MAN page from RedHat Other perl-5.004-0.1.i386.rpm

PERLOP

Section: Perl Programmers Reference Guide (1)
Updated: perl 5.004, patch 04
Index

NAME

perlop - Perl operators and precedence

SYNOPSIS

Perl operators have the following associativity and precedence,listed from highest precedence to lowest. Note that all operatorsborrowed from C keep the same precedence relationship with each other,even where C's precedence is slightly screwy. (This makes learningPerl easier for C folks.) With very few exceptions, these alloperate 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.

DESCRIPTION

Terms and List Operators (Leftward)

A TERM has the highest precedence in Perl. They includes variables,quote and quote-like operators, any expression in parentheses,and any function whose arguments are parenthesized. Actually, therearen't really functions in this sense, just list operators and unaryoperators behaving as functions because you put parentheses aroundthe 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 andarguments 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 asprint, sort, or chmod is either very high or very low depending onwhether 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, butthe commas on the left are evaluated after. In other words, listoperators tend to gobble up all the arguments that follow them, andthen act like a simple TERM with regard to the preceding expression.Note that you have to 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. Seethe section on Named Unary Operators for more discussion of this.

Also parsed as terms are the do {} and eval {} constructs, aswell as subroutine and method calls, and the anonymousconstructors [] and {}.

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

The Arrow Operator

Just as in C and C++, ``->'' is an infix dereference operator. If theright side is either a [...] or {...} subscript, then the left sidemust be either a hard or symbolic reference to an array or hash (ora location capable of holding a hard reference, if it's an lvalue (assignable)).See the perlref manpage.

Otherwise, the right side is a method name or a simple scalar variablecontaining the method name, and the left side must either be 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, theyincrement or decrement the variable before returning the value, and ifplaced after, increment or decrement the variable after returning the value.

The auto-increment operator has a little extra builtin magic to it. Ifyou increment a variable that is numeric, or that has ever been used ina numeric context, you get a normal increment. If, however, thevariable has been used in only string contexts since it was set, andhas a value that is not null and matches the pattern/^[a-zA-Z]*[0-9]*$/, the increment is done as a string, preserving eachcharacter 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. Note that it binds even moretightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This isimplemented using C's pow(3) function, which actually works on doublesinternally.)

Symbolic Unary Operators

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

Unary ``-'' performs arithmetic negation if the operand is numeric. Ifthe operand is an identifier, a string consisting of a minus signconcatenated with the identifier is returned. Otherwise, if the stringstarts with a plus or minus, a string starting with the opposite signis returned. One effect of these rules is that -bareword is equivalentto "-bareword".

Unary ``~'' performs bitwise negation, i.e., 1's complement.(See also the section on Integer Arithmetic.)

Unary ``+'' has no effect whatsoever, even on strings. It is usefulsyntactically for separating a function name from a parenthesized expressionthat would otherwise be interpreted as the complete list of functionarguments. (See examples above under the section on Terms and List Operators (Leftward).)

Unary ``\'' creates a reference to whatever follows it. See the perlref manpage.Do not confuse this behavior with the behavior of backslash within astring, although both forms do convey the notion of protecting the nextthing from interpretation.

Binding Operators

Binary ``=~'' binds a scalar expression to a pattern match. Certain operationssearch or modify the string $_ by default. This operator makes that kindof operation work on some other string. The right argument is a searchpattern, substitution, or translation. The left argument is what issupposed to be searched, substituted, or translated instead of the default$_. The return value indicates the success of the operation. (If theright argument is an expression rather than a search pattern,substitution, or translation, it is interpreted as a search pattern at runtime. This can be is less efficient than an explicit search, because thepattern must be compiled every time the expression is evaluated.

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

Multiplicative Operators

Binary ``*'' multiplies two numbers.

Binary ``/'' divides two numbers.

Binary ``%'' computes the modulus of two numbers. Given integeroperands $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 thesmallest multiple of $b that is not less than $a (i.e. theresult will be less than or equal to zero).

Binary ``x'' is the repetition operator. In a scalar context, itreturns a string consisting of the left operand repeated the number oftimes specified by the right operand. In a list context, if the leftoperand is a list 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 thenumber of bits specified by the right argument. Arguments should beintegers. (See also the section on Integer Arithmetic.)

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

Named Unary Operators

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

    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 the section on Terms and List Operators (Leftward).

Relational Operators

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

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

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

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

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

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

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

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

Equality Operators

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

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

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

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

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

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

``lt'', ``le'', ``ge'', ``gt'' and ``cmp'' use the collation (sort) order specifiedby 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 the section on Integer Arithmetic.)

Bitwise Or and Exclusive Or

Binary ``|'' returns its operators ORed together bit by bit.(See also the section on Integer Arithmetic.)

Binary ``^'' returns its operators XORed together bit by bit.(See also the section on Integer Arithmetic.)

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 itis 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 itis evaluated.

The || and && operators differ from C's in that, rather than returning0 or 1, they return the last value evaluated. Thus, a reasonably portableway 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";

As more readable alternatives to && and ||, Perl provides ``and'' and``or'' operators (see below). The short-circuit behavior is identical. Theprecedence of ``and'' and ``or'' is much lower, however, so that you cansafely use them after a list operator without the need forparentheses:

    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);

Range Operator

Binary ``..'' is the range operator, which is really two differentoperators depending on the context. In a list context, it returns anarray of values counting (by ones) from the left value to the rightvalue. This is useful for writing for (1..10) loops and for doingslice operations on arrays. Be aware that under the current implementation,a temporary array is created, so you'll burn a lot of memory if youwrite something like this:

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

In a scalar context, ``..'' returns a boolean value. The operator isbistable, like a flip-flop, and emulates the line-range (comma) operatorof sed, awk, and various editors. Each ``..'' operator maintains itsown 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 theright operand is true, AFTER which the range operator becomes falseagain. (It doesn't become false till the next time the range operator isevaluated. It can test the right operand and become false on the sameevaluation 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), use three dots (''...") instead of two.) The rightoperand is not evaluated while the operator is in the ``false'' state, andthe left operand is not evaluated while the operator is in the ``true''state. The precedence is a little lower than || and &&. The valuereturned is either the null string for false, or a sequence number(beginning with 1) for true. The sequence number is reset for each rangeencountered. The final sequence number in a range has the string ``E0''appended to it, which doesn't affect its numeric value, but gives yousomething to search for if you want to exclude the endpoint. You canexclude the beginning point by waiting for the sequence number to begreater than 1. If either operand of scalar ``..'' is a numeric literal,that operand is implicitly compared to the $. variable, the currentline 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

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 a list context) makes use of the magicalauto-increment algorithm if the operands are strings. Youcan say

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

to get all the 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 notin the sequence that the magical increment would produce, the sequencegoes until the next value would be longer than the final valuespecified.

Conditional Operator

Ternary ``?:'' is the conditional operator, just as in C. It works muchlike an if-then-else. If the argument before the ? is true, theargument 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 2ndor 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 arelegal lvalues (meaning that you can assign to them):

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

This is not necessarily guaranteed to contribute to the readability of your program.

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 lvaluemight trigger, such as from tie(). Other assignment operators work similarly.The following are recognized:

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

Note that while these are grouped by family, they all have the precedenceof assignment.

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

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

Likewise,

    ($a += 2) *= 3;

is equivalent to

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

Comma Operator

Binary ``,'' is the comma operator. In a scalar context it evaluatesits left argument, throws that value away, then evaluates its rightargument and returns that value. This is just like C's comma operator.

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

The => digraph is mostly just a synonym for the comma operator. It's useful fordocumenting arguments that come in pairs. As of release 5.001, it also forcesany 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 listoperators without the need for extra parentheses:

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

See also discussion of list operators in the section on 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 surroundingexpressions. It's equivalent to && except for the very lowprecedence. This means that it short-circuits: i.e., the rightexpression is evaluated only if the left expression is true.

Logical or and Exclusive Or

Binary ``or'' returns the logical disjunction of the two surroundingexpressions. It's equivalent to || except for the very lowprecedence. This means that it short-circuits: i.e., the rightexpression is evaluated only if the left expression is false.

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 dereferencingoperators are typed: $, @, %, and &.)
(TYPE): Type casting operator.

Quote and Quote-like Operators

While we usually think of quotes as literal values, in Perl theyfunction as operators, providing various kinds of interpolating andpattern matching capabilities. Perl provides customary quote charactersfor these behaviors, but also provides a way for you to choose yourquote character for any of them. In the following table, a {} representsany pair of delimiters you choose. Non-bracketing delimiters usethe same character fore and aft, but the 4 sorts of brackets(round, angle, square, curly) will all nest.

