perlop - Perl operators and precedence
-=head1 SYNOPSIS
+=head1 DESCRIPTION
+
+=head2 Operator Precedence and Associativity
+
+Operator precedence and associativity work in Perl more or less like
+they do in mathematics.
+
+I<Operator precedence> means some operators are evaluated before
+others. For example, in C<2 + 4 * 5>, the multiplication has higher
+precedence so C<4 * 5> is evaluated first yielding C<2 + 20 ==
+22> and not C<6 * 5 == 30>.
+
+I<Operator associativity> defines what happens if a sequence of the
+same operators is used one after another: whether the evaluator will
+evaluate the left operations first or the right. For example, in C<8
+- 4 - 2>, subtraction is left associative so Perl evaluates the
+expression left to right. C<8 - 4> is evaluated first making the
+expression C<4 - 2 == 2> and not C<8 - 2 == 6>.
Perl operators have the following associativity and precedence,
listed from highest precedence to lowest. Operators borrowed from
left &
left | ^
left &&
- left ||
+ left || //
nonassoc .. ...
right ?:
right = += -= *= etc.
nonassoc list operators (rightward)
right not
left and
- left or xor
+ left or xor err
In the following sections, these operators are covered in precedence order.
Many operators can be overloaded for objects. See L<overload>.
-=head1 DESCRIPTION
-
=head2 Terms and List Operators (Leftward)
A TERM has the highest precedence in Perl. They include variables,
print ($foo & 255) + 1, "\n";
-probably doesn't do what you expect at first glance. See
-L<Named Unary Operators> for more discussion of this.
+probably doesn't do what you expect at first glance. The parentheses
+enclose the argument list for C<print> which is evaluated (printing
+the result of C<$foo & 255>). Then one is added to the return value
+of C<print> (usually 1). The result is something like this:
+
+ 1 + 1, "\n"; # Obviously not what you meant.
+
+To do what you meant properly, you must write:
+
+ print(($foo & 255) + 1, "\n");
+
+See L<Named Unary Operators> for more discussion of this.
Also parsed as terms are the C<do {}> and C<eval {}> constructs, as
well as subroutine and method calls, and the anonymous
=head2 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.
+"++" and "--" work as in C. That is, if placed before a variable,
+they increment or decrement the variable by one before returning the
+value, and if placed after, increment or decrement after returning the
+value.
+
+ $i = 0; $j = 0;
+ print $i++; # prints 0
+ print ++$j; # prints 1
+
+Note that just as in C, Perl doesn't define B<when> the variable is
+incremented or decremented. You just know it will be done sometime
+before or after the value is returned. This also means that modifying
+a variable twice in the same statement will lead to undefined behaviour.
+Avoid statements like:
+
+ $i = $i ++;
+ print ++ $i + $i ++;
+
+Perl will not guarantee what the result of the above statements is.
The auto-increment operator has a little extra builtin magic to it. If
you increment a variable that is numeric, or that has ever been used in
a numeric context, you get a normal increment. If, however, the
variable has been used in only string contexts since it was set, and
has a value that is not the empty string and matches the pattern
-C</^[a-zA-Z]*[0-9]*$/>, the increment is done as a string, preserving each
+C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each
character within its range, with carry:
print ++($foo = '99'); # prints '100'
print ++($foo = 'Az'); # prints 'Ba'
print ++($foo = 'zz'); # prints 'aaa'
+C<undef> is always treated as numeric, and in particular is changed
+to C<0> before incrementing (so that a post-increment of an undef value
+will return C<0> rather than C<undef>).
+
The auto-decrement operator is not magical.
=head2 Exponentiation
L<Bitwise String Operators>.) Note that the width of the result is
platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
bits wide on a 64-bit platform, so if you are expecting a certain bit
-width, remember use the & operator to mask off the excess bits.
+width, remember to use the & operator to mask off the excess bits.
Unary "+" has no effect whatsoever, even on strings. It is useful
syntactically for separating a function name from a parenthesized expression
of operation work on some other string. The right argument is a search
pattern, substitution, or transliteration. The left argument is what is
supposed to be searched, substituted, or transliterated instead of the default
-$_. The return value indicates the success of the operation. If the
-right argument is an expression rather than a search pattern,
+$_. When used in scalar context, the return value generally indicates the
+success of the operation. Behavior in list context depends on the particular
+operator. See L</"Regexp Quote-Like Operators"> for details.
+
+If the right argument is an expression rather than a search pattern,
substitution, or transliteration, it is interpreted as a search pattern at run
-time. This can be less efficient than an explicit search, because the
-pattern must be compiled every time the expression is evaluated.
+time.
Binary "!~" is just like "=~" except the return value is negated in
the logical sense.
C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
smallest multiple of C<$b> that is not less than C<$a> (i.e. the
result will be less than or equal to zero).
-Note than when C<use integer> is in scope, "%" give you direct access
+Note that when C<use integer> is in scope, "%" gives 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.
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.
+parentheses, it repeats the list. If the right operand is zero or
+negative, it returns an empty string or an empty list, depending on the
+context.
print '-' x 80; # print row of dashes
the number of bits specified by the right argument. Arguments should
be integers. (See also L<Integer Arithmetic>.)
+Note that both "<<" and ">>" in Perl are implemented directly using
+"<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is
+in force then signed C integers are used, else unsigned C integers are
+used. Either way, the implementation isn't going to generate results
+larger than the size of the integer type Perl was built with (32 bits
+or 64 bits).
+
+The result of overflowing the range of the integers is undefined
+because it is undefined also in C. In other words, using 32-bit
+integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
+of bits is also undefined.
+
=head2 Named Unary Operators
The various named unary operators are treated as functions with one
-argument, with optional parentheses. These include the filetest
-operators, like C<-f>, C<-M>, etc. See L<perlfunc>.
+argument, with optional parentheses.
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:
+just like a normal function call. For example,
+because named unary operators are higher precedence than ||:
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 ||:
+but, because * is higher precedence than named operators:
chdir $foo * 20; # chdir ($foo * 20)
chdir($foo) * 20; # (chdir $foo) * 20
rand (10) * 20; # (rand 10) * 20
rand +(10) * 20; # rand (10 * 20)
+Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
+treated like named unary operators, but they don't follow this functional
+parenthesis rule. That means, for example, that C<-f($file).".bak"> is
+equivalent to C<-f "$file.bak">.
+
See also L<"Terms and List Operators (Leftward)">.
=head2 Relational Operators
Binary "<=>" returns -1, 0, or 1 depending on whether the left
argument is numerically less than, equal to, or greater than the right
-argument.
+argument. If your platform supports NaNs (not-a-numbers) as numeric
+values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
+"<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
+returns true, as does NaN != anything else. If your platform doesn't
+support NaNs then NaN is just a string with numeric value 0.
+
+ perl -le '$a = NaN; print "No NaN support here" if $a == $a'
+ perl -le '$a = NaN; print "NaN support here" if $a != $a'
Binary "eq" returns true if the left argument is stringwise equal to
the right argument.
Binary "ne" returns true if the left argument is stringwise not equal
to the right argument.
-Binary "cmp" returns -1, 0, or 1 depending on whether the left argument is stringwise
-less than, equal to, or greater than the right argument.
+Binary "cmp" returns -1, 0, or 1 depending on whether the left
+argument is stringwise less than, equal to, or greater than the right
+argument.
"lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
by the current locale if C<use locale> is in effect. See L<perllocale>.
=head2 Bitwise And
-Binary "&" returns its operators ANDed together bit by bit.
+Binary "&" returns its operands ANDed together bit by bit.
(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
+Note that "&" has lower priority than relational operators, so for example
+the brackets are essential in a test like
+
+ print "Even\n" if ($x & 1) == 0;
+
=head2 Bitwise Or and Exclusive Or
-Binary "|" returns its operators ORed together bit by bit.
+Binary "|" returns its operands ORed together bit by bit.
(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
-Binary "^" returns its operators XORed together bit by bit.
+Binary "^" returns its operands XORed together bit by bit.
(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
+Note that "|" and "^" have lower priority than relational operators, so
+for example the brackets are essential in a test like
+
+ print "false\n" if (8 | 2) != 10;
+
=head2 C-style Logical And
Binary "&&" performs a short-circuit logical AND operation. That is,
Scalar or list context propagates down to the right operand if it
is evaluated.
-The C<||> and C<&&> 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:
+=head2 C-style Logical Defined-Or
+
+Although it has no direct equivalent in C, Perl's C<//> operator is related
+to its C-style or. In fact, it's exactly the same as C<||>, except that it
+tests the left hand side's definedness instead of its truth. Thus, C<$a // $b>
+is similar to C<defined($a) || $b> (except that it returns the value of C<$a>
+rather than the value of C<defined($a)>) and is exactly equivalent to
+C<defined($a) ? $a : $b>. This is very useful for providing default values
+for variables. If you actually want to test if at least one of C<$a> and
+C<$b> is defined, use C<defined($a // $b)>.
+
+The C<||>, C<//> and C<&&> operators return the last value evaluated
+(unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
+portable way to find out the home directory might be:
- $home = $ENV{'HOME'} || $ENV{'LOGDIR'} ||
- (getpwuid($<))[7] || die "You're homeless!\n";
+ $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 = scalar(@b) || @c; # really meant this
@a = @b ? @b : @c; # this works fine, though
-As more readable alternatives to C<&&> and C<||> when used for
-control flow, Perl provides C<and> and C<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
+As more readable alternatives to C<&&>, C<//> and C<||> when used for
+control flow, Perl provides C<and>, C<err> and C<or> operators (see below).
+The short-circuit behavior is identical. The precedence of "and", "err"
+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"
=head2 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
+operators depending on the context. In list context, it returns a
+list 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
-C<foreach (1..10)> loops and for doing slice operations on arrays. In
+returns the empty list. The range operator is useful for writing
+C<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 C<foreach> loops, but older
versions of Perl might burn a lot of memory when you write something
# code
}
+The range operator also works on strings, using the magical auto-increment,
+see below.
+
In scalar context, ".." returns a boolean value. The operator is
bistable, like a flip-flop, and emulates the line-range (comma) operator
of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
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 C<$.> variable, the
-current line number. Examples:
+than 1.
+
+If either operand of scalar ".." is a constant expression,
+that operand is considered true if it is equal (C<==>) to the current
+input line number (the C<$.> variable).
+
+To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
+but that is only an issue if you use a floating point expression; when
+implicitly using C<$.> as described in the previous paragraph, the
+comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.>
+is set to a floating point value and you are not reading from a file.
+Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
+you want in scalar context because each of the operands are evaluated
+using their integer representation.
+
+Examples:
As a scalar operator:
- if (101 .. 200) { print; } # print 2nd hundred lines
- next line if (1 .. /^$/); # skip header lines
+ if (101 .. 200) { print; } # print 2nd hundred lines, short for
+ # if ($. == 101 .. $. == 200) ...
+ next line if (1 .. /^$/); # skip header lines, short for
+ # ... if ($. == 1 .. /^$/);
s/^/> / if (/^$/ .. eof()); # quote body
# parse mail messages
while (<>) {
$in_header = 1 .. /^$/;
- $in_body = /^$/ .. eof();
- # do something based on those
+ $in_body = /^$/ .. eof;
+ if ($in_header) {
+ # ...
+ } else { # in body
+ # ...
+ }
} continue {
- close ARGV if eof; # reset $. each file
+ close ARGV if eof; # reset $. each file
+ }
+
+Here's a simple example to illustrate the difference between
+the two range operators:
+
+ @lines = (" - Foo",
+ "01 - Bar",
+ "1 - Baz",
+ " - Quux");
+
+ foreach(@lines)
+ {
+ if (/0/ .. /1/)
+ {
+ print "$_\n";
+ }
}
-As a list operator:
+This program will print only the line containing "Bar". If
+the range operator is changed to C<...>, it will also print the
+"Baz" line.
+
+And now some examples as a list operator:
for (101 .. 200) { print; } # print $_ 100 times
@foo = @foo[0 .. $#foo]; # an expensive no-op
@alphabet = ('A' .. 'Z');
-to get all normal letters of the alphabet, or
+to get all normal letters of the English alphabet, or
$hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
goes until the next value would be longer than the final value
specified.
+Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
+return two elements in list context.
+
+ @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
+
=head2 Conditional Operator
Ternary "?:" is the conditional operator, just as in C. It works much
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.
+The C<< => >> operator is a synonym for the comma, but forces any word
+to its left to be interpreted as a string (as of 5.001). It is helpful
+in documenting the correspondence between keys and values in hashes,
+and other paired elements in lists.
=head2 List Operators (Rightward)
precedence. This means that it short-circuits: i.e., the right
expression is evaluated only if the left expression is true.
-=head2 Logical or and Exclusive Or
+=head2 Logical or, Defined 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.
@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.
+Then again, you could always use parentheses.
+
+Binary "err" is equivalent to C<//>--it's just like binary "or", except it tests
+its left argument's definedness instead of its truth. There are two ways to
+remember "err": either because many functions return C<undef> on an B<err>or,
+or as a sort of correction: C<$a=($b err 'default')>
Binary "xor" returns the exclusive-OR of the two surrounding expressions.
It cannot short circuit, of course.
Customary Generic Meaning Interpolates
'' q{} Literal no
"" qq{} Literal yes
- `` qx{} Command yes (unless '' is delimiter)
+ `` qx{} Command yes*
qw{} Word list no
- // m{} Pattern match yes (unless '' is delimiter)
- qr{} Pattern yes (unless '' is delimiter)
- s{}{} Substitution yes (unless '' is delimiter)
+ // m{} Pattern match yes*
+ qr{} Pattern yes*
+ s{}{} Substitution yes*
tr{}{} Transliteration no (but see below)
+ <<EOF here-doc yes*
+
+ * unless the delimiter is ''.
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
$s = q{ if($a eq "}") ... }; # WRONG
-is a syntax error. The C<Text::Balanced> module on CPAN is able to do this
-properly.
+is a syntax error. The C<Text::Balanced> module (from CPAN, and
+starting from Perl 5.8 part of the standard distribution) is able
+to do this properly.
There can be whitespace between the operator and the quoting
characters, except when C<#> is being used as the quoting character.
s {foo} # Replace foo
{bar} # with bar.
-For constructs that do interpolate, variables beginning with "C<$>"
-or "C<@>" are interpolated, as are the following escape sequences. Within
-a transliteration, the first eleven of these sequences may be used.
+The following escape sequences are available in constructs that interpolate
+and in transliterations.
\t tab (HT, TAB)
\n newline (NL)
\x1b hex char (ESC)
\x{263a} wide hex char (SMILEY)
\c[ control char (ESC)
- \N{name} named char
+ \N{name} named Unicode character
+
+B<NOTE>: Unlike C and other languages, Perl has no \v escape sequence for
+the vertical tab (VT - ASCII 11).
+
+The following escape sequences are available in constructs that interpolate
+but not in transliterations.
\l lowercase next char
\u uppercase next char
\E end case modification
\Q quote non-word characters till \E
-If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u>
-and C<\U> is taken from the current locale. See L<perllocale>. For
-documentation of C<\N{name}>, see L<charnames>.
+If C<use locale> is in effect, the case map used by C<\l>, C<\L>,
+C<\u> and C<\U> is taken from the current locale. See L<perllocale>.
+If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or
+beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and
+C<\U> is as defined by Unicode. For documentation of C<\N{name}>,
+see L<charnames>.
All systems use the virtual C<"\n"> to represent a line terminator,
called a "newline". There is no such thing as an unvarying, physical
printing C<"\n"> may emit no actual data. In general, use C<"\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 (C<"\012\015"> or C<"\cJ\cM">) for line terminators,
+and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
and although they often accept just C<"\012">, they seldom tolerate just
C<"\015">. If you get in the habit of using C<"\n"> for networking,
you may be burned some day.
+For constructs that do interpolate, variables beginning with "C<$>"
+or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
+C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
+But method calls such as C<< $obj->meth >> are not.
+
+Interpolating an array or slice interpolates the elements in order,
+separated by the value of C<$">, so is equivalent to interpolating
+C<join $", @array>. "Punctuation" arrays such as C<@+> are only
+interpolated if the name is enclosed in braces C<@{+}>.
+
You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
An unescaped C<$> or C<@> interpolates the corresponding variable,
while escaping will cause the literal string C<\$> to be inserted.
reset if eof; # clear ?? status for next file
}
-This usage is vaguely depreciated, which means it just might possibly
+This usage is vaguely deprecated, which means it just might possibly
be removed in some distant future version of Perl, perhaps somewhere
around the year 2168.
