell instead:

    $perl_info  = qx(ps $$);            # that's Perl's $$
    $shell_info = qx'ps $$';            # that's the new shell's $$

How that string gets evaluated is entirely subject to the command interpreter on your system. On most platforms, you will have to protect shell metacharacters if you want them treated literally. This is in practice difficult to do, as it's unclear how to escape which characters. See the perlsec manpage for a clean and safe example of a manual fork() and exec() to emulate backticks safely.

On some platforms (notably DOS-like ones), the shell may not be capable of dealing with multiline commands, so putting newlines in the string may not get you what you want. You may be able to evaluate multiple commands in a single line by separating them with the command separator character, if your shell supports that (e.g. ; on many Unix shells; & on the Windows NT cmd shell).

Beginning with v5.6.0, Perl will attempt to flush all files opened for output before starting the child process, but this may not be supported on some platforms (see the perlport manpage). To be safe, you may need to set $| ($AUTOFLUSH in English) or call the autoflush() method of IO::Handle on any open handles.

Beware that some command shells may place restrictions on the length of the command line. You must ensure your strings don't exceed this limit after any necessary interpolations. See the platform-specific release notes for more details about your particular environment.

Using this operator can lead to programs that are difficult to port, because the shell commands called vary between systems, and may in fact not be present at all. As one example, the type command under the POSIX shell is very different from the type command under DOS. That doesn't mean you should go out of your way to avoid backticks when they're the right way to get something done. Perl was made to be a glue language, and one of the things it glues together is commands. Just understand what you're getting yourself into.

See I/O Operators for more discussion.

qw/STRING/

Evaluates to a list of the words extracted out of STRING, using embedded whitespace as the word delimiters. It can be understood as being roughly equivalent to:

    split(' ', q/STRING/);

the difference being that it generates a real list at compile time. So this expression:

    qw(foo bar baz)

is semantically equivalent to the list:

    'foo', 'bar', 'baz'

Some frequently seen examples:

    use POSIX qw( setlocale localeconv )
    @EXPORT = qw( foo bar baz );

A common mistake is to try to separate the words with comma or to put comments into a multi-line qw-string. For this reason, the use warnings pragma and the -w switch (that is, the $^W variable) produces warnings if the STRING contains the ``,'' or the ``#'' character.

s/PATTERN/REPLACEMENT/egimosx

Searches a string for a pattern, and if found, replaces that pattern with the replacement text and returns the number of substitutions made. Otherwise it returns false (specifically, the empty string).

If no string is specified via the =~ or !~ operator, the $_ variable is searched and modified. (The string specified with =~ must be scalar variable, an array element, a hash element, or an assignment to one of those, i.e., an lvalue.)

If the delimiter chosen is a single quote, no interpolation is done on either the PATTERN or the REPLACEMENT. Otherwise, if the PATTERN contains a $ that looks like a variable rather than an end-of-string test, the variable will be interpolated into the pattern at run-time. If you want the pattern compiled only once the first time the variable is interpolated, use the /o option. If the pattern evaluates to the empty string, the last successfully executed regular expression is used instead. See the perlre manpage for further explanation on these. See the perllocale manpage for discussion of additional considerations that apply when use locale is in effect.

Options are:

    e   Evaluate the right side as an expression.
    g   Replace globally, i.e., all occurrences.
    i   Do case-insensitive pattern matching.
    m   Treat string as multiple lines.
    o   Compile pattern only once.
    s   Treat string as single line.
    x   Use extended regular expressions.

Any non-alphanumeric, non-whitespace delimiter may replace the slashes. If single quotes are used, no interpretation is done on the replacement string (the /e modifier overrides this, however). Unlike Perl 4, Perl 5 treats backticks as normal delimiters; the replacement text is not evaluated as a command. If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own pair of quotes, which may or may not be bracketing quotes, e.g., s(foo)(bar) or s<foo>/bar/. A /e will cause the replacement portion to be treated as a full-fledged Perl expression and evaluated right then and there. It is, however, syntax checked at compile-time. A second e modifier will cause the replacement portion to be evaled before being run as a Perl expression.

Examples:

    s/\bgreen\b/mauve/g;                # don't change wintergreen

    $path =~ s|/usr/bin|/usr/local/bin|;

    s/Login: $foo/Login: $bar/; # run-time pattern

    ($foo = $bar) =~ s/this/that/;      # copy first, then change

    $count = ($paragraph =~ s/Mister\b/Mr./g);  # get change-count

    $_ = 'abc123xyz';
    s/\d+/$&*2/e;               # yields 'abc246xyz'
    s/\d+/sprintf("%5d",$&)/e;  # yields 'abc  246xyz'
    s/\w/$& x 2/eg;             # yields 'aabbcc  224466xxyyzz'

    s/%(.)/$percent{$1}/g;      # change percent escapes; no /e
    s/%(.)/$percent{$1} || $&/ge;       # expr now, so /e
    s/^=(\w+)/&pod($1)/ge;      # use function call