    Customary  Generic     Meaning    Interpolates        ''       q{}       Literal         no        ""      qq{}       Literal         yes        ``      qx{}       Command         yes                qw{}      Word list        no        //       m{}    Pattern match      yes                 s{}{}   Substitution      yes                tr{}{}   Translation       no

Note that there can be whitespace between the operator and the quotingcharacters, except when # is being used as the quoting character.q#foo# is parsed as being the string foo, which q #foo# is theoperator q followed by a comment. Its argument will be taken from thenext line. This allows you to write:

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

For constructs that do interpolation, variables beginning with ``$'' or ``@''are interpolated, as are the following sequences:

    \t          tab             (HT, TAB)    \n          newline         (LF, NL)    \r          return          (CR)    \f          form feed       (FF)    \b          backspace       (BS)    \a          alarm (bell)    (BEL)    \e          escape          (ESC)    \033        octal char    \x1b        hex char    \c[         control char    \l          lowercase next char    \u          uppercase next char    \L          lowercase till \E    \U          uppercase till \E    \E          end case modification    \Q          quote regexp metacharacters till \E

If use locale is in effect, the case map used by \l, \L, \uand <\U> is taken from the current locale. See the perllocale manpage.

Patterns are subject to an additional level of interpretation as aregular expression. This is done as a second pass, after variables areinterpolated, so that regular expressions may be incorporated into thepattern from the variables. If this is not what you want, use \Q tointerpolate a variable literally.

Apart from the above, there are no multiple levels of interpolation. Inparticular, contrary to the expectations of shell programmers, back-quotesdo NOT interpolate within double quotes, nor do single quotes impedeevaluation of variables when used within double quotes.

Regexp Quote-Like Operators

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

?PATTERN?

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

This usage is vaguely deprecated, and may be removed in some futureversion of Perl.

m/PATTERN/cgimosx

/PATTERN/cgimosx

Searches a string for a pattern match, and in a scalar context returnstrue (1) or false (''). 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 expressionevaluation, but remember the =~ binds rather tightly.) See alsothe perlre manpage.See the perllocale manpage for discussion of additional considerations which applywhen 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 myou can use any pair of non-alphanumeric, non-whitespace characters asdelimiters. This is particularly useful for matching Unix path namesthat contain ``/'', to avoid LTS (leaning toothpick syndrome). If ``?'' isthe delimiter, then the match-only-once rule of ?PATTERN? applies.

PATTERN may contain variables, which will be interpolated (and thepattern recompiled) every time the pattern search is evaluated. (Notethat $) and $| might not be interpolated because they look likeend-of-string tests.) If you want such a pattern to be compiled onlyonce, add a /o after the trailing delimiter. This avoids expensiverun-time recompilations, and is useful when the value you areinterpolating 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.

If the PATTERN evaluates to a null string, the lastsuccessfully executed regular expression is used instead.

If used in a context that requires a list value, a pattern match returns alist consisting of the subexpressions matched by the parentheses in thepattern, i.e., ($1, $2, $3...). (Note that here $1 etc. are also set, andthat this differs from Perl 4's behavior.) If the match fails, a nullarray is returned. If the match succeeds, but there were no parentheses,a list value of (1) is returned.

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 theremainder of the line, and assigns those three fields to $F1, $F2, and$Etc. The conditional is true if any variables were assigned, i.e., ifthe pattern matched.

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

In a scalar context, m//g iterates through the string, returning TRUEeach time it matches, and FALSE when it eventually runs out of matches.(In other words, it remembers where it left off last time and restartsthe search at that point. You can actually find the current matchposition of a string or set it using the pos() function; seethe pos entry in the perlfunc manpage.) A failed match normally resets the search position tothe beginning of the string, but you can avoid that by adding the /cmodifier (e.g. m//gc). Modifying the target string also resets thesearch position.

You can intermix m//g matches with m/\G.../g, where \G is azero-width assertion that matches the exact position where the previousm//g, if any, left off. The \G assertion is not supported withoutthe /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 cancombine several regexps like this to process a string part-by-part,doing different actions depending on which regexp matched. Eachregexp 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 backslashunless followed by the delimiter or another backslash, in which casethe 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 /(tcl|rexx|python)/;      # :-)    $baz = "\n";                # a one-character string

qx/STRING/

`STRING`

A string which is interpolated and then executed as a system command.The collected standard output of the command is returned. In scalarcontext, it comes back as a single (potentially multi-line) string.In list context, returns a list of lines (however you've defined lineswith $/ or $INPUT_RECORD_SEPARATOR).