PATTERN may contain variables, which will be interpolated (and the
pattern recompiled) every time the pattern search is evaluated, except
-for when the delimiter is a single quote. (Note that C<$)> and C<$|>
-might not be interpolated because they look like end-of-string tests.)
+for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
+C<$|> are not interpolated because they look like end-of-string tests.)
If you want such a pattern to be compiled only once, add a C</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 C</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 L<"qr//">.
+Perl won't even notice. See also L<"qr/STRING/imosx">.
If the PATTERN evaluates to the empty string, the last
-I<successfully> matched regular expression is used instead.
+I<successfully> matched regular expression is used instead. In this
+case, only the C<g> and C<c> flags on the empty pattern is honoured -
+the other flags are taken from the original pattern. If no match has
+previously succeeded, this will (silently) act instead as a genuine
+empty pattern (which will always match).
+
+Note that it's possible to confuse Perl into thinking C<//> (the empty
+regex) is really C<//> (the defined-or operator). Perl is usually pretty
+good about this, but some pathological cases might trigger this, such as
+C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //>
+(C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl
+will assume you meant defined-or. If you meant the empty regex, just
+use parentheses or spaces to disambiguate, or even prefix the empty
+regex with an C<m> (so C<//> becomes C<m//>).
If the C</g> option is not used, C<m//> in list context returns a
list consisting of the subexpressions matched by the parentheses in the
You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
zero-width assertion that matches the exact position where the previous
-C<m//g>, if any, left off. The C<\G> assertion is not supported without
-the C</g> modifier. (Currently, without C</g>, C<\G> behaves just like
-C<\A>, but that's accidental and may change in the future.)
+C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion
+still anchors at pos(), but the match is of course only attempted once.
+Using C<\G> without C</g> on a target string that has not previously had a
+C</g> match applied to it is the same as using the C<\A> assertion to match
+the beginning of the string. Note also that, currently, C<\G> is only
+properly supported when anchored at the very beginning of the pattern.
Examples:
($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
# scalar context
- $/ = ""; $* = 1; # $* deprecated in modern perls
+ $/ = "";
while (defined($paragraph = <>)) {
while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
$sentences++;
print "3: '";
print $1 while /(p)/gc; print "', pos=", pos, "\n";
}
+ print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
The last example should print:
1: '', pos=7
2: 'q', pos=8
3: '', pos=8
+ Final: 'q', pos=8
+
+Notice that the final match matched C<q> instead of C<p>, which a match
+without the C<\G> anchor would have done. Also note that the final match
+did not update C<pos> -- C<pos> is only updated on a C</g> match. If the
+final match did indeed match C<p>, it's a good bet that you're running an
+older (pre-5.6.0) Perl.
A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
combine several regexps like this to process a string part-by-part,
=item qr/STRING/imosx
-This operators quotes--and compiles--its I<STRING> as a regular
+This operator quotes (and possibly compiles) its I<STRING> as a regular
expression. I<STRING> is interpolated the same way as I<PATTERN>
in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
is done. Returns a Perl value which may be used instead of the
=item `STRING`
-A string which is (possibly) interpolated and then executed as a system
-command with C</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).
+A string which is (possibly) interpolated and then executed as a
+system command with C</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, or undef if the command failed. In list context, returns a
+list of lines (however you've defined lines with $/ or
+$INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
Because backticks do not affect standard error, use shell file descriptor
syntax (assuming the shell supports this) if you care to address this.
$output = `cmd 3>&1 1>&2 2>&3 3>&-`;
To read both a command's STDOUT and its STDERR separately, it's easiest
-and safest to redirect them separately to files, and then read from those
-files when the program is done:
+to redirect them separately to files, and then read from those files
+when the program is done:
- system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");
+ system("program args 1>program.stdout 2>program.stderr");
Using single-quote as a delimiter protects the command from Perl's
double-quote interpolation, passing it on to the shell instead:
separator character, if your shell supports that (e.g. C<;> on many Unix
shells; C<&> on the Windows NT C<cmd> shell).