    # expand variables in $_, but dynamics only, using
    # symbolic dereferencing
    s/\$(\w+)/${$1}/g;

    # Add one to the value of any numbers in the string
    s/(\d+)/1 + $1/eg;

    # This will expand any embedded scalar variable
    # (including lexicals) in $_ : First $1 is interpolated
    # to the variable name, and then evaluated
    s/(\$\w+)/$1/eeg;

    # Delete (most) C comments.
    $program =~ s {
        /\*     # Match the opening delimiter.
        .*?     # Match a minimal number of characters.
        \*/     # Match the closing delimiter.
    } []gsx;

    s/^\s*(.*?)\s*$/$1/;        # trim white space in $_, expensively

    for ($variable) {           # trim white space in $variable, cheap
        s/^\s+//;
        s/\s+$//;
    }

    s/([^ ]*) *([^ ]*)/$2 $1/;  # reverse 1st two fields

Note the use of $ instead of \ in the last example. Unlike sed, we use the \<digit> form in only the left hand side. Anywhere else it's $<digit>.

Occasionally, you can't use just a /g to get all the changes to occur that you might want. Here are two common cases:

    # put commas in the right places in an integer
    1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;

    # expand tabs to 8-column spacing
    1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;

tr/SEARCHLIST/REPLACEMENTLIST/cdsUC

y/SEARCHLIST/REPLACEMENTLIST/cdsUC

Transliterates all occurrences of the characters found in the search list with the corresponding character in the replacement list. It returns the number of characters replaced or deleted. If no string is specified via the =~ or !~ operator, the $_ string is transliterated. (The string specified with =~ must be a scalar variable, an array element, a hash element, or an assignment to one of those, i.e., an lvalue.)

A character range may be specified with a hyphen, so tr/A-J/0-9/ does the same replacement as tr/ACEGIBDFHJ/0246813579/. For sed devotees, y is provided as a synonym for tr. If the SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has its own pair of quotes, which may or may not be bracketing quotes, e.g., tr[A-Z][a-z] or tr(+\-*/)/ABCD/.

Note also that the whole range idea is rather unportable between character sets--and even within character sets they may cause results you probably didn't expect. A sound principle is to use only ranges that begin from and end at either alphabets of equal case (a-e, A-E), or digits (0-4). Anything else is unsafe. If in doubt, spell out the character sets in full.

Options:

    c   Complement the SEARCHLIST.
    d   Delete found but unreplaced characters.
    s   Squash duplicate replaced characters.
    U   Translate to/from UTF-8.
    C   Translate to/from 8-bit char (octet).

If the /c modifier is specified, the SEARCHLIST character set is complemented. If the /d modifier is specified, any characters specified by SEARCHLIST not found in REPLACEMENTLIST are deleted. (Note that this is slightly more flexible than the behavior of some tr programs, which delete anything they find in the SEARCHLIST, period.) If the /s modifier is specified, sequences of characters that were transliterated to the same character are squashed down to a single instance of the character.

If the /d modifier is used, the REPLACEMENTLIST is always interpreted exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter than the SEARCHLIST, the final character is replicated till it is long enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated. This latter is useful for counting characters in a class or for squashing character sequences in a class.

The first /U or /C modifier applies to the left side of the translation. The second one applies to the right side. If present, these modifiers override the current utf8 state.

Examples:

    $ARGV[1] =~ tr/A-Z/a-z/;    # canonicalize to lower case

    $cnt = tr/*/*/;             # count the stars in $_

    $cnt = $sky =~ tr/*/*/;     # count the stars in $sky

    $cnt = tr/0-9//;            # count the digits in $_

    tr/a-zA-Z//s;               # bookkeeper -> bokeper

    ($HOST = $host) =~ tr/a-z/A-Z/;

    tr/a-zA-Z/ /cs;             # change non-alphas to single space

    tr [\200-\377]
       [\000-\177];             # delete 8th bit

    tr/\0-\xFF//CU;             # change Latin-1 to Unicode
    tr/\0-\x{FF}//UC;           # change Unicode to Latin-1

If multiple transliterations are given for a character, only the first one is used:

    tr/AAA/XYZ/

will transliterate any A to X.

Because the transliteration table is built at compile time, neither the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote interpolation. That means that if you want to use variables, you must use an eval():

    eval "tr/$oldlist/$newlist/";
    die $@ if $@;

    eval "tr/$oldlist/$newlist/, 1" or die $@;

Gory details of parsing quoted constructs

When presented with something that might have several different interpretations, Perl uses the DWIM (that's ``Do What I Mean'') principle to pick the most probable interpretation. This strategy is so successful that Perl programmers often do not suspect the ambivalence of what they write. But from time to time, Perl's notions differ substantially from what the author honestly meant.

This section hopes to clarify how Perl handles quoted constructs. Although the most common reason to learn this is to unravel labyrinthine regular expressions, because the initial steps of parsing are the same for all quoting operators, they are all discussed together.

The most important Perl parsing rule is the first one discussed below: when processing a quoted construct, Perl first finds the end of that construct, then interprets its contents. If you understand this rule, you may skip the rest of this section on the first reading. The other rules are likely to contradict the user's expectations much less frequently than this first one.

Some passes discussed below are performed concurrently, but because their results are the same, we consider them individually. For different quoting constructs, Perl performs different numbers of passes, from one to f