    $today = qx{ date };

Note that how the string gets evaluated is entirely subject to thecommand interpreter on your system. On most platforms, you will haveto protect shell metacharacters if you want them treated literally.On some platforms (notably DOS-like ones), the shell may not becapable of dealing with multiline commands, so putting newlines inthe string may not get you what you want. You may be able to evaluatemultiple commands in a single line by separating them with the commandseparator character, if your shell supports that (e.g. ; on many Unixshells; & on the Windows NT cmd shell).

Beware that some command shells may place restrictions on the lengthof the command line. You must ensure your strings don't exceed thislimit after any necessary interpolations. See the platform-specificrelease notes for more details about your particular environment.

Also realize that using this operator frequently leads to unportableprograms.

See the section on I/O Operators for more discussion.

qw/STRING/

Returns a list of the words extracted out of STRING, using embeddedwhitespace as the word delimiters. It is exactly equivalent to

    split(' ', q/STRING/);

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 putcomments into a multi-line qw-string. For this reason the -wswitch produce warnings if the STRING contains the ``,'' or the ``#''character.

s/PATTERN/REPLACEMENT/egimosx

Searches a string for a pattern, and if found, replaces that patternwith the replacement text and returns the number of substitutionsmade. 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 =~ mustbe a scalar variable, an array element, a hash element, or an assignmentto one of those, i.e., an lvalue.)

If the delimiter chosen is single quote, no variable interpolation isdone on either the PATTERN or the REPLACEMENT. Otherwise, if thePATTERN contains a $ that looks like a variable rather than anend-of-string test, the variable will be interpolated into the patternat run-time. If you want the pattern compiled only once the first timethe variable is interpolated, use the /o option. If the patternevaluates to a null string, the last successfully executed regularexpression is used instead. See the perlre manpage for further explanation on these.See the perllocale manpage for discussion of additional considerations which applywhen 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 theslashes. If single quotes are used, no interpretation is done on thereplacement string (the /e modifier overrides this, however). UnlikePerl 4, Perl 5 treats backticks as normal delimiters; the replacementtext is not evaluated as a command. If thePATTERN is delimited by bracketing quotes, the REPLACEMENT has its ownpair of quotes, which may or may not be bracketing quotes, e.g.,s(foo)(bar) or s<foo>/bar/. A /e will cause thereplacement portion to be interpreter as a full-fledged Perl expressionand eval()ed right then and there. It is, however, syntax checked atcompile-time.

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/;

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

    $_ = 'abc123xyz';    s/\d+/$&*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

    # /e's can even nest;  this will expand    # simple embedded variables in $_    s/(\$\w+)/$1/eeg;

    # Delete 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

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

Note the use of $ instead of \ in the last example. Unlikesed, 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 changesto occur. Here are two common cases:

    # put commas in the right places in an integer    1 while s/(.*\d)(\d\d\d)/$1,$2/g;      # perl4    1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;  # perl5

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

tr/SEARCHLIST/REPLACEMENTLIST/cds

y/SEARCHLIST/REPLACEMENTLIST/cds

Translates all occurrences of the characters found in the search listwith the corresponding character in the replacement list. It returnsthe number of characters replaced or deleted. If no string isspecified via the =~ or !~ operator, the $_ string is translated. (Thestring specified with =~ must be a scalar variable, an array element, ahash element, or an assignment to one of those, i.e., an lvalue.)For sed devotees, y is provided as a synonym for tr. If theSEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST hasits own pair of quotes, which may or may not be bracketing quotes,e.g., tr[A-Z][a-z] or tr(+-*/)/ABCD/.

Options:

    c   Complement the SEARCHLIST.    d   Delete found but unreplaced characters.    s   Squash duplicate replaced characters.

If the /c modifier is specified, the SEARCHLIST character set iscomplemented. If the /d modifier is specified, any characters specifiedby SEARCHLIST not found in REPLACEMENTLIST are deleted. (Notethat this is slightly more flexible than the behavior of some trprograms, which delete anything they find in the SEARCHLIST, period.)If the /s modifier is specified, sequences of characters that weretranslated to the same character are squashed down to a single instance of thecharacter.