+Beginning with v5.6.0, Perl will attempt to flush all files opened for
+output before starting the child process, but this may not be supported
+on some platforms (see L<perlport>). To be safe, you may need to set
+C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
+C<IO::Handle> on any open handles.
+
Beware that some command shells may place restrictions on the length
of the command line. You must ensure your strings don't exceed this
limit after any necessary interpolations. See the platform-specific
split(' ', q/STRING/);
-the difference being that it generates a real list at compile time. So
+the differences being that it generates a real list at compile time, and
+in scalar context it returns the last element in the list. So
this expression:
qw(foo bar baz)
A common mistake is to try to separate the words with comma or to
put comments into a multi-line C<qw>-string. For this reason, the
-B<-w> switch (that is, the C<$^W> variable) produces warnings if
-the STRING contains the "," or the "#" character.
+C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
+produces warnings if the STRING contains the "," or the "#" character.
=item s/PATTERN/REPLACEMENT/egimosx
PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
pair of quotes, which may or may not be bracketing quotes, e.g.,
C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the
-replacement portion to be interpreted as a full-fledged Perl expression
-and eval()ed right then and there. It is, however, syntax checked at
-compile-time.
+replacement portion to be treated as a full-fledged Perl expression
+and evaluated right then and there. It is, however, syntax checked at
+compile-time. A second C<e> modifier will cause the replacement portion
+to be C<eval>ed before being run as a Perl expression.
Examples:
# symbolic dereferencing
s/\$(\w+)/${$1}/g;
- # /e's can even nest; this will expand
- # any embedded scalar variable (including lexicals) in $_
+ # 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+)/$1/eeg;
# Delete (most) C comments.
# expand tabs to 8-column spacing
1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
-=item tr/SEARCHLIST/REPLACEMENTLIST/cdsUC
+=item tr/SEARCHLIST/REPLACEMENTLIST/cds
-=item y/SEARCHLIST/REPLACEMENTLIST/cdsUC
+=item y/SEARCHLIST/REPLACEMENTLIST/cds
Transliterates all occurrences of the characters found in the search list
with the corresponding character in the replacement list. It returns
its own pair of quotes, which may or may not be bracketing quotes,
e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
+Note that C<tr> does B<not> do regular expression character classes
+such as C<\d> or C<[:lower:]>. The <tr> operator is not equivalent to
+the tr(1) utility. If you want to map strings between lower/upper
+cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider
+using the C<s> operator if you need regular expressions.
+
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
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</c> modifier is specified, the SEARCHLIST character set
is complemented. If the C</d> modifier is specified, any characters
This latter is useful for counting characters in a class or for
squashing character sequences in a class.
-The first C</U> or C</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:
$ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
tr [\200-\377]
[\000-\177]; # delete 8th bit
- tr/\0-\xFF//CU; # change Latin-1 to Unicode
- tr/\0-\x{FF}//UC; # change Unicode to Latin-1
-
If multiple transliterations are given for a character, only the
first one is used:
eval "tr/$oldlist/$newlist/, 1" or die $@;
+=item <<EOF
+
+A line-oriented form of quoting is based on the shell "here-document"
+syntax. Following a C<< << >> you specify a string to terminate
+the quoted material, and all lines following the current line down to
+the terminating string are the value of the item. The terminating
+string may be either an identifier (a word), or some quoted text. If
+quoted, the type of quotes you use determines the treatment of the
+text, just as in regular quoting. An unquoted identifier works like
+double quotes. There must be no space between the C<< << >> and
+the identifier, unless the identifier is quoted. (If you put a space it
+will be treated as a null identifier, which is valid, and matches the first
+empty line.) The terminating string must appear by itself (unquoted and
+with no surrounding whitespace) on the terminating line.
+
+ print <<EOF;
+ The price is $Price.
+ EOF
+
+ print << "EOF"; # same as above
+ The price is $Price.
+ EOF
+
+ print << `EOC`; # execute commands
+ echo hi there
+ echo lo there
+ EOC
+
+ print <<"foo", <<"bar"; # you can stack them
+ I said foo.
+ foo
+ I said bar.
+ bar
+
+ myfunc(<< "THIS", 23, <<'THAT');
+ Here's a line
+ or two.
+ THIS
+ and here's another.
