If any list operator (print(), etc.) or any unary operator (chdir(), etc.) is followed by a left parenthesis as the next token, the operator and arguments within parentheses are taken to be of highest precedence, just like a normal function call.
In the absence of parentheses, the precedence of list operators such as print , sort , or 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. 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
then act like a simple TERM with regard to the preceding expression.
Note that you have to be careful with parens:
# These evaluate exit before doing the print:
print($foo, exit); # Obviously not what you want.
print $foo, exit; # Nor is this.
# These do the print before evaluating exit:
(print $foo), exit; # This is what you want.
print($foo), exit; # Or this.
print ($foo), exit; # Or even this.
Also note that
print ($foo & 255) + 1, "\n";
probably doesn't do what you expect at first glance. See
Named Unary Operators
for more discussion of this.
Also parsed as terms are the do {} and eval {} constructs, as well as subroutine and method calls, and the anonymous constructors [] and {}.
See also Quote and Quotelike Operators toward the end of this section, as well as I/O Operators .
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 the perlobj manpage .
The autoincrement operator has a little extra built-in 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 has a value that is not null and matches the pattern /^[a-zA-Z]*[0-9]*$/, the increment is done as a string, preserving each character within its range, with carry:
print ++($foo = '99'); # prints '100'
print ++($foo = 'a0'); # prints 'a1'
print ++($foo = 'Az'); # prints 'Ba'
print ++($foo = 'zz'); # prints 'aaa'
The autodecrement operator is not magical.
Unary "-" performs arithmetic negation if the operand is numeric. If the operand is an identifier, a string consisting of a minus sign concatenated with the identifier is returned. Otherwise, if the string starts with a plus or minus, a string starting with the opposite sign is returned. One effect of these rules is that -bareword is equivalent to "-bareword".
Unary "~" performs bitwise negation, i.e. 1's complement.
Unary "+" has no effect whatsoever, even on strings. It is useful syntactically for separating a function name from a parenthesized expression that would otherwise be interpreted as the complete list of function arguments. (See examples above under List Operators.)
Unary "\" creates a reference to whatever follows it. See the perlref manpage . Do not confuse this behavior with the behavior of backslash within a string, although both forms do convey the notion of protecting the next thing from interpretation.
Binary "!~" is just like "=~" except the return value is negated in the logical sense.
Binary "/" divides two numbers.
Binary "%" computes the modulus of the two numbers.
Binary "x" is the repetition operator. In a scalar context, it returns a string consisting of the left operand repeated the number of times specified by the right operand. In a list context, if the left operand is a list in parens, it repeats the list.
print '-' x 80; # print row of dashes
print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
@ones = (1) x 80; # a list of 80 1's
@ones = (5) x @ones; # set all elements to 5
Binary "-" returns the difference of two numbers.
Binary "." concatenates two strings.
Binary >>" returns the value of its left argument shifted right by the number of bits specified by the right argument. Arguments should be integers.
If any list operator (print(), etc.) or any unary operator (chdir(), etc.) is followed by a left parenthesis as the next token, the operator and arguments within parentheses are taken to be of highest precedence, just like a normal function call. Examples:
chdir $foo || die; # (chdir $foo) || die
chdir($foo) || die; # (chdir $foo) || die
chdir ($foo) || die; # (chdir $foo) || die
chdir +($foo) || die; # (chdir $foo) || die
but, