=head1 SYNOPSIS
Perl operators have the following associativity and precedence,
-listed from highest precedence to lowest. Note that all 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.
+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 =~ !~
left * / % x
left + - .
left << >>
left | ^
left &&
left ||
- nonassoc ..
+ nonassoc .. ...
right ?:
right = += -= *= etc.
left , =>
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)
-Any TERM is of highest precedence of Perl. These includes variables,
+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
In the absence of parentheses, the precedence of list operators such as
C<print>, C<sort>, or C<chmod> is either very high or very low depending on
-whether you look at the left side of operator or the right side of it.
+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 the arguments that follow them, and
+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.
-Note that you have to be careful with parentheses:
+Be careful with parentheses:
# These evaluate exit before doing the print:
print($foo, exit); # Obviously not what you want.
print ($foo & 255) + 1, "\n";
-probably doesn't do what you expect at first glance. See
+probably doesn't do what you expect at first glance. 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
+well as subroutine and method calls, and the anonymous
constructors C<[]> and C<{}>.
-See also L<Quote and Quote-Like Operators> toward the end of this section,
+See also L<Quote and Quote-like Operators> toward the end of this section,
as well as L<"I/O Operators">.
=head2 The Arrow Operator
-Just as in C and C++, "C<-E<gt>>" is an infix dereference operator. If the
-right side is either a C<[...]> or C<{...}> subscript, then the left side
-must be either a hard or symbolic reference to an array or hash (or
-a location capable of holding a hard reference, if it's an lvalue (assignable)).
-See L<perlref>.
+"C<-E<gt>>" is an infix dereference operator, just as it is in C
+and C++. If the right side is either a C<[...]>, C<{...}>, or a
+C<(...)> 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 L<perlreftut> and L<perlref>.
-Otherwise, the right side is a method name or a simple scalar variable
-containing 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 L<perlobj>.
+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 L<perlobj>.
=head2 Auto-increment and Auto-decrement
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 built-in magic to it. If
+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 null and matches the pattern
+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
character within its range, with carry:
=head2 Exponentiation
-Binary "**" is the exponentiation operator. Note that it binds even more
+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.)
is returned. One effect of these rules is that C<-bareword> is equivalent
to C<"-bareword">.
-Unary "~" performs bitwise negation, i.e., 1's complement.
-(See also L<Integer Arithmetic>.)
+Unary "~" performs bitwise negation, i.e., 1's complement. For example,
+C<0666 &~ 027> is 0640. (See also L<Integer Arithmetic> and L<Bitwise
+String Operators>.)
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 L<Terms and List Operators (Leftward)>.)
-Unary "\" creates a reference to whatever follows it. See L<perlref>.
-Do not confuse this behavior with the behavior of backslash within a
-string, although both forms do convey the notion of protecting the next
-thing from interpretation.
+Unary "\" creates a reference to whatever follows it. See L<perlreftut>
+and L<perlref>. Do not confuse this behavior with the behavior of
+backslash within a string, although both forms do convey the notion
+of protecting the next thing from interpolation.
=head2 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 translation. The left argument is what is
-supposed to be searched, substituted, or translated instead of the default
+pattern, substitution, or transliteration. The left argument is what is
+supposed to be searched, substituted, or transliterated instead of the default
$_. The return value indicates the success of the operation. (If the
right argument is an expression rather than a search pattern,
-substitution, or translation, it is interpreted as a search pattern at run
-time. This is less efficient than an explicit search, because the pattern
-must be compiled every time the expression is evaluated--unless you've
-used C</o>.)
+substitution, or transliteration, it is interpreted as a search pattern at run
+time. This can be is less efficient than an explicit search, because the
+pattern must be compiled every time the expression is evaluated).
Binary "!~" is just like "=~" except the return value is negated in
the logical sense.
Binary "/" divides two numbers.
-Binary "%" computes the modulus of the two numbers.
-
-Binary "x" is the repetition operator. In a scalar context, it
-returns a string consisting of the left operand repeated the number of
-times specified by the right operand. In a list context, if the left
-operand is a list in parentheses, it repeats the list.
+Binary "%" computes the modulus of two numbers. Given integer
+operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
+C<$a> minus the largest multiple of C<$b> that is not greater than
+C<$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
+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
=head2 Bitwise And
Binary "&" returns its operators ANDed together bit by bit.
-(See also L<Integer Arithmetic>.)
+(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
=head2 Bitwise Or and Exclusive Or
Binary "|" returns its operators ORed together bit by bit.
-(See also L<Integer Arithmetic>.)
+(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
Binary "^" returns its operators XORed together bit by bit.
-(See also L<Integer Arithmetic>.)
+(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
=head2 C-style Logical And
$home = $ENV{'HOME'} || $ENV{'LOGDIR'} ||
(getpwuid($<))[7] || die "You're homeless!\n";
-As more readable alternatives to C<&&> and C<||>, 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:
+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 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
+list operator without the need for parentheses:
unlink "alpha", "beta", "gamma"
or gripe(), next LINE;
unlink("alpha", "beta", "gamma")
|| (gripe(), next LINE);
-=head2 Range Operator
+Use "or" for assignment is unlikely to do what you want; see below.
+
+=head2 Range Operators
Binary ".." is the range operator, which is really two different
-operators depending on the context. In a list context, it returns an
-array of values counting (by ones) from the left value to the right
-value. This is useful for writing C<for (1..10)> loops and for doing
-slice operations on arrays. Be aware that under the current implementation,
-a temporary array is created, so you'll burn a lot of memory if you
-write something like this:
+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
+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
+like this:
for (1 .. 1_000_000) {
# code
- }
+ }
-In a scalar context, ".." returns a boolean value. The operator is
+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
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, I<AFTER> which the range operator becomes false
-again. (It doesn't become false till the next time the range operator is
+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 B<awk>), but it still returns true once.
-If you don't want it to test the right operand till the next evaluation
-(as in B<sed>), use three dots ("...") instead of two.) 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 null 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 numeric literal,
-that operand is implicitly compared to the C<$.> variable, the current
-line number. Examples:
+If you don't want it to test the right operand till the next
+evaluation, as in B<sed>, just use three dots ("...") instead of
+two. In all other regards, "..." behaves just like ".." does.
+
+The right operand is not evaluated while the operator is in the
+"false" state, and the left operand is not evaluated while the
+operator is in the "true" state. The precedence is a little lower
+than || and &&. The value returned is either the empty string for
+false, or a sequence number (beginning with 1) for true. The
+sequence number is reset for each range encountered. The final
+sequence number in a range has the string "E0" appended to it, which
+doesn't affect its numeric value, but gives you something to search
+for if you want to exclude the endpoint. You can exclude the
+beginning point by waiting for the sequence number to be greater
+than 1. If either operand of scalar ".." is a constant expression,
+that operand is implicitly compared to the C<$.> variable, the
+current line number. Examples:
As a scalar operator:
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 $_ 100 times
- @foo = @foo[$[ .. $#foo]; # an expensive no-op
+ @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 magical
+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 the letters of the alphabet, or
+to get all normal letters of the alphabet, or
$hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
argument before the : is returned, otherwise the argument after the :
is returned. For example:
- printf "I have %d dog%s.\n", $n,
+ 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.
+or 3rd argument, whichever is selected.
$a = $ok ? $b : $c; # get a scalar
@a = $ok ? @b : @c; # get an array
($a_or_b ? $a : $b) = $c;
-This is not necessarily guaranteed to contribute to the readability of your program.
+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;
=head2 Assignment Operators
$a = $a + 2;
although without duplicating any side effects that dereferencing the lvalue
-might trigger, such as from tie(). Other assignment operators work similarly.
-The following are recognized:
+might trigger, such as from tie(). Other assignment operators work similarly.
+The following are recognized:
**= += *= &= <<= &&=
-= /= |= >>= ||=
.= %= ^=
x=
-Note that while these are grouped by family, they all have the precedence
+Although these are grouped by family, they all have the precedence
of assignment.
-Unlike in C, the assignment operator produces a valid lvalue. Modifying
-an assignment is equivalent to doing the assignment and then modifying
-the variable that was assigned to. This is useful for modifying
-a copy of something, like this:
+Unlike in C, the scalar assignment operator produces a valid lvalue.
+Modifying an assignment is equivalent to doing the assignment and
+then modifying the variable that was assigned to. This is useful
+for modifying a copy of something, like this:
($tmp = $global) =~ tr [A-Z] [a-z];
$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.
+
=head2 Comma Operator
-Binary "," is the comma operator. In a scalar context it evaluates
+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 a list context, it's just the list argument separator, and inserts
+In list context, it's just the list argument separator, and inserts
both its arguments into the list.
The =E<gt> digraph is mostly just a synonym for the comma operator. It's useful for
=head2 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 means that it short-circuits: i.e., the right
-expression is evaluated only if the left expression is false.
+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.
=item unary *
-Dereference-address operator. (Perl's prefix dereferencing
+Dereference-address operator. (Perl's prefix dereferencing
operators are typed: $, @, %, and &.)
=item (TYPE)
-Type casting operator.
+Type-casting operator.
=back
pattern matching capabilities. Perl provides customary quote characters
for these behaviors, but also provides a way for you to choose your
quote character for any of them. In the following table, a C<{}> represents
-any pair of delimiters you choose. Non-bracketing delimiters use
-the same character fore and aft, but the 4 sorts of brackets
-(round, angle, square, curly) will all nest.
-
- Customary Generic Meaning Interpolates
- '' q{} Literal no
- "" qq{} Literal yes
- `` qx{} Command yes
- qw{} Word list no
- // m{} Pattern match yes
- s{}{} Substitution yes
- tr{}{} Translation no
-
-For constructs that do interpolation, variables beginning with "C<$>" or "C<@>"
-are interpolated, as are the following sequences:
+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 C<Text::Balanced> module on CPAN 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.
+C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
+operator C<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 "C<$>"
+or "C<@>" are interpolated, as are the following escape sequences. Within
+a transliteration, the first eleven of these sequences may be used.
\t tab (HT, TAB)
- \n newline (LF, NL)
+ \n newline (NL)
\r return (CR)
\f form feed (FF)
\b backspace (BS)
\a alarm (bell) (BEL)
\e escape (ESC)
- \033 octal char
- \x1b hex char
- \c[ control char
+ \033 octal char (ESC)
+ \x1b hex char (ESC)
+ \x{263a} wide hex char (SMILEY)
+ \c[ control char (ESC)
+ \N{name} named char
+
\l lowercase next char
\u uppercase next char
\L lowercase till \E
\U uppercase till \E
\E end case modification
- \Q quote regexp metacharacters till \E
+ \Q quote non-word characters till \E
If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u>
-and <\U> is taken from the current locale. See L<perllocale>.
+and C<\U> is taken from the current locale. See L<perllocale>. For
+documentation of C<\N{name}>, see L<charnames>.
+
+All systems use the virtual C<"\n"> to represent a line terminator,
+called a "newline". There is no such thing as an unvarying, physical
+newline character. It is only an illusion that the operating system,
+device drivers, C libraries, and Perl all conspire to preserve. Not all
+systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
+on a Mac, these are reversed, and on systems without line terminator,
+printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
+you mean a "newline" for your system, but use the literal ASCII when you
+need an exact character. For example, most networking protocols expect
+and prefer a CR+LF (C<"\012\015"> or C<"\cJ\cM">) for line terminators,
+and although they often accept just C<"\012">, they seldom tolerate just
+C<"\015">. If you get in the habit of using C<"\n"> for networking,
+you may be burned some day.
+
+You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
+An unescaped C<$> or C<@> interpolates the corresponding variable,
+while escaping will cause the literal string C<\$> to be inserted.
+You'll need to write something like C<m/\Quser\E\@\Qhost/>.
Patterns are subject to an additional level of interpretation as a
regular expression. This is done as a second pass, after variables are
pattern from the variables. If this is not what you want, use C<\Q> to
interpolate a variable literally.
-Apart from the above, there are no multiple levels of interpolation. In
-particular, contrary to the expectations of shell programmers, back-quotes
-do I<NOT> interpolate within double quotes, nor do single quotes impede
-evaluation of variables when used within double quotes.
+Apart from the behavior described above, Perl does not expand
+multiple levels of interpolation. In particular, contrary to the
+expectations of shell programmers, back-quotes do I<NOT> interpolate
+within double quotes, nor do single quotes impede evaluation of
+variables when used within double quotes.
=head2 Regexp Quote-Like Operators
something in each file of a set of files, for instance. Only C<??>
patterns local to the current package are reset.
-This usage is vaguely deprecated, and may be removed in some future
-version of Perl.
+ while (<>) {
+ if (?^$?) {
+ # 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.
-=item m/PATTERN/gimosx
+=item m/PATTERN/cgimosx
-=item /PATTERN/gimosx
+=item /PATTERN/cgimosx
-Searches a string for a pattern match, and in a scalar context returns
-true (1) or false (''). If no string is specified via the C<=~> or
-C<!~> operator, the $_ string is searched. (The string specified with
-C<=~> need not be an lvalue--it may be the result of an expression
-evaluation, but remember the C<=~> binds rather tightly.) See also
-L<perlre>.
-See L<perllocale> for discussion of additional considerations which apply
-when C<use locale> is in effect.
+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 C<=~> or C<!~> operator, the $_ string is searched. (The
+string specified with C<=~> need not be an lvalue--it may be the
+result of an expression evaluation, but remember the C<=~> binds
+rather tightly.) See also L<perlre>. See L<perllocale> for
+discussion of additional considerations that apply when C<use locale>
+is in effect.
Options are:
+ c Do not reset search position on a failed match when /g is in effect.
g Match globally, i.e., find all occurrences.
i Do case-insensitive pattern matching.
m Treat string as multiple lines.
x Use extended regular expressions.
If "/" is the delimiter then the initial C<m> is optional. With the C<m>
-you can use any pair of non-alphanumeric, non-whitespace characters as
-delimiters. This is particularly useful for matching Unix path names
-that contain "/", to avoid LTS (leaning toothpick syndrome).
+you can use any pair of non-alphanumeric, non-whitespace characters
+as delimiters. This is particularly useful for matching path names
+that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
+the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
+If "'" is the delimiter, no interpolation is performed on the PATTERN.
PATTERN may contain variables, which will be interpolated (and the
-pattern recompiled) every time the pattern search is evaluated. (Note
-that C<$)> and C<$|> might not be interpolated because they look like
-end-of-string tests.) If you want such a pattern to be compiled only
-once, add a C</o> after the trailing delimiter. This avoids expensive
-run-time recompilations, and is useful when the value you are
-interpolating won't change over the life of the script. However, mentioning
-C</o> constitutes a promise that you won't change the variables in the pattern.
-If you change them, Perl won't even notice.
-
-If the PATTERN evaluates to a null string, the last
-successfully executed regular expression is used instead.
-
-If used in a context that requires a list value, a pattern match returns a
+pattern recompiled) every time the pattern search is evaluated, except
+for when the delimiter is a single quote. (Note that C<$)> and C<$|>
+might not be interpolated because they look like end-of-string tests.)
+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//>.
+
+If the PATTERN evaluates to the empty string, the last
+I<successfully> matched regular expression is used instead.
+
+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
-pattern, i.e., (C<$1>, $2, $3...). (Note that here $1 etc. are also set, and
-that this differs from Perl 4's behavior.) If the match fails, a null
-array is returned. If the match succeeds, but there were no parentheses,
-a list value of (1) is returned.
+pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$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 C<(1)> for
+success. With or without parentheses, an empty list is returned upon
+failure.
Examples:
$Etc. The conditional is true if any variables were assigned, i.e., if
the pattern matched.
-The C</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 a list context, it returns a list of all the
-substrings matched by all the 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.
+The C</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 C<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 L<perlfunc/pos>. A failed match normally resets the
+search position to the beginning of the string, but you can avoid that
+by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
+string also resets the search position.
+
+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.)
-In a scalar context, C<m//g> iterates through the string, returning TRUE
-each time it matches, and FALSE when it eventually runs out of
-matches. (In other words, it remembers where it left off last time and
-restarts the search at that point. You can actually find the current
-match position of a string or set it using the pos() function--see
-L<perlfunc/pos>.) Note that you can use this feature to stack C<m//g>
-matches or intermix C<m//g> matches with C<m/\G.../>.
-
-If you modify the string in any way, the match position is reset to the
-beginning. Examples:
+Examples:
# list context
($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
# scalar context
$/ = ""; $* = 1; # $* deprecated in modern perls
- while ($paragraph = <>) {
+ while (defined($paragraph = <>)) {
while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
$sentences++;
}
}
print "$sentences\n";
- # using m//g with \G
- $_ = "ppooqppq";
+ # using m//gc with \G
+ $_ = "ppooqppqq";
while ($i++ < 2) {
print "1: '";
- print $1 while /(o)/g; print "', pos=", pos, "\n";
+ print $1 while /(o)/gc; print "', pos=", pos, "\n";
print "2: '";
- print $1 if /\G(q)/; print "', pos=", pos, "\n";
+ print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
print "3: '";
- print $1 while /(p)/g; print "', pos=", pos, "\n";
+ print $1 while /(p)/gc; print "', pos=", pos, "\n";
}
The last example should print:
1: 'oo', pos=4
- 2: 'q', pos=4
+ 2: 'q', pos=5
3: 'pp', pos=7
1: '', pos=7
- 2: 'q', pos=7
- 3: '', pos=7
+ 2: 'q', pos=8
+ 3: '', pos=8
-Note how C<m//g> matches change the value reported by C<pos()>, but the
-non-global match doesn't.
-
-A useful idiom for C<lex>-like scanners is C</\G.../g>. You can
+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,
-doing different actions depending on which regexp matched. The next
-regexp would step in at the place the previous one left off.
+doing different actions depending on which regexp matched. Each
+regexp tries to match where the previous one leaves off.
- $_ = <<'EOL';
+ $_ = <<'EOL';
$url = new URI::URL "http://www/"; die if $url eq "xXx";
-EOL
- LOOP:
+ EOL
+ LOOP:
{
- print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/g;
- print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/g;
- print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/g;
- print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/g;
- print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/g;
- print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/g;
- print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/g;
+ 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";
}
=item C<'STRING'>
-A single-quoted, literal string. Backslashes are ignored, unless
-followed by the delimiter or another backslash, in which case the
-delimiter or backslash is interpolated.
+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
=item qq/STRING/
$_ .= qq
(*** The previous line contains the naughty word "$1".\n)
- if /(tcl|rexx|python)/; # :-)
+ if /\b(tcl|java|python)\b/i; # :-)
+ $baz = "\n"; # a one-character string
+
+=item qr/STRING/imosx
+
+This operators quotes--and 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
+corresponding C</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/;
+ }
+ $success;
+ } @_;
+ }
+
+Precompilation of the pattern into an internal representation at
+the moment of qr() avoids a need to recompile the pattern every
+time a match C</$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 L<perlre> for additional information on valid syntax for STRING, and
+for a detailed look at the semantics of regular expressions.
=item qx/STRING/
=item `STRING`
-A string which is interpolated and then executed as a system command.
-The collected standard output of the command is returned. 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. 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`;
- $today = qx{ date };
+To capture a command's STDOUT but discard its STDERR:
-See L<I/O Operators> for more discussion.
+ $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 L<perlsec> 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. C<;> on many Unix
+shells; C<&> on the Windows NT C<cmd> shell).
+
+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 C<type> command under
+the POSIX shell is very different from the C<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 L<"I/O Operators"> for more discussion.
=item qw/STRING/
-Returns a list of the words extracted out of STRING, using embedded
-whitespace as the word delimiters. It is exactly equivalent to
+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 exactly 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 C<qw>-string. For this reason, the
+B<-w> switch (that is, the C<$^W> variable) produces warnings if
+the STRING contains the "," or the "#" character.
+
=item s/PATTERN/REPLACEMENT/egimosx
Searches a string for a pattern, and if found, replaces that pattern
If no string is specified via the C<=~> or C<!~> operator, the C<$_>
variable is searched and modified. (The string specified with C<=~> must
-be a scalar variable, an array element, a hash element, or an assignment
+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 single quote, no variable interpolation is
+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 C</o> option. If the pattern
-evaluates to a null string, the last successfully executed regular
+evaluates to the empty string, the last successfully executed regular
expression is used instead. See L<perlre> for further explanation on these.
-See L<perllocale> for discussion of additional considerations which apply
+See L<perllocale> for discussion of additional considerations that apply
when C<use locale> is in effect.
Options are:
Any non-alphanumeric, non-whitespace delimiter may replace the
slashes. If single quotes are used, no interpretation is done on the
replacement string (the C</e> modifier overrides this, however). Unlike
-Perl 4, Perl 5 treats back-ticks as normal delimiters; the replacement
+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.,
C<s(foo)(bar)> or C<sE<lt>fooE<gt>/bar/>. A C</e> will cause the
-replacement portion to be interpreter as a full-fledged Perl expression
+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.
s/Login: $foo/Login: $bar/; # run-time pattern
- ($foo = $bar) =~ s/this/that/;
+ ($foo = $bar) =~ s/this/that/; # copy first, then change
- $count = ($paragraph =~ s/Mister\b/Mr./g);
+ $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
$_ = 'abc123xyz';
s/\d+/$&*2/e; # yields 'abc246xyz'
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;
+
# /e's can even nest; this will expand
- # simple embedded variables in $_
+ # any embedded scalar variable (including lexicals) in $_
s/(\$\w+)/$1/eeg;
- # Delete C comments.
+ # Delete (most) C comments.
$program =~ s {
/\* # Match the opening delimiter.
.*? # Match a minimal number of characters.
\*/ # Match the closing delimiter.
} []gsx;
- s/^\s*(.*?)\s*$/$1/; # trim white space
+ s/^\s*(.*?)\s*$/$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
+Note the use of $ instead of \ in the last example. Unlike
B<sed>, we use the \E<lt>I<digit>E<gt> form in only the left hand side.
Anywhere else it's $E<lt>I<digit>E<gt>.
Occasionally, you can't use just a C</g> to get all the changes
-to occur. Here are two common cases:
+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)/$1,$2/g; # perl4
- 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; # perl5
+ 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;
+=item tr/SEARCHLIST/REPLACEMENTLIST/cdsUC
-=item tr/SEARCHLIST/REPLACEMENTLIST/cds
+=item y/SEARCHLIST/REPLACEMENTLIST/cdsUC
-=item y/SEARCHLIST/REPLACEMENTLIST/cds
-
-Translates all occurrences of the characters found in the search list
+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 translated. (The
-string specified with =~ must be a scalar variable, an array element,
-or an assignment to one of those, i.e., an lvalue.) For B<sed> devotees,
-C<y> is provided as a synonym for C<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., C<tr[A-Z][a-z]>
-or C<tr(+-*/)/ABCD/>.
+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 C<tr/A-J/0-9/>
+does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
+For B<sed> devotees, C<y> is provided as a synonym for C<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., C<tr[A-Z][a-z]> or C<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.
-
-If the C</c> modifier is specified, the SEARCHLIST character set is
-complemented. If the C</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 B<tr>
-programs, which delete anything they find in the SEARCHLIST, period.)
-If the C</s> modifier is specified, sequences of characters that were
-translated to the same character are squashed down to a single instance of the
-character.
+ 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
+specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
+(Note that this is slightly more flexible than the behavior of some
+B<tr> programs, which delete anything they find in the SEARCHLIST,
+period.) If the C</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 C</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 null, the SEARCHLIST is replicated.
+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 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:
$ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
tr [\200-\377]
[\000-\177]; # delete 8th bit
-If multiple translations are given for a character, only the first one is used:
+ tr/\0-\xFF//CU; # change Latin-1 to Unicode
+ tr/\0-\x{FF}//UC; # change Unicode to Latin-1
+
+If multiple transliterations are given for a character, only the
+first one is used:
tr/AAA/XYZ/
-will translate any A to X.
+will transliterate any A to X.
-Note that because the translation table is built at compile time, neither
+Because the transliteration table is built at compile time, neither
the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
-interpolation. That means that if you want to use variables, you must use
-an eval():
+interpolation. That means that if you want to use variables, you
+must use an eval():
eval "tr/$oldlist/$newlist/";
die $@ if $@;
=back
+=head2 Gory details of parsing quoted constructs
+
+When presented with something that might have several different
+interpretations, Perl uses the B<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 five, but these passes are always performed in the same order.
+
+=over
+
+=item Finding the end
+
+The first pass is finding the end of the quoted construct, whether
+it be a multicharacter delimiter C<"\nEOF\n"> in the C<<<EOF>
+construct, a C</> that terminates a C<qq//> construct, a C<]> which
+terminates C<qq[]> construct, or a C<E<gt>> which terminates a
+fileglob started with C<E<lt>>.
+
+When searching for single-character non-pairing delimiters, such
+as C</>, combinations of C<\\> and C<\/> are skipped. However,
+when searching for single-character pairing delimiter like C<[>,
+combinations of C<\\>, C<\]>, and C<\[> are all skipped, and nested
+C<[>, C<]> are skipped as well. When searching for multicharacter
+delimiters, nothing is skipped.
+
+For constructs with three-part delimiters (C<s///>, C<y///>, and
+C<tr///>), the search is repeated once more.
+
+During this search no attention is paid to the semantics of the construct.
+Thus:
+
+ "$hash{"$foo/$bar"}"
+
+or:
+
+ m/
+ bar # NOT a comment, this slash / terminated m//!
+ /x
+
+do not form legal quoted expressions. The quoted part ends on the
+first C<"> and C</>, and the rest happens to be a syntax error.
+Because the slash that terminated C<m//> was followed by a C<SPACE>,
+the example above is not C<m//x>, but rather C<m//> with no C</x>
+modifier. So the embedded C<#> is interpreted as a literal C<#>.
+
+=item Removal of backslashes before delimiters
+
+During the second pass, text between the starting and ending
+delimiters is copied to a safe location, and the C<\> is removed
+from combinations consisting of C<\> and delimiter--or delimiters,
+meaning both starting and ending delimiters will should these differ.
+This removal does not happen for multi-character delimiters.
+Note that the combination C<\\> is left intact, just as it was.
+
+Starting from this step no information about the delimiters is
+used in parsing.
+
+=item Interpolation
+
+The next step is interpolation in the text obtained, which is now
+delimiter-independent. There are four different cases.
+
+=over
+
+=item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///>
+
+No interpolation is performed.
+
+=item C<''>, C<q//>
+
+The only interpolation is removal of C<\> from pairs C<\\>.
+
+=item C<"">, C<``>, C<qq//>, C<qx//>, C<<file*globE<gt>>
+
+C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
+converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
+is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
+The other combinations are replaced with appropriate expansions.
+
+Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
+is interpolated in the usual way. Something like C<"\Q\\E"> has
+no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
+result is the same as for C<"\\\\E">. As a general rule, backslashes
+between C<\Q> and C<\E> may lead to counterintuitive results. So,
+C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
+as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
+
+ $str = '\t';
+ return "\Q$str";
+
+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:
+
+ $foo . " XXX '" . (join $", @arr) . "'";
+
+All operations above are performed simultaneously, left to right.
+
+Because the result of C<"\Q STRING \E"> has all metacharacters
+quoted, there is no way to insert a literal C<$> or C<@> inside a
+C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
+C<"\\\$">; if not, it is interpreted as the start of an interpolated
+scalar.
+
+Note also that the interpolation code needs to make a decision on
+where the interpolated scalar ends. For instance, whether
+C<"a $b -E<gt> {c}"> really means:
+
+ "a " . $b . " -> {c}";
+
+or:
+
+ "a " . $b -> {c};
+
+Most of the time, the longest possible text that does not include
+spaces between components and which contains matching braces or
+brackets. because the outcome may be determined by voting based
+on heuristic estimators, the result is not strictly predictable.
+Fortunately, it's usually correct for ambiguous cases.
+
+=item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
+
+Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
+happens (almost) as with C<qq//> constructs, but the substitution
+of C<\> followed by RE-special chars (including C<\>) is not
+performed. Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
+a C<#>-comment in a C<//x>-regular expression, no processing is
+performed whatsoever. This is the first step at which the presence
+of the C<//x> modifier is relevant.
+
+Interpolation has several quirks: C<$|>, C<$(>, and C<$)> are not
+interpolated, and constructs C<$var[SOMETHING]> are voted (by several
+different estimators) to be either an array element or C<$var>
+followed by an RE alternative. This is where the notation
+C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
+array element C<-9>, not as a regular expression from the variable
+C<$arr> followed by a digit, which would be the interpretation of
+C</$arr[0-9]/>. Since voting among different estimators may occur,
+the result is not predictable.
+
+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.
+
+The lack of processing of C<\\> creates specific restrictions on
+the post-processed text. If the delimiter is C</>, one cannot get
+the combination C<\/> into the result of this step. C</> will
+finish the regular expression, C<\/> will be stripped to C</> on
+the previous step, and C<\\/> will be left as is. Because C</> is
+equivalent to C<\/> inside a regular expression, this does not
+matter unless the delimiter happens to be character special to the
+RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
+alphanumeric char, as in:
+
+ m m ^ a \s* b mmx;
+
+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
+reason you're encouraged to restrict your delimiters to non-alphanumeric,
+non-whitespace choices.
+
+=back
+
+This step is the last one for all constructs except regular expressions,
+which are processed further.
+
+=item Interpolation of regular expressions
+
+Previous steps were performed during the compilation of Perl code,
+but this one happens at run time--although it may be optimized to
+be calculated at compile time if appropriate. After preprocessing
+described above, and possibly after evaluation if catenation,
+joining, casing translation, or metaquoting are involved, the
+resulting I<string> is passed to the RE engine for compilation.
+
+Whatever happens in the RE engine might be better discussed in L<perlre>,
+but for the sake of continuity, we shall do so here.
+
+This is another step where the presence of the C<//x> modifier is
+relevant. The RE engine scans the string from left to right and
+converts it to a finite automaton.
+
+Backslashed characters are either replaced with corresponding
+literal strings (as with C<\{>), or else they generate special nodes
+in the finite automaton (as with C<\b>). Characters special to the
+RE engine (such as C<|>) generate corresponding nodes or groups of
+nodes. C<(?#...)> comments are ignored. All the rest is either
+converted to literal strings to match, or else is ignored (as is
+whitespace and C<#>-style comments if C<//x> is present).
+
+Parsing of the bracketed character class construct, C<[...]>, is
+rather different than the rule used for the rest of the pattern.
+The terminator of this construct is found using the same rules as
+for finding the terminator of a C<{}>-delimited construct, the only
+exception being that C<]> immediately following C<[> is treated as
+though preceded by a backslash. Similarly, the terminator of
+C<(?{...})> is found using the same rules as for finding the
+terminator of a C<{}>-delimited construct.
+
+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>.
+
+=item Optimization of regular expressions
+
+This step is listed for completeness only. Since it does not change
+semantics, details of this step are not documented and are subject
+to change without notice. This step is performed over the finite
+automaton that was generated during the previous pass.
+
+It is at this stage that C<split()> silently optimizes C</^/> to
+mean C</^/m>.
+
+=back
+
=head2 I/O Operators
-There are several I/O operators you should know about.
-A string is enclosed by back-ticks (grave accents) first undergoes
-variable substitution just like a double quoted string. It is then
-interpreted as a command, and the output of that command is the value
-of the pseudo-literal, like in a shell. In a scalar context, a single
-string consisting of all the output is returned. In a list context,
-a list of values is returned, one for each line of output. (You can
-set C<$/> to use a different line terminator.) The command is executed
+There are several I/O operators you should know about.
+
+A string enclosed by backticks (grave accents) first undergoes
+double-quote interpolation. It is then interpreted as an external
+command, and the output of that command is the value of the
+pseudo-literal, j
+string consisting of all output is returned. In list context, a
+list of values is returned, one per line of output. (You can set
+C<$/> 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 $ through to the shell you need to hide it with a backslash.
-The generalized form of back-ticks is C<qx//>. (Because back-ticks
-always undergo shell expansion as well, see L<perlsec> for
-security concerns.)
-
-Evaluating a filehandle in angle brackets yields the next line from
-that file (newline included, so it's never false until end of file, at
-which time an undefined value is returned). Ordinarily you must assign
-that value to a variable, but there is one situation where an automatic
-assignment happens. I<If and ONLY if> the input symbol is the only
-thing inside the conditional of a C<while> loop, the value is
-automatically assigned to the variable C<$_>. The assigned value is
-then tested to see if it is defined. (This may seem like an odd thing
-to you, but you'll use the construct in almost every Perl script you
-write.) Anyway, the following lines are equivalent to each other:
+To pass a literal dollar-sign through to the shell you need to hide
+it with a backslash. The generalized form of backticks is C<qx//>.
+(Because backticks always undergo shell expansion as well, see
+L<perlsec> for security concerns.)
+
+In scalar context, evaluating a filehandle in angle brackets yields
+the next line from that file (the newline, if any, included), or
+C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
+(sometimes known as file-slurp mode) and the file is empty, it
+returns C<''> the first time, followed by C<undef> subsequently.
+
+Ordinarily you must assign the returned value to a variable, but
+there is one situation where an automatic assignment happens. If
+and only if the input symbol is the only thing inside the conditional
+of a C<while> statement (even if disguised as a C<for(;;)> loop),
+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.
+You'll have to put a C<local $_;> before the loop if you want that
+to happen.
+
+The following lines are equivalent:
while (defined($_ = <STDIN>)) { print; }
+ while ($_ = <STDIN>) { print; }
while (<STDIN>) { print; }
for (;<STDIN>;) { print; }
print while defined($_ = <STDIN>);
+ print while ($_ = <STDIN>);
print while <STDIN>;
+This also behaves similarly, but avoids $_ :
+
+ while (my $line = <STDIN>) { print $line }
+
+In these loop constructs, the assigned value (whether assignment
+is automatic or explicit) is then tested to see whether it is
+defined. The defined test avoids problems where line has a string
+value that would be treated as false by Perl, for example a "" or
+a "0" with no trailing newline. If you really mean for such values
+to terminate the loop, they should be tested for explicitly:
+
+ while (($_ = <STDIN>) ne '0') { ... }
+ while (<STDIN>) { last unless $_; ... }
+
+In other boolean contexts, C<E<lt>I<filehandle>E<gt>> without an
+explicit C<defined> test or comparison elicit a warning if the B<-w>
+command-line switch (the C<$^W> variable) is in effect.
+
The filehandles STDIN, STDOUT, and STDERR are predefined. (The
-filehandles C<stdin>, C<stdout>, and C<stderr> will also work except in
-packages, where they would be interpreted as local identifiers rather
-than global.) Additional filehandles may be created with the open()
-function. See L<perlfunc/open()> for details on this.
+filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
+in packages, where they would be interpreted as local identifiers
+rather than global.) Additional filehandles may be created with
+the open() function, amongst others. See L<perlopentut> and
+L<perlfunc/open> for details on this.
-If a E<lt>FILEHANDLEE<gt> is used in a context that is looking for a list, a
-list consisting of all the input lines is returned, one line per list
-element. It's easy to make a I<LARGE> data space this way, so use with
-care.
+If a E<lt>FILEHANDLEE<gt> is used in a context that is looking for
+a list, a list comprising all input lines is returned, one line per
+list element. It's easy to grow to a rather large data space this
+way, so use with care.
-The null filehandle E<lt>E<gt> is special and can be used to emulate the
+E<lt>FILEHANDLEE<gt> may also be spelled C<readline(*FILEHANDLE)>.
+See L<perlfunc/readline>.
+
+The null filehandle E<lt>E<gt> is special: it can be used to emulate the
behavior of B<sed> and B<awk>. Input from E<lt>E<gt> comes either from
standard input, or from each file listed on the command line. Here's
how it works: the first time E<lt>E<gt> is evaluated, the @ARGV array is
-checked, and if it is null, C<$ARGV[0]> is set to "-", which when opened
+checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
gives you standard input. The @ARGV array is then processed as a list
of filenames. The loop
is equivalent to the following Perl-like pseudo code:
- unshift(@ARGV, '-') if $#ARGV < $[;
+ unshift(@ARGV, '-') unless @ARGV;
while ($ARGV = shift) {
open(ARGV, $ARGV);
while (<ARGV>) {
}
}
-except that it isn't so cumbersome to say, and will actually work. It
-really does shift array @ARGV and put the current filename into variable
-$ARGV. It also uses filehandle I<ARGV> internally--E<lt>E<gt> is just a
-synonym for E<lt>ARGVE<gt>, which is magical. (The pseudo code above
-doesn't work because it treats E<lt>ARGVE<gt> as non-magical.)
+except that it isn't so cumbersome to say, and will actually work.
+It really does shift the @ARGV array and put the current filename
+into the $ARGV variable. It also uses filehandle I<ARGV>
+internally--E<lt>E<gt> is just a synonym for E<lt>ARGVE<gt>, which
+is magical. (The pseudo code above doesn't work because it treats
+E<lt>ARGVE<gt> as non-magical.)
You can modify @ARGV before the first E<lt>E<gt> as long as the array ends up
containing the list of filenames you really want. Line numbers (C<$.>)
-continue as if the input were one big happy file. (But see example
-under eof() for how to reset line numbers on each file.)
+continue as though the input were one big happy file. See the example
+in L<perlfunc/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.
+This sets @ARGV to all plain text files if no @ARGV was given:
-If you want to set @ARGV to your own list of files, go right ahead. If
-you want to pass switches into your script, you can use one of the
+ @ARGV = grep { -f && -T } glob('*') unless @ARGV;
+
+You can even set them to pipe commands. For example, this automatically
+filters compressed arguments through B<gzip>:
+
+ @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
+
+If you want to pass switches into your script, you can use one of the
Getopts modules or put a loop on the front like this:
while ($_ = $ARGV[0], /^-/) {
last if /^--$/;
if (/^-D(.*)/) { $debug = $1 }
if (/^-v/) { $verbose++ }
- ... # other switches
+ # ... # other switches
}
+
while (<>) {
- ... # code for each line
+ # ... # code for each line
}
-The E<lt>E<gt> symbol will return FALSE only once. If you call it again after
-this it will assume you are processing another @ARGV list, and if you
-haven't set @ARGV, will input from STDIN.
+The E<lt>E<gt> symbol will return C<undef> for end-of-file only once.
+If you call it again after this, it will assume you are processing another
+@ARGV list, and if you haven't set @ARGV, will read input from STDIN.
-If the string inside the angle brackets is a reference to a scalar
-variable (e.g., E<lt>$fooE<gt>), then that variable contains the name of the
-filehandle to input from, or a reference to the same. For example:
+If angle brackets contain is a simple scalar variable (e.g.,
+E<lt>$fooE<gt>), then that variable contains the name of the
+filehandle to input from, or its typeglob, or a reference to the
+same. For example:
$fh = \*STDIN;
$line = <$fh>;
-If the string inside angle brackets is not a filehandle or a scalar
-variable containing a filehandle name or reference, then it is interpreted
-as a filename pattern to be globbed, and either a list of filenames or the
-next filename in the list is returned, depending on context. One level of
-$ interpretation is done first, but you can't say C<E<lt>$fooE<gt>>
-because that's an indirect filehandle as explained in the previous
-paragraph. (In older versions of Perl, programmers would insert curly
-brackets to force interpretation as a filename glob: C<E<lt>${foo}E<gt>>.
-These days, it's considered cleaner to call the internal function directly
-as C<glob($foo)>, which is probably the right way to have done it in the
-first place.) Example:
+If what's within the angle brackets is neither a filehandle nor a simple
+scalar variable containing a filehandle name, typeglob, or typeglob
+reference, it is interpreted as a filename pattern to be globbed, and
+either a list of filenames or the next filename in the list is returned,
+depending on context. This distinction is determined on syntactic
+grounds alone. That means C<E<lt>$xE<gt>> is always a readline() from
+an indirect handle, but C<E<lt>$hash{key}E<gt>> is always a glob().
+That's because $x is a simple scalar variable, but C<$hash{key}> is
+not--it's a hash element.
+
+One level of double-quote interpretation is done first, but you can't
+say C<E<lt>$fooE<gt>> because that's an indirect filehandle as explained
+in the previous paragraph. (In older versions of Perl, programmers
+would insert curly brackets to force interpretation as a filename glob:
+C<E<lt>${foo}E<gt>>. These days, it's considered cleaner to call the
+internal function directly as C<glob($foo)>, which is probably the right
+way to have done it in the first place.) For example:
while (<*.c>) {
chmod 0644, $_;
chmod 0644, $_;
}
-In fact, it's currently implemented that way. (Which means it will not
-work on filenames with spaces in them unless you have csh(1) on your
-machine.) Of course, the shortest way to do the above is:
+In fact, it's currently implemented that way, but this is expected
+to be made completely internal in the near future. (Which means
+it will not work on filenames with spaces in them unless you have
+csh(1) on your machine.) 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() yourself
-and do your own grep() on the filenames. Furthermore, due to its current
-implementation of using a shell, the glob() routine may get "Arg list too
-long" errors (unless you've installed tcsh(1L) as F</bin/csh>).
-
-A glob evaluates its (embedded) argument only when it is starting a new
-list. All values must be read before it will start over. In a list
-context this isn't important, because you automatically get them all
-anyway. In a scalar context, however, the operator returns the next value
-each 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 from
-a glob, it is much better to say
+Because globbing currently invokes a shell, it's often faster to
+call readdir() yourself and do your own grep() on the filenames.
+Furthermore, due to its current implementation of using a shell,
+the glob() routine may get "Arg list too long" errors (unless you've
+installed tcsh(1L) as F</bin/csh> or hacked your F<config.sh>).
+
+A (file)glob evaluates its (embedded) argument only when it is
+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
+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
+terminate the loop. Again, C<undef> is returned only once. So if
+you're expecting a single value from a glob, it is much better to
+say
($file) = <blurch*>;
$file = <blurch*>;
because the latter will alternate between returning a filename and
-returning FALSE.
+returning false.
It you're trying to do variable interpolation, it's definitely better
to use the glob() function, because the older notation can cause people
=head2 Constant Folding
Like C, Perl does a certain amount of expression evaluation at
-compile time, whenever it determines that all of the arguments to an
+compile time whenever it determines that all arguments to an
operator are static and have no side effects. In particular, string
concatenation happens at compile time between literals that don't do
-variable substitution. Backslash interpretation also happens at
+variable substitution. Backslash interpolation also happens at
compile 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, if
+and this all reduces to one string internally. Likewise, if
you say
foreach $file (@filenames) {
- if (-s $file > 5 + 100 * 2**16) { ... }
- }
+ if (-s $file > 5 + 100 * 2**16) { }
+ }
+
+the compiler will precompute the number which that expression
+represents so that the interpreter won't have to.
+
+=head2 Bitwise String Operators
+
+Bitstrings of any size may be manipulated by the bitwise operators
+(C<~ | & ^>).
+
+If the operands to a binary bitwise op are strings of different
+sizes, B<|> and B<^> ops act as though the shorter operand had
+additional zero bits on the right, while the B<&> op acts as though
+the longer operand were truncated to the length of the shorter.
+The granularity for such extension or truncation is one or more
+bytes.
-the compiler will pre-compute the number that
-expression represents so that the interpreter
-won't have to.
+ # ASCII-based examples
+ print "j p \n" ^ " a h"; # prints "JAPH\n"
+ print "JA" | " ph\n"; # prints "japh\n"
+ print "japh\nJunk" & '_____'; # prints "JAPH\n";
+ print 'p N$' ^ " E<H\n"; # prints "Perl\n";
+If you are intending to manipulate bitstrings, be certain that
+you're supplying bitstrings: If an operand is a number, that will imply
+a B<numeric> bitwise operation. You may explicitly show which type of
+operation you intend by using C<""> or C<0+>, as in the examples below.
+
+ $foo = 150 | 105 ; # yields 255 (0x96 | 0x69 is 0xFF)
+ $foo = '150' | 105 ; # yields 255
+ $foo = 150 | '105'; # yields 255
+ $foo = '150' | '105'; # yields string '155' (under ASCII)
+
+ $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
+ $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
+
+See L<perlfunc/vec> for information on how to manipulate individual bits
+in a bit vector.
=head2 Integer Arithmetic
-By default Perl assumes that it must do most of its arithmetic in
+By default, Perl assumes that it must do most of its arithmetic in
floating point. But by saying
use integer;
you may tell the compiler that it's okay to use integer operations
-from here to the end of the enclosing BLOCK. An inner BLOCK may
-countermand this by saying
+(if it feels like it) from here to the end of the enclosing BLOCK.
+An inner BLOCK may countermand this by saying
no integer;
-which lasts until the end of that BLOCK.
+which lasts until the end of that BLOCK. Note that this doesn't
+mean everything is only an integer, merely that Perl may use integer
+operations if it is so inclined. For example, even under C<use
+integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
+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
+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
+integral value. However, C<use integer; ~0> is C<-1> on twos-complement
+machines.
+
+=head2 Floating-point Arithmetic
+
+While C<use integer> provides integer-only arithmetic, there is no
+analogous mechanism to provide automatic rounding or truncation to a
+certain number of decimal places. For rounding to a certain number
+of digits, sprintf() or printf() is usually the easiest route.
+See L<perlfaq4>.
+
+Floating-point numbers are only approximations to what a mathematician
+would call real numbers. There are infinitely more reals than floats,
+so some corners must be cut. For example:
+
+ printf "%.20g\n", 123456789123456789;
+ # produces 123456789123456784
+
+Testing for exact equality of floating-point equality or inequality is
+not a good idea. Here's a (relatively expensive) work-around to compare
+whether two floating-point numbers are equal to a particular number of
+decimal places. See Knuth, volume II, for a more robust treatment of
+this topic.
+
+ sub fp_equal {
+ my ($X, $Y, $POINTS) = @_;
+ my ($tX, $tY);
+ $tX = sprintf("%.${POINTS}g", $X);
+ $tY = sprintf("%.${POINTS}g", $Y);
+ return $tX eq $tY;
+ }
+
+The POSIX module (part of the standard perl distribution) implements
+ceil(), floor(), and other mathematical and trigonometric functions.
+The Math::Complex module (part of the standard perl distribution)
+defines mathematical functions that work on both the reals and the
+imaginary numbers. Math::Complex not as efficient as POSIX, but
+POSIX can't work with complex numbers.
+
+Rounding in financial applications can have serious implications, and
+the rounding method used should be specified precisely. In these
+cases, it probably pays not to trust whichever system rounding is
+being used by Perl, but to instead implement the rounding function you
+need yourself.
+
+=head2 Bigger Numbers
+
+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
+considerable speed, they avoid the normal pitfalls associated with
+limited-precision representations.
+
+ use Math::BigInt;
+ $x = Math::BigInt->new('123456789123456789');
+ print $x * $x;
+
+ # prints +15241578780673678515622620750190521
-The bitwise operators ("&", "|", "^", "~", "<<", and ">>") always
-produce integral results. However, C<use integer> still has meaning
-for them. By default, their results are interpreted as unsigned
-integers. However, if C<use integer> is in effect, their results are
-interpreted as signed integers. For example, C<~0> usually evaluates
-to a large integral value. However, C<use integer; ~0> is -1.
+The non-standard modules SSLeay::BN and Math::Pari provide
+equivalent functionality (and much more) with a substantial
+performance savings.
https://perl5.git.perl.org / perl5.git / blobdiff