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)
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]*$/>, the increment is done as a string, preserving each
character within its range, with carry:
is returned. One effect of these rules is that C<-bareword> is equivalent
to C<"-bareword">.
-Unary "~" performs bitwise negation, i.e., 1's complement.
-(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
+Unary "~" performs bitwise negation, i.e., 1's complement. For example,
+C<0666 &~ 027> is 0640. (See also L<Integer Arithmetic> and L<Bitwise
+String Operators>.)
Unary "+" has no effect whatsoever, even on strings. It is useful
syntactically for separating a function name from a parenthesized expression
C<$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).
+result will be less than or equal to zero). If C<use integer> is
+in effect, the native hardware will be used instead of this rule,
+which may be construed a bug that will be fixed at some point.
-Binary "x" is the repetition operator. In a scalar context, it
+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, 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
+times specified by the right operand. In list context, if the left
operand is a list in parentheses, it repeats the list.
print '-' x 80; # print row of dashes
$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
+operators depending on the context. In 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:
+value. This 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.
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
+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 numeric literal,
+greater than 1. If either operand of scalar ".." is a constant expression,
that operand is implicitly compared to the C<$.> variable, the current
line number. Examples:
next line if (1 .. /^$/); # skip header lines
s/^/> / if (/^$/ .. eof()); # quote body
+ # parse mail messages
+ while (<>) {
+ $in_header = 1 .. /^$/;
+ $in_body = /^$/ .. eof();
+ # do something based on those
+ } continue {
+ close ARGV if eof; # reset $. each file
+ }
+
As a list operator:
for (101 .. 200) { print; } # print $_ 100 times
@foo = @foo[0 .. $#foo]; # an expensive no-op
@foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
-The range operator (in 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
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)
+
=head2 Assignment Operators
"=" is the ordinary assignment operator.
=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($file) or die; # better, now @info gets its due
+
+Then again, you could always use parentheses.
Binary "xor" returns the exclusive-OR of the two surrounding expressions.
It cannot short circuit, of course.
Customary Generic Meaning Interpolates
'' q{} Literal no
"" qq{} Literal yes
- `` qx{} Command yes
+ `` qx{} Command yes (unless '' is delimiter)
qw{} Word list no
// m{} Pattern match yes
s{}{} Substitution yes
a transliteration, the first ten of these sequences may be used.
\t tab (HT, TAB)
- \n newline (LF, NL)
+ \n newline (NL)
\r return (CR)
\f form feed (FF)
\b backspace (BS)
\L lowercase till \E
\U uppercase till \E
\E end case modification
- \Q quote regexp metacharacters till \E
+ \Q quote non-word characters till \E
If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u>
-and <\U> is taken from the current locale. See L<perllocale>.
+and C<\U> is taken from the current locale. See L<perllocale>.
+
+All systems use the virtual C<"\n"> to represent a line terminator,
+called a "newline". There is no such thing as an unvarying, physical
+newline character. It is an illusion that the operating system,
+device drivers, C libraries, and Perl all conspire to preserve. Not all
+systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
+on a Mac, these are reversed, and on systems without line terminator,
+printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
+you mean a "newline" for your system, but use the literal ASCII when you
+need an exact character. For example, most networking protocols expect
+and prefer a CR+LF (C<"\012\015"> or C<"\cJ\cM">) for line terminators,
+and although they often accept just C<"\012">, they seldom tolerate just
+C<"\015">. If you get in the habit of using C<"\n"> for networking,
+you may be burned some day.
+
+You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
+An unescaped C<$> or C<@> interpolates the corresponding variable,
+while escaping will cause the literal string C<\$> to be inserted.
+You'll need to write something like C<m/\Quser\E\@\Qhost/>.
Patterns are subject to an additional level of interpretation as a
regular expression. This is done as a second pass, after variables are
Here are the quote-like operators that apply to pattern
matching and related activities.
+Most of this section is related to use of regular expressions from Perl.
+Such a use may be considered from two points of view: Perl handles a
+a string and a "pattern" to RE (regular expression) engine to match,
+RE engine finds (or does not find) the match, and Perl uses the findings
+of RE engine for its operation, possibly asking the engine for other matches.
+
+RE engine has no idea what Perl is going to do with what it finds,
+similarly, the rest of Perl has no idea what a particular regular expression
+means to RE engine. This creates a clean separation, and in this section
+we discuss matching from Perl point of view only. The other point of
+view may be found in L<perlre>.
+
=over 8
=item ?PATTERN?
something in each file of a set of files, for instance. Only C<??>
patterns local to the current package are reset.
+ while (<>) {
+ if (?^$?) {
+ # blank line between header and body
+ }
+ } continue {
+ reset if eof; # clear ?? status for next file
+ }
+
This usage is vaguely deprecated, and may be removed in some future
version of Perl.
=item /PATTERN/cgimosx
-Searches a string for a pattern match, and in a scalar context returns
+Searches a string for a pattern match, and in 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
+See L<perllocale> for discussion of additional considerations that apply
when C<use locale> is in effect.
Options are:
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
+you can use any pair of non-alphanumeric, non-whitespace characters
+as delimiters (if single quotes are used, no interpretation is done
+on the replacement string. Unlike Perl 4, Perl 5 treats backticks as normal
+delimiters; the replacement text is not evaluated as a command).
+This is particularly useful for matching Unix path names
that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
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 matched regular expression is used instead.
+If the PATTERN evaluates to the empty string, the last
+I<successfully> matched regular expression is used instead.
-If used in a context that requires a list value, a pattern match returns a
+If the C</g> option is not used, C<m//> in a 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.) If there are no parentheses,
+the return value is the list C<(1)> for success or C<('')> upon failure.
+With parentheses, C<()> is returned upon failure.
Examples:
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
+the context. In 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.
-In a scalar context, C<m//g> iterates through the string, returning TRUE
+In 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
=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:
- $today = qx{ date };
+ $output = `cmd 2>&1`;
+
+To capture a command's STDOUT but discard its STDERR:
+
+ $output = `cmd 2>/dev/null`;
+
+To capture a command's STDERR but discard its STDOUT (ordering is
+important here):
+
+ $output = `cmd 2>&1 1>/dev/null`;
+
+To exchange a command's STDOUT and STDERR in order to capture the STDERR
+but leave its STDOUT to come out the old STDERR:
+
+ $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
+
+To read both a command's STDOUT and its STDERR separately, it's easiest
+and safest to redirect them separately to files, and then read from those
+files when the program is done:
+
+ system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");
+
+Using single-quote as a delimiter protects the command from Perl's
+double-quote interpolation, passing it on to the shell instead:
+
+ $perl_info = qx(ps $$); # that's Perl's $$
+ $shell_info = qx'ps $$'; # that's the new shell's $$
+
+Note that how the 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.
-Note that how the 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.
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
limit after any necessary interpolations. See the platform-specific
release notes for more details about your particular environment.
-Also realize that using this operator frequently leads to unportable
-programs.
+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.
split(' ', q/STRING/);
+This equivalency means that if used in scalar context, you'll get split's
+(unfortunate) scalar context behavior, complete with mysterious warnings.
+
Some frequently seen examples:
use POSIX qw( setlocale localeconv )
@EXPORT = qw( foo bar baz );
A common mistake is to try to separate the words with comma or to put
-comments into a multi-line qw-string. For this reason the C<-w>
+comments into a multi-line C<qw>-string. For this reason the C<-w>
switch produce warnings if the STRING contains the "," or the "#"
character.
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
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:
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
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.
=back
+=head2 Gory details of parsing quoted constructs
+
+When presented with something which may have several different
+interpretations, Perl uses the principle B<DWIM> (expanded to Do What I Mean
+- not what I wrote) to pick up the most probable interpretation of the
+source. This strategy is so successful that Perl users usually do not
+suspect ambivalence of what they write. However, time to time Perl's ideas
+differ from what the author meant.
+
+The target of this section is to clarify the Perl's way of interpreting
+quoted constructs. The most frequent reason one may have to want to know the
+details discussed in this section is hairy regular expressions. However, the
+first steps of parsing are the same for all Perl quoting operators, so here
+they are discussed together.
+
+Some of the passes discussed below are performed concurrently, but as
+far as results are the same, we consider them one-by-one. For different
+quoting constructs Perl performs different number of passes, from
+one to five, but they are always performed in the same order.
+
+=over
+
+=item Finding the end
+
+First pass is finding the end of the quoted construct, be it multichar ender
+C<"\nEOF\n"> of C<<<EOF> construct, C</> which terminates C<qq/> construct,
+C<E<]>> which terminates C<qq[> construct, or C<E<gt>> which terminates a
+fileglob started with C<<>.
+
+When searching for multichar construct no skipping is performed. When
+searching for one-char non-matching delimiter, such as C</>, combinations
+C<\\> and C<\/> are skipped. When searching for one-char matching delimiter,
+such as C<]>, combinations C<\\>, C<\]> and C<\[> are skipped, and
+nested C<[>, C<]> are skipped as well.
+
+For 3-parts constructs C<s///> etc. the search is repeated once more.
+
+During this search no attension is paid to the semantic of the construct, thus
+
+ "$hash{"$foo/$bar"}"
+
+or
+
+ m/
+ bar # This is not a comment, this slash / terminated m//!
+ /x
+
+do not form legal quoted expressions. Note that since the slash which
+terminated C<m//> was followed by a C<SPACE>, this is not C<m//x>,
+thus C<#> was interpreted as a literal C<#>.
+
+=item Removal of backslashes before delimiters
+
+During the second pass the text between the starting delimiter and
+the ending delimiter is copied to a safe location, and the C<\> is
+removed from combinations consisting of C<\> and delimiter(s) (both starting
+and ending delimiter if they differ).
+
+The removal does not happen for multi-char delimiters.
+
+Note that the combination C<\\> is left as it was!
+
+Starting from this step no information about the delimiter(s) is used in the
+parsing.
+
+=item Interpolation
+
+Next step is interpolation in the obtained delimiter-independent text.
+There are many different cases.
+
+=over
+
+=item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///>
+
+No interpolation is performed.
+
+=item C<''>, C<q//>
+
+The only interpolation is removal of C<\> from pairs C<\\>.
+
+=item C<"">, C<``>, C<qq//>, C<qx//>, C<<file*globE<gt>>
+
+C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are converted
+to corresponding Perl constructs, thus C<"$foo\Qbaz$bar"> is converted to
+
+ $foo . (quotemeta("baz" . $bar));
+
+Other combinations of C<\> with following chars are substituted with
+appropriate expansions.
+
+Interpolated scalars and arrays are converted to C<join> and C<.> Perl
+constructs, thus C<"'@arr'"> becomes
+
+ "'" . (join $", @arr) . "'";
+
+Since all three above steps are performed simultaneously left-to-right,
+the is no way to insert a literal C<$> or C<@> inside C<\Q\E> pair: it
+cannot be protected by C<\>, since any C<\> (except in C<\E>) is
+interpreted as a literal inside C<\Q\E>, and any $ is
+interpreted as starting an interpolated scalar.
+
+Note also that the interpolating code needs to make decision where the
+interpolated scalar ends, say, whether C<"a $b -> {c}"> means
+
+ "a " . $b . " -> {c}";
+
+or
+
+ "a " . $b -> {c};
+
+Most the time the decision is to take the longest possible text which does
+not include spaces between components and contains matching braces/brackets.
+
+=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 qq// constructs, but I<the substitution of C<\> followed by
+other chars is not performed>! Moreover, inside C<(?{BLOCK})> no processing
+is performed at all.
