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.)
+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 , =>
nonassoc list operators (rightward)
- left not
+ right not
left and
left or xor
In the following sections, these operators are covered in precedence order.
-=head1 DESCRIPTIONS
+=head1 DESCRIPTION
=head2 Terms and List Operators (Leftward)
-Any TERM is of highest precedence of Perl. These includes variables,
-quote and quotelike operators, any expression in parentheses,
+A TERM has the highest precedence in Perl. They includes variables,
+quote and quote-like operators, any expression in parentheses,
and any function whose arguments are parenthesized. Actually, there
aren't really functions in this sense, just list operators and unary
operators behaving as functions because you put parentheses around
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);
the commas on the left are evaluated after. In other words, list
operators tend to gobble up all the arguments that follow them, and
then act like a simple TERM with regard to the preceding expression.
-Note that you have to be careful with parens:
+Note that you have to be careful with parentheses:
# These evaluate exit before doing the print:
print($foo, exit); # Obviously not what you want.
print ($foo & 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 Quotelike Operators> toward the end of this section,
-as well as L<I/O Operators>.
+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
(a blessed reference) or a class name (that is, a package name).
See L<perlobj>.
-=head2 Autoincrement and Autodecrement
+=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.
-The autoincrement 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 only been used in string contexts since it was set, and
+variable has been used in only string contexts since it was set, and
has a value that is not null and matches the pattern
C</^[a-zA-Z]*[0-9]*$/>, the increment is done as a string, preserving each
character within its range, with carry:
print ++($foo = 'Az'); # prints 'Ba'
print ++($foo = 'zz'); # prints 'aaa'
-The autodecrement operator is not magical.
+The auto-decrement operator is not magical.
=head2 Exponentiation
Binary "**" is the exponentiation operator. Note that it binds even more
-tightly than unary minus, so -2**4 is -(2**4), not (-2)**4.
+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.)
=head2 Symbolic Unary Operators
-Unary "!" performs logical negation, i.e. "not". See also C<not> for a lower
+Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower
precedence version of this.
Unary "-" performs arithmetic negation if the operand is numeric. If
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.
+Unary "~" performs bitwise negation, i.e., 1's complement.
+(See also L<Integer Arithmetic>.)
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<List Operators>.)
+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
=head2 Binding Operators
-Binary "=~" binds an 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 $_. 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, since the pattern must be compiled every time
-the expression is evaluated--unless you've used C</o>.)
+Binary "=~" binds a scalar expression to a pattern match. Certain operations
+search or modify the string $_ by default. This operator makes that kind
+of operation work on some other string. The right argument is a search
+pattern, substitution, or translation. The left argument is what is
+supposed to be searched, substituted, or translated instead of the default
+$_. 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 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 "%" 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).
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 parens, it repeats the list.
+operand is a list in parentheses, it repeats the list.
print '-' x 80; # print row of dashes
=head2 Shift Operators
Binary "<<" returns the value of its left argument shifted left by the
-number of bits specified by the right argument. Arguments should be
-integers.
+number of bits specified by the right argument. Arguments should be
+integers. (See also L<Integer Arithmetic>.)
-Binary ">>" returns the value of its left argument shifted right by the
-number of bits specified by the right argument. Arguments should be
-integers.
+Binary ">>" returns the value of its left argument shifted right by
+the number of bits specified by the right argument. Arguments should
+be integers. (See also L<Integer Arithmetic>.)
=head2 Named Unary Operators
rand (10) * 20; # (rand 10) * 20
rand +(10) * 20; # rand (10 * 20)
-See also L<"List Operators">.
+See also L<"Terms and List Operators (Leftward)">.
=head2 Relational Operators
-Binary "<" returns true if the left argument is numerically less than
+Binary "E<lt>" returns true if the left argument is numerically less than
the right argument.
-Binary ">" returns true if the left argument is numerically greater
+Binary "E<gt>" returns true if the left argument is numerically greater
than the right argument.
-Binary "<=" returns true if the left argument is numerically less than
+Binary "E<lt>=" returns true if the left argument is numerically less than
or equal to the right argument.
-Binary ">=" returns true if the left argument is numerically greater
+Binary "E<gt>=" returns true if the left argument is numerically greater
than or equal to the right argument.
Binary "lt" returns true if the left argument is stringwise less than
Binary "!=" returns true if the left argument is numerically not equal
to the right argument.
-Binary "<=>" returns -1, 0, or 1 depending on whether the left argument is numerically
-less than, equal to, or greater than the right argument.