If the /d modifier is used, the REPLACEMENTLIST is always interpretedexactly as specified. Otherwise, if the REPLACEMENTLIST is shorterthan the SEARCHLIST, the final character is replicated till it is longenough. If the REPLACEMENTLIST is null, the SEARCHLIST is replicated.This latter is useful for counting characters in a class or forsquashing character sequences in a class.

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

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

    tr/AAA/XYZ/

will translate any A to X.

Note that because the translation table is built at compile time, neitherthe SEARCHLIST nor the REPLACEMENTLIST are subjected to double quoteinterpolation. That means that if you want to use variables, you must usean eval():

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

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

I/O Operators

There are several I/O operators you should know about.A string is enclosed by backticks (grave accents) first undergoesvariable substitution just like a double quoted string. It is theninterpreted as a command, and the output of that command is the valueof the pseudo-literal, like in a shell. In a scalar context, a singlestring consisting of all the output is returned. In a list context,a list of values is returned, one for each line of output. (You canset $/ to use a different line terminator.) The command is executedeach time the pseudo-literal is evaluated. The status value of thecommand is returned in $? (see the perlvar manpage for the interpretationof $?). Unlike in csh, no translation is done on the returndata---newlines remain newlines. Unlike in any of the shells, singlequotes do not hide variable names in the command from interpretation.To pass a $ through to the shell you need to hide it with a backslash.The generalized form of backticks is qx//. (Because backticksalways undergo shell expansion as well, see the perlsec manpage forsecurity concerns.)

Evaluating a filehandle in angle brackets yields the next line fromthat file (newline, if any, included), or undef at end of file.Ordinarily you must assign that value to a variable, but there is onesituation where an automatic assignment happens. If and ONLY if theinput symbol is the only thing inside the conditional of a while orfor(;;) loop, the value is automatically assigned to the variable$_. The assigned value is then tested to see if it is defined.(This may seem like an odd thing to you, but you'll use the constructin almost every Perl script you write.) Anyway, the following linesare equivalent to each other:

    while (defined($_ = <STDIN>)) { print; }    while (<STDIN>) { print; }    for (;<STDIN>;) { print; }    print while defined($_ = <STDIN>);    print while <STDIN>;

The filehandles STDIN, STDOUT, and STDERR are predefined. (Thefilehandles stdin, stdout, and stderr will also work except inpackages, where they would be interpreted as local identifiers ratherthan global.) Additional filehandles may be created with the open()function. See the open() entry in the perlfunc manpage for details on this.

If a <FILEHANDLE> is used in a context that is looking for a list, alist consisting of all the input lines is returned, one line per listelement. It's easy to make a LARGE data space this way, so use withcare.

The null filehandle <> is special and can be used to emulate thebehavior of sed and awk. Input from <> comes either fromstandard input, or from each file listed on the command line. Here'show it works: the first time <> is evaluated, the @ARGV array ischecked, and if it is null, $ARGV[0] is set to ``-'', which when openedgives you standard input. The @ARGV array is then processed as a listof filenames. The loop

    while (<>) {        ...                     # code for each line    }

is equivalent to the following Perl-like pseudo code:

    unshift(@ARGV, '-') unless @ARGV;    while ($ARGV = shift) {        open(ARGV, $ARGV);        while (<ARGV>) {            ...         # code for each line        }    }

except that it isn't so cumbersome to say, and will actually work. Itreally does shift array @ARGV and put the current filename into variable$ARGV. It also uses filehandle ARGV internally--<> is just asynonym for <ARGV>, which is magical. (The pseudo code abovedoesn't work because it treats <ARGV> as non-magical.)

You can modify @ARGV before the first <> as long as the array ends upcontaining the list of filenames you really want. Line numbers ($.)continue as if the input were one big happy file. (But see exampleunder eof() for how to reset line numbers on each file.)

If you want to set @ARGV to your own list of files, go right ahead. Ifyou want to pass switches into your script, you can use one of theGetopts modules or put a loop on the front like this:

    while ($_ = $ARGV[0], /^-/) {        shift;        last if /^--$/;        if (/^-D(.*)/) { $debug = $1 }        if (/^-v/)     { $verbose++  }        ...             # other switches    }    while (<>) {        ...             # code for each line    }

The <> symbol will return FALSE only once. If you call it again afterthis it will assume you are processing another @ARGV list, and if youhaven't set @ARGV, will input from STDIN.

If the string inside the angle brackets is a reference to a scalarvariable (e.g., <$foo>), then that variable contains the name of thefilehandle to input from, or a reference to the same. For example:

    $fh = \*STDIN;    $line = <$fh>;

If the string inside angle brackets is not a filehandle or a scalarvariable containing a filehandle name or reference, then it is interpretedas a filename pattern to be globbed, and either a list of filenames or thenext filename in the list is returned, depending on context. One level of$ interpretation is done first, but you can't say <$foo>because that's an indirect filehandle as explained in the previousparagraph. (In older versions of Perl, programmers would insert curlybrackets to force interpretation as a filename glob: <${foo}>.These days, it's considered cleaner to call the internal function directlyas glob($foo), which is probably the right way to have done it in thefirst place.) Example:

    while (<*.c>) {        chmod 0644, $_;    }

is equivalent to

    open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");    while (<FOO>) {        chop;        chmod 0644, $_;    }

In fact, it's currently implemented that way. (Which means it will notwork on filenames with spaces in them unless you have csh(1) on yourmachine.) Of course, the shortest way to do the above is:

    chmod 0644, <*.c>;

Because globbing invokes a shell, it's often faster to call readdir() yourselfand do your own grep() on the filenames. Furthermore, due to its currentimplementation of using a shell, the glob() routine may get ``Arg list toolong'' errors (unless you've installed tcsh(1L) as /bin/csh).

A glob evaluates its (embedded) argument only when it is starting a newlist. All values must be read before it will start over. In a listcontext this isn't important, because you automatically get them allanyway. In a scalar context, however, the operator returns the next valueeach time it is called, or a FALSE value if you've just run out. Again,FALSE is returned only once. So if you're expecting a single value froma glob, it is much better to say

    ($file) = <blurch*>;

than

    $file = <blurch*>;

because the latter will alternate between returning a filename andreturning FALSE.

It you're trying to do variable interpolation, it's definitely betterto use the glob() function, because the older notation can cause peopleto become confused with the indirect filehandle notation.

    @files = glob("$dir/*.[ch]");    @files = glob($files[$i]);

Constant Folding

Like C, Perl does a certain amount of expression evaluation atcompile time, whenever it determines that all of the arguments to anoperator are static and have no side effects. In particular, stringconcatenation happens at compile time between literals that don't dovariable substitution. Backslash interpretation also happens atcompile time. You can say

    'Now is the time for all' . "\n" .        'good men to come to.'

and this all reduces to one string internally. Likewise, ifyou say

    foreach $file (@filenames) {        if (-s $file > 5 + 100 * 2**16) { ... }    }

the compiler will precompute the number thatexpression represents so that the interpreterwon't have to.

Integer Arithmetic

By default Perl assumes that it must do most of its arithmetic infloating point. But by saying

    use integer;

you may tell the compiler that it's okay to use integer operationsfrom here to the end of the enclosing BLOCK. An inner BLOCK maycountermand this by saying

    no integer;

which lasts until the end of that BLOCK.

The bitwise operators ("&'', ``|'', ``^'', ``~'', ``<<'', and ``>>") alwaysproduce integral results. However, use integer still has meaningfor them. By default, their results are interpreted as unsignedintegers. However, if use integer is in effect, their results areinterpreted as signed integers. For example, ~0 usually evaluatesto a large integral value. However, use integer; ~0 is -1.

Floating-point Arithmetic

While use integer provides integer-only arithmetic, there is nosimilar ways to provide rounding or truncation at a certain number ofdecimal places. For rounding to a certain number of digits, sprintf()or printf() is usually the easiest route.

The POSIX module (part of the standard perl distribution) implementsceil(), floor(), and a number of other mathematical and trigonometricfunctions. The Math::Complex module (part of the standard perldistribution) defines a number of mathematical functions that can alsowork on real numbers. Math::Complex not as efficient as POSIX, butPOSIX can't work with complex numbers.

Rounding in financial applications can have serious implications, andthe rounding method used should be specified precisely. In thesecases, it probably pays not to trust whichever system rounding isbeing used by Perl, but to instead implement the rounding function youneed yourself.

This document was created byman2html,using the manual pages.