+
+Interpolation has several quirks: $|, $( and $) are not interpolated, and
+constructs C<$var[SOMETHING]> are I<voted> (by several different estimators)
+to be an array element or $var followed by a RE alternative. This is
+the place where the notation C<${arr[$bar]}> comes handy: C</${arr[0-9]}/>
+is interpreted as an array element -9, not as a regular expression from
+variable $arr followed by a digit, which is the interpretation of
+C</$arr[0-9]/>.
+
+Note that absense of processing of C<\\> creates specific restrictions on the
+post-processed text: if the delimeter 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. Since C</> is equivalent to C<\/> inside a regular expression, this
+does not matter unless the delimiter is special character for RE engine, as
+in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>.
+
+=back
+
+This step is the last one for all the constructs except regular expressions,
+which are processed further.
+
+=item Interpolation of regular expressions
+
+All the previous steps were performed during the compilation of Perl code,
+this one happens in run time (though it may be optimized to be calculated
+at compile time if appropriate). After all the preprocessing performed
+above (and possibly after evaluation if catenation, joining, up/down-casing
+and quotemeta()ing are involved) the resulting I<string> is passed to RE
+engine for compilation.
+
+Whatever happens in the RE engine is better be discussed in L<perlre>,
+but for the sake of continuity let us do it here.
+
+This is the first step where presense of the C<//x> switch is relevant.
+RE engine scans the string left-to-right, and converts it to a finite
+automaton.
+
+Backslashed chars are either substituted by corresponding literal
+strings, or generate special nodes of the finite automaton. Characters
+which are special to RE engine generate corresponding nodes. C<(?#...)>
+comments are ignored. All the rest is either converted to literal strings
+to match, or is ignored (as is whitespace and C<#>-style comments if
+C<//x> is present).
+
+Note that the parsing of the construct C<[...]> is performed using
+absolutely different rules than the rest of the regular expression.
+Similarly, the C<(?{...})> is only checked for matching braces.
+
+=item Optimization of regular expressions
+
+This step is listed for compeleteness only. Since it does not change
+semantics, details of this step are not documented and are subject
+to change.
+
+=back
+
=head2 I/O Operators
There are several I/O operators you should know about.
-A string is enclosed by backticks (grave accents) first undergoes
+A string 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
-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,
+of the pseudo-literal, like in a shell. In scalar context, a single
+string consisting of all the output is returned. In list context,
a list of values is returned, one for each line of output. (You can
set C<$/> to use a different line terminator.) The command is executed
each time the pseudo-literal is evaluated. The status value of the
situation where an automatic assignment happens. I<If and ONLY if> the
input symbol is the only thing inside the conditional of a C<while> or
C<for(;;)> loop, the value is automatically assigned to the variable
-C<$_>. 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:
+C<$_>. In these loop constructs, the assigned value (whether assignment
+is automatic or explicit) is then tested to see if it is defined.
+The defined test avoids problems where line has a string value
+that would be treated as false by perl e.g. "" or "0" with no trailing
+newline. (This may seem like an odd thing to you, but you'll use the
+construct in almost every Perl script you write.) Anyway, the following
+lines are equivalent to each other:
while (defined($_ = <STDIN>)) { print; }
+ while ($_ = <STDIN>) { print; }
while (<STDIN>) { print; }
for (;<STDIN>;) { print; }
print while defined($_ = <STDIN>);
+ print while ($_ = <STDIN>);
print while <STDIN>;
+and this also behaves similarly, but avoids the use of $_ :
+
+ while (my $line = <STDIN>) { print $line }
+
+If you really mean such values to terminate the loop they should be
+tested for explicitly:
+
+ while (($_ = <STDIN>) ne '0') { ... }
+ while (<STDIN>) { last unless $_; ... }
+
+In other boolean contexts, C<E<lt>I<filehandle>E<gt>> without explicit C<defined>
+test or comparison will solicit a warning if C<-w> is in effect.
+
The filehandles STDIN, STDOUT, and STDERR are predefined. (The
filehandles C<stdin>, C<stdout>, and C<stderr> will also work except in
packages, where they would be interpreted as local identifiers rather
behavior of B<sed> and B<awk>. Input from E<lt>E<gt> comes either from
standard input, or from each file listed on the command line. Here's
how it works: the first time E<lt>E<gt> is evaluated, the @ARGV array is
-checked, and if it is null, C<$ARGV[0]> is set to "-", which when opened
+checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
gives you standard input. The @ARGV array is then processed as a list
of filenames. The loop
You can modify @ARGV before the first E<lt>E<gt> as long as the array ends up
containing the list of filenames you really want. Line numbers (C<$.>)
continue as if the input were one big happy file. (But see example
-under eof() for how to reset line numbers on each file.)
+under C<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:
+
+ @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 < $_ |" : $_ } @ARGV;
-If you want to set @ARGV to your own list of files, go right ahead. If
-you want to pass switches into your script, you can use one of the
+If you want to pass switches into your script, you can use one of the
Getopts modules or put a loop on the front like this:
while ($_ = $ARGV[0], /^-/) {
last if /^--$/;
if (/^-D(.*)/) { $debug = $1 }
if (/^-v/) { $verbose++ }
- ... # other switches
+ # ... # other switches
}
+
while (<>) {
- ... # code for each line
+ # ... # code for each line
}
-The E<lt>E<gt> symbol will return FALSE only once. If you call it again after
-this it will assume you are processing another @ARGV list, and if you
-haven't set @ARGV, will input from STDIN.
+The E<lt>E<gt> symbol will return C<undef> for end-of-file only once.
+If you call it again after this it will assume you are processing another
+@ARGV list, and if you haven't set @ARGV, will input from STDIN.
If the string inside the angle brackets is a reference to a scalar
variable (e.g., E<lt>$fooE<gt>), then that variable contains the name of the
-filehandle to input from, or a reference to the same. For example:
+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.) Example:
while (<*.c>) {
chmod 0644, $_;
A glob evaluates its (embedded) argument only when it is starting a new
list. All values must be read before it will start over. In a list
context this isn't important, because you automatically get them all
-anyway. In a scalar context, however, the operator returns the next value
-each time it is called, or a FALSE value if you've just run out. Again,
-FALSE is returned only once. So if you're expecting a single value from
-a glob, it is much better to say
+anyway. In scalar context, however, the operator returns the next value
+each time it is called, or a C<undef> value if you've just run out. As
+for filehandles an automatic C<defined> is generated when the glob
+occurs in the test part of a C<while> or C<for> - because legal glob returns
+(e.g. a file called F<0>) would otherwise terminate the loop.
+Again, C<undef> is returned only once. So if you're expecting a single value
+from a glob, it is much better to say
($file) = <blurch*>;
=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
you say
foreach $file (@filenames) {
- if (-s $file > 5 + 100 * 2**16) { ... }
+ if (-s $file > 5 + 100 * 2**16) { }
}
the compiler will precompute the number that
for them. By default, their results are interpreted as unsigned
integers. However, if C<use integer> is in effect, their results are
interpreted as signed integers. For example, C<~0> usually evaluates
-to a large integral value. However, C<use integer; ~0> is -1.
+to a large integral value. However, C<use integer; ~0> is -1 on twos-complement machines.
=head2 Floating-point Arithmetic
decimal places. For rounding to a certain number of digits, sprintf()
or printf() is usually the easiest route.
+Floating-point numbers are only approximations to what a mathematician
+would call real numbers. There are infinitely more reals than floats,
+so some corners must be cut. For example:
+
+ printf "%.20g\n", 123456789123456789;
+ # produces 123456789123456784
+
+Testing for exact equality of floating-point equality or inequality is
+not a good idea. Here's a (relatively expensive) work-around to compare
+whether two floating-point numbers are equal to a particular number of
+decimal places. See Knuth, volume II, for a more robust treatment of
+this topic.
+
+ sub fp_equal {
+ my ($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 a number of other mathematical and trigonometric
functions. The Math::Complex module (part of the standard perl
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.
+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
projects / p5sagit/p5-mst-13.2.git / blobdiff