+Binary "E<lt>=E<gt>" returns -1, 0, or 1 depending on whether the left
+argument is numerically less than, equal to, or greater than the right
+argument.
Binary "eq" returns true if the left argument is stringwise equal to
the right argument.
Binary "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.
+(See also L<Integer Arithmetic>.)
=head2 Bitwise Or and Exclusive Or
Binary "|" returns its operators ORed together bit by bit.
+(See also L<Integer Arithmetic>.)
Binary "^" returns its operators XORed together bit by bit.
+(See also L<Integer Arithmetic>.)
=head2 C-style Logical And
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
+a temporary array is created, so you'll burn a lot of memory if you
write something like this:
for (1 .. 1_000_000) {
# code
- }
+ }
In a scalar context, ".." returns a boolean value. The operator is
bistable, like a flip-flop, and emulates the line-range (comma) operator
@foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
The range operator (in a list context) makes use of the magical
-autoincrement algorithm if the operaands are strings. You
+auto-increment algorithm if the operands are strings. You
can say
@alphabet = ('A' .. 'Z');
Ternary "?:" is the conditional operator, just as in C. It works much
like an if-then-else. If the argument before the ? is true, the
argument before the : is returned, otherwise the argument after the :
-is returned. Scalar or list context propagates downward into the 2nd
-or 3rd argument, whichever is selected. The operator may be assigned
-to if both the 2nd and 3rd arguments are legal lvalues (meaning that you
-can assign to them):
+is returned. For example:
+
+ printf "I have %d dog%s.\n", $n,
+ ($n == 1) ? '' : "s";
+
+Scalar or list context propagates downward into the 2nd
+or 3rd argument, whichever is selected.
+
+ $a = $ok ? $b : $c; # get a scalar
+ @a = $ok ? @b : @c; # get an array
+ $a = $ok ? @b : @c; # oops, that's just a count!
+
+The operator may be assigned to if both the 2nd and 3rd arguments are
+legal lvalues (meaning that you can assign to them):
($a_or_b ? $a : $b) = $c;
-Note that this is not guaranteed to contribute to the readability of
-your program.
+This is not necessarily guaranteed to contribute to the readability of your program.
-=head2 Assigment Operators
+=head2 Assignment Operators
"=" is the ordinary assignment operator.
$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:
**= += *= &= <<= &&=
-= /= |= >>= ||=
$a += 2;
$a *= 3;
-=head2
+=head2 Comma Operator
Binary "," is the comma operator. In a scalar context it evaluates
its left argument, throws that value away, then evaluates its right
In a 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
+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.
+
=head2 List Operators (Rightward)
On the right side of a list operator, it has very low precedence,
open HANDLE, "filename"
or die "Can't open: $!\n";
-See also discussion of list operators in L<List Operators (Leftward)>.
+See also discussion of list operators in L<Terms and List Operators (Leftward)>.
=head2 Logical Not
Binary "and" returns the logical conjunction of the two surrounding
expressions. It's equivalent to && except for the very low
-precedence. This means that it short-circuits: i.e. the right
+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
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
+precedence. This means that it short-circuits: i.e., the right
expression is evaluated only if the left expression is false.
Binary "xor" returns the exclusive-OR of the two surrounding expressions.
=item unary *
-Dereference-address operator. (Perl's prefix dereferencing
+Dereference-address operator. (Perl's prefix dereferencing
operators are typed: $, @, %, and &.)
=item (TYPE)
-Type casting operator.
+Type casting operator.
=back
-=head2 Quote and Quotelike Operators
+=head2 Quote and Quote-like Operators
While we usually think of quotes as literal values, in Perl they
function as operators, providing various kinds of interpolating and
for these behaviors, but also provides a way for you to choose your
quote character for any of them. In the following table, a C<{}> represents
any pair of delimiters you choose. Non-bracketing delimiters use
-the same character fore and aft, but the 4 sorts of brackets
+the same character fore and aft, but the 4 sorts of brackets
(round, angle, square, curly) will all nest.
Customary Generic Meaning Interpolates
s{}{} Substitution yes
tr{}{} Translation no
-For constructs that do interpolation, variables beginning with "C<$> or "C<@>"
+For constructs that do interpolation, variables beginning with "C<$>" or "C<@>"
are interpolated, as are the following sequences:
- \t tab
- \n newline
- \r return
- \f form feed
- \v vertical tab, whatever that is
- \b backspace
- \a alarm (bell)
- \e escape
+ \t tab (HT, TAB)
+ \n newline (LF, NL)
+ \r return (CR)
+ \f form feed (FF)
+ \b backspace (BS)
+ \a alarm (bell) (BEL)
+ \e escape (ESC)
\033 octal char
\x1b hex char
\c[ control char
\E end case modification
\Q quote regexp metacharacters 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>.
+
Patterns are subject to an additional level of interpretation as a
regular expression. This is done as a second pass, after variables are
interpolated, so that regular expressions may be incorporated into the
interpolate a variable literally.
Apart from the above, there are no multiple levels of interpolation. In
-particular, contrary to the expectations of shell programmers, backquotes
+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
+
+Here are the quote-like operators that apply to pattern
+matching and related activities.
+
=over 8
=item ?PATTERN?
This is just like the C</pattern/> search, except that it matches only
once between calls to the reset() operator. This is a useful
-optimization when you only want to see the first occurrence of
+optimization when you want to see only the first occurrence of
something in each file of a set of files, for instance. Only C<??>
patterns local to the current package are reset.
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.
Options are:
- g Match globally, i.e. find all occurrences.
+ g Match globally, i.e., find all occurrences.
i Do case-insensitive pattern matching.
m Treat string as multiple lines.
- o Only compile pattern once.
+ o Compile pattern only once.
s Treat string as single line.
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).
+that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
+the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
PATTERN may contain variables, which will be interpolated (and the
pattern recompiled) every time the pattern search is evaluated. (Note
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 most recently executed
-(and successfully compiled) regular expression is used instead.
+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
list consisting of the subexpressions matched by the parentheses in the
-pattern, i.e. ($1, $2, $3...). (Note that here $1 etc. are also set, and
+pattern, i.e., (C<$1>, $2, $3...). (Note that here $1 etc. are also set, and
that this differs from Perl 4's behavior.) If the match fails, a null
array is returned. If the match succeeds, but there were no parentheses,
a list value of (1) is returned.
if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
This last example splits $foo into the first two words and the
-remainder of the line, and assigns those three fields to $F1, $F2 and
-$Etc. The conditional is true if any variables were assigned, i.e. if
+remainder of the line, and assigns those three fields to $F1, $F2, and
+$Etc. The conditional is true if any variables were assigned, i.e., if
the pattern matched.
The C</g> modifier specifies global pattern matching--that is, matching
strings, as if there were parentheses around the whole pattern.
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 using the pos() function--see L<perlfunc>.)
-If you modify the string in any way, the match position is reset to the
-beginning. Examples:
+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>.) A failed match normally resets the search position to
+the beginning of the string, but you can avoid that by adding the "c"
+modifier (e.g. 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.
+
+Examples:
# list context
($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
# scalar context
- $/ = ""; $* = 1; # $* deprecated in Perl 5
- while ($paragraph = <>) {
+ $/ = ""; $* = 1; # $* deprecated in modern perls
+ while (defined($paragraph = <>)) {
while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
$sentences++;
}
}
print "$sentences\n";
+ # using m//gc with \G
+ $_ = "ppooqppqq";
+ while ($i++ < 2) {
+ print "1: '";
+ print $1 while /(o)/gc; print "', pos=", pos, "\n";
+ print "2: '";
+ print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
+ print "3: '";
+ print $1 while /(p)/gc; print "', pos=", pos, "\n";
+ }
+
+The last example should print:
+
+ 1: 'oo', pos=4
+ 2: 'q', pos=5
+ 3: 'pp', pos=7
+ 1: '', pos=7
+ 2: 'q', pos=8
+ 3: '', pos=8
+
+A useful idiom for 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. Each
+regexp tries to match where the previous one leaves off.
+
+ $_ = <<'EOL';
+ $url = new URI::URL "http://www/"; die if $url eq "xXx";
+ EOL
+ LOOP:
+ {
+ print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
+ print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
+ print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
+ print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
+ print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
+ print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
+ print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
+ print ". That's all!\n";
+ }
+
+Here is the output (split into several lines):
+
+ line-noise lowercase line-noise lowercase UPPERCASE line-noise
+ UPPERCASE line-noise lowercase line-noise lowercase line-noise
+ lowercase lowercase line-noise lowercase lowercase line-noise
+ MiXeD line-noise. That's all!
+
=item q/STRING/
=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)/; # :-)
+ $baz = "\n"; # a one-character string
=item qx/STRING/
$today = qx{ date };
-See L<I/O Operators> for more discussion.
+See L<"I/O Operators"> for more discussion.
=item qw/STRING/
use POSIX qw( setlocale localeconv )
@EXPORT = qw( foo bar baz );
+A common mistake is to try to separate the words with comma or to put
+comments into a multi-line qw-string. For this reason the C<-w>
+switch produce warnings if the STRING contains the "," or the "#"
+character.
+
=item s/PATTERN/REPLACEMENT/egimosx
Searches a string for a pattern, and if found, replaces that pattern
with the replacement text and returns the number of substitutions
-made. Otherwise it returns false (0).
+made. Otherwise it returns false (specifically, the empty string).
If no string is specified via the C<=~> or C<!~> operator, the C<$_>
variable is searched and modified. (The string specified with C<=~> must
be a scalar variable, an array element, a hash element, or an assignment
-to one of those, i.e. an lvalue.)
+to one of those, i.e., an lvalue.)
If the delimiter chosen is single quote, no variable 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 only want the pattern compiled once the first time
+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 most recently executed (and successfully compiled) regular
+evaluates to a null 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
+when C<use locale> is in effect.
Options are:
e Evaluate the right side as an expression.
- g Replace globally, i.e. all occurrences.
+ g Replace globally, i.e., all occurrences.
i Do case-insensitive pattern matching.
m Treat string as multiple lines.
- o Only compile pattern once.
+ o Compile pattern only once.
s Treat string as single line.
x Use extended regular expressions.
Any non-alphanumeric, non-whitespace delimiter may replace the
slashes. If single quotes are used, no interpretation is done on the
-replacement string (the C</e> modifier overrides this, however). If
-backquotes are used, the replacement string is a command to execute
-whose output will be used as the actual replacement text. If the
+replacement string (the C</e> modifier overrides this, however). Unlike
+Perl 4, Perl 5 treats backticks as normal delimiters; the replacement
+text is not evaluated as a command. If the
PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
-pair of quotes, which may or may not be bracketing quotes, e.g.
+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
and eval()ed right then and there. It is, however, syntax checked at
# Delete C comments.
$program =~ s {
- /\* (?# Match the opening delimiter.)
- .*? (?# Match a minimal number of characters.)
- \*/ (?# Match the closing delimiter.)
+ /\* # Match the opening delimiter.
+ .*? # Match a minimal number of characters.
+ \*/ # Match the closing delimiter.
} []gsx;
s/^\s*(.*?)\s*$/$1/; # trim white space
s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
-Note the use of $ instead of \ in the last example. Unlike
-B<sed>, we only use the \<I<digit>> form in the left hand side.
-Anywhere else it's $<I<digit>>.
+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 just use a C</g> to get all the changes
+Occasionally, you can't use just a C</g> to get all the changes
to occur. Here are two common cases:
# put commas in the right places in an integer
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/>.
+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.)
+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/>.
Options:
tr [\200-\377]
[\000-\177]; # delete 8th bit
+If multiple translations are given for a character, only the first one is used:
+
+ tr/AAA/XYZ/
+
+will translate any A to X.
+
Note that because the translation table is built at compile time, neither
the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
interpolation. That means that if you want to use variables, you must use
=head2 I/O Operators
-There are several I/O operators you should know about.
+There are several I/O operators you should know about.
A string is enclosed by backticks (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
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 backticks is C<qx//>.
+The generalized form of backticks is C<qx//>. (Because backticks
+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<$_>. (This may seem like an
-odd thing to you, but you'll use the construct in almost every Perl
-script you write.) Anyway, the following lines are equivalent to each
-other:
-
- while ($_ = <STDIN>) { print; }
+that file (newline, if any, included), or C<undef> at end of file.
+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> or
+C<for(;;)> loop, the value is automatically assigned to the variable
+C<$_>. The assigned value is then tested to see if it is defined.
+(This may seem like an odd thing to you, but you'll use the construct
+in almost every Perl script you write.) Anyway, the following lines
+are equivalent to each other:
+
+ while (defined($_ = <STDIN>)) { print; }
while (<STDIN>) { print; }
for (;<STDIN>;) { print; }
- print while $_ = <STDIN>;
+ print while defined($_ = <STDIN>);
print while <STDIN>;
-The filehandles STDIN, STDOUT and STDERR are predefined. (The
-filehandles C<stdin>, C<stdout> and C<stderr> will also work except in
+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.
+function. See L<perlfunc/open()> for details on this.
-If a <FILEHANDLE> is used in a context that is looking for a list, a
+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.
-The null filehandle <> is special and can be used to emulate the
-behavior of B<sed> and B<awk>. Input from <> comes either from
+The null filehandle E<lt>E<gt> is special and 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 <> is evaluated, the @ARGV array is
+how it works: the first time E<lt>E<gt> is evaluated, the @ARGV array is
checked, and if it is null, C<$ARGV[0]> is set to "-", which when opened
gives you standard input. The @ARGV array is then processed as a list
of filenames. The loop
except that it isn't so cumbersome to say, and will actually work. It
really does shift array @ARGV and put the current filename into variable
-$ARGV. It also uses filehandle I<ARGV> internally--<> is just a synonym
-for <ARGV>, which is magical. (The pseudo code above doesn't work
-because it treats <ARGV> as non-magical.)
+$ARGV. It also uses filehandle I<ARGV> internally--E<lt>E<gt> is just a
+synonym for E<lt>ARGVE<gt>, which is magical. (The pseudo code above
+doesn't work because it treats E<lt>ARGVE<gt> as non-magical.)
-You can modify @ARGV before the first <> as long as the array ends up
+You can modify @ARGV before the first E<lt>E<gt> as long as the array ends up
containing the list of filenames you really want. Line numbers (C<$.>)
continue as if the input were one big happy file. (But see example
under eof() for how to reset line numbers on each file.)
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
+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 ($_ = $ARGV[0], /^-/) {
... # code for each line
}
-The <> symbol will return FALSE only once. If you call it again after
+The E<lt>E<gt> symbol will return FALSE only once. If you call it again after
this it will assume you are processing another @ARGV list, and if you
haven't set @ARGV, will input from STDIN.
If the string inside the angle brackets is a reference to a scalar
-variable (e.g. <$foo>), then that variable contains the name of the
-filehandle to input from.
-
-If the string inside angle brackets is not a filehandle, 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>$fooE<gt>> because that's an indirect filehandle as explained in the
-previous paragraph. You could insert curly brackets to force
-interpretation as a filename glob: C<E<lt>${foo}E<gt>>. (Alternately, you can
-call the internal function directly as C<glob($foo)>, which is probably
-the right way to have done it in the first place.) Example:
+variable (e.g., E<lt>$fooE<gt>), then that variable contains the name of the
+filehandle to input from, or a reference to the same. For example:
+
+ $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:
while (<*.c>) {
chmod 0644, $_;
chmod 0644, <*.c>;
Because globbing invokes a shell, it's often faster to call readdir() yourself
-and just do your own grep() on the filenames. Furthermore, due to its current
-implementation of using a shell, the glob() routine may get "Arg list too
+and do your own grep() on the filenames. Furthermore, due to its current
+implementation of using a shell, the glob() routine may get "Arg list too
long" errors (unless you've installed tcsh(1L) as F</bin/csh>).
+A glob evaluates its (embedded) argument only when it is starting a new
+list. All values must be read before it will start over. In a list
+context this isn't important, because you automatically get them all
+anyway. In a scalar context, however, the operator returns the next value
+each time it is called, or a FALSE value if you've just run out. Again,
+FALSE is returned only once. So if you're expecting a single value from
+a glob, it is much better to say
+
+ ($file) = <blurch*>;
+
+than
+
+ $file = <blurch*>;
+
+because the latter will alternate between returning a filename and
+returning FALSE.
+
+It you're trying to do variable interpolation, it's definitely better
+to use the glob() function, because the older notation can cause people
+to become confused with the indirect filehandle notation.
+
+ @files = glob("$dir/*.[ch]");
+ @files = glob($files[$i]);
+
=head2 Constant Folding
Like C, Perl does a certain amount of expression evaluation at
'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) { ... }
- }
+ }
-the compiler will pre-compute the number that
+the compiler will precompute the number that
expression represents so that the interpreter
won't have to.
-=head2 Integer arithmetic
+=head2 Integer Arithmetic
By default Perl assumes that it must do most of its arithmetic in
floating point. But by saying
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
+countermand this by saying
no integer;
which lasts until the end of that BLOCK.
+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.
+
+=head2 Floating-point Arithmetic
+
+While C<use integer> provides integer-only arithmetic, there is no
+similar ways to provide rounding or truncation at a certain number of
+decimal places. For rounding to a certain number of digits, sprintf()
+or printf() is usually the easiest route.
+
+The POSIX module (part of the standard perl distribution) implements
+ceil(), floor(), and a number of other mathematical and trigonometric
+functions. The Math::Complex module (part of the standard perl
+distribution) defines a number of mathematical functions that can also
+work on real 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.
projects / p5sagit/p5-mst-13.2.git / blobdiff