+ THAT
+
+Just don't forget that you have to put a semicolon on the end
+to finish the statement, as Perl doesn't know you're not going to
+try to do this:
+
+ print <<ABC
+ 179231
+ ABC
+ + 20;
+
+If you want your here-docs to be indented with the
+rest of the code, you'll need to remove leading whitespace
+from each line manually:
+
+ ($quote = <<'FINIS') =~ s/^\s+//gm;
+ The Road goes ever on and on,
+ down from the door where it began.
+ FINIS
+
+If you use a here-doc within a delimited construct, such as in C<s///eg>,
+the quoted material must come on the lines following the final delimiter.
+So instead of
+
+ s/this/<<E . 'that'
+ the other
+ E
+ . 'more '/eg;
+
+you have to write
+
+ s/this/<<E . 'that'
+ . 'more '/eg;
+ the other
+ E
+
+If the terminating identifier is on the last line of the program, you
+must be sure there is a newline after it; otherwise, Perl will give the
+warning B<Can't find string terminator "END" anywhere before EOF...>.
+
+Additionally, the quoting rules for the identifier are not related to
+Perl's quoting rules -- C<q()>, C<qq()>, and the like are not supported
+in place of C<''> and C<"">, and the only interpolation is for backslashing
+the quoting character:
+
+ print << "abc\"def";
+ testing...
+ abc"def
+
+Finally, quoted strings cannot span multiple lines. The general rule is
+that the identifier must be a string literal. Stick with that, and you
+should be safe.
+
=back
=head2 Gory details of parsing quoted constructs
quoting constructs, Perl performs different numbers of passes, from
one to five, but these passes are always performed in the same order.
-=over
+=over 4
=item Finding the end
The next step is interpolation in the text obtained, which is now
delimiter-independent. There are four different cases.
-=over
+=over 4
=item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///>
may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
Interpolated scalars and arrays are converted internally to the C<join> and
-C<.> catentation operations. Thus, C<"$foo XXX '@arr'"> becomes:
+C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
$foo . " XXX '" . (join $", @arr) . "'";
It is at this step that C<\1> is begrudgingly converted to C<$1> in
the replacement text of C<s///> to correct the incorrigible
I<sed> hackers who haven't picked up the saner idiom yet. A warning
-is emitted if the B<-w> command-line flag (that is, the C<$^W> variable)
-was set.
+is emitted if the C<use warnings> pragma or the B<-w> command-line flag
+(that is, the C<$^W> variable) was set.
The lack of processing of C<\\> creates specific restrictions on
the post-processed text. If the delimiter is C</>, one cannot get
In the RE above, which is intentionally obfuscated for illustration, the
delimiter is C<m>, the modifier is C<mx>, and after backslash-removal the
-RE is the same as for C<m/ ^ a s* b /mx>). There's more than one
+RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
reason you're encouraged to restrict your delimiters to non-alphanumeric,
non-whitespace choices.
It is possible to inspect both the string given to RE engine and the
resulting finite automaton. See the arguments C<debug>/C<debugcolor>
in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
-switch documented in L<perlrun/Switches>.
+switch documented in L<perlrun/"Command Switches">.
=item Optimization of regular expressions
A string enclosed by backticks (grave accents) first undergoes
double-quote interpolation. It is then interpreted as an external
command, and the output of that command is the value of the
-pseudo-literal, j
-string consisting of all output is returned. In list context, a
-list of values is returned, one per line of output. (You can set
-C<$/> to use a different line terminator.) The command is executed
-each time the pseudo-literal is evaluated. The status value of the
-command is returned in C<$?> (see L<perlvar> for the interpretation
-of C<$?>). Unlike in B<csh>, no translation is done on the return
-data--newlines remain newlines. Unlike in any of the shells, single
-quotes do not hide variable names in the command from interpretation.
-To pass a literal dollar-sign through to the shell you need to hide
-it with a backslash. The generalized form of backticks is C<qx//>.
-(Because backticks always undergo shell expansion as well, see
-L<perlsec> for security concerns.)
+backtick string, like in a shell. In scalar context, a single string
+consisting of all output is returned. In list context, a list of
+values is returned, one per line of output. (You can set C<$/> to use
+a different line terminator.) The command is executed each time the
+pseudo-literal is evaluated. The status value of the command is
+returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
+Unlike in B<csh>, no translation is done on the return data--newlines
+remain newlines. Unlike in any of the shells, single quotes do not
+hide variable names in the command from interpretation. To pass a
+literal dollar-sign through to the shell you need to hide it with a
+backslash. The generalized form of backticks is C<qx//>. (Because
+backticks always undergo shell expansion as well, see L<perlsec> for
+security concerns.)
In scalar context, evaluating a filehandle in angle brackets yields
the next line from that file (the newline, if any, included), or
the value is automatically assigned to the global variable $_,
destroying whatever was there previously. (This may seem like an
odd thing to you, but you'll use the construct in almost every Perl
-script you write.) The $_ variables is not implicitly localized.
+script you write.) The $_ variable is not implicitly localized.
You'll have to put a C<local $_;> before the loop if you want that
to happen.
while (<STDIN>) { last unless $_; ... }
In other boolean contexts, C<< <I<filehandle>> >> without an
-explicit C<defined> test or comparison elicit a warning if the B<-w>
+explicit C<defined> test or comparison elicit a warning if the
+C<use warnings> pragma or the B<-w>
command-line switch (the C<$^W> variable) is in effect.
The filehandles STDIN, STDOUT, and STDERR are predefined. (The
If you call it again after this, it will assume you are processing another
@ARGV list, and if you haven't set @ARGV, will read input from STDIN.
-If angle brackets contain is a simple scalar variable (e.g.,
+If what the angle brackets contain is a simple scalar variable (e.g.,
<$foo>), then that variable contains the name of the
filehandle to input from, or its typeglob, or a reference to the
same. For example:
open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
while (<FOO>) {
- chop;
+ chomp;
chmod 0644, $_;
}
starting a new list. All values must be read before it will start
over. In list context, this isn't important because you automatically
get them all anyway. However, in scalar context the operator returns
-the next value each time it's called, or C
+the next value each time it's called, or C<undef> when the list has
run out. As with filehandle reads, an automatic C<defined> is
generated when the glob occurs in the test part of a C<while>,
because legal glob returns (e.g. a file called F<0>) would otherwise
because the latter will alternate between returning a filename and
returning false.
-It you're trying to do variable interpolation, it's definitely better
+If you're trying to do variable interpolation, it's definitely better
to use the glob() function, because the older notation can cause people
to become confused with the indirect filehandle notation.
or so.
Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
-and ">>") always produce integral results. (But see also L<Bitwise
-String Operators>.) However, C<use integer> still has meaning for
+and ">>") always produce integral results. (But see also
+L<Bitwise String Operators>.) However, C<use integer> still has meaning for
them. By default, their results are interpreted as unsigned integers, but
if C<use integer> is in effect, their results are interpreted
as signed integers. For example, C<~0> usually evaluates to a large
The standard Math::BigInt and Math::BigFloat modules provide
variable-precision arithmetic and overloaded operators, although
-they're currently pretty slow. At the cost of some space and
+they're currently pretty slow. At the cost of some space and
considerable speed, they avoid the normal pitfalls associated with
limited-precision representations.
# prints +15241578780673678515622620750190521
-The non-standard modules SSLeay::BN and Math::Pari provide
-equivalent functionality (and much more) with a substantial
-performance savings.
+There are several modules that let you calculate with (bound only by
+memory and cpu-time) unlimited or fixed precision. There are also
+some non-standard modules that provide faster implementations via
+external C libraries.
+
+Here is a short, but incomplete summary:
+
+ Math::Fraction big, unlimited fractions like 9973 / 12967
+ Math::String treat string sequences like numbers
+ Math::FixedPrecision calculate with a fixed precision
+ Math::Currency for currency calculations
+ Bit::Vector manipulate bit vectors fast (uses C)
+ Math::BigIntFast Bit::Vector wrapper for big numbers
+ Math::Pari provides access to the Pari C library
+ Math::BigInteger uses an external C library
+ Math::Cephes uses external Cephes C library (no big numbers)
+ Math::Cephes::Fraction fractions via the Cephes library
+ Math::GMP another one using an external C library
+
+Choose wisely.
=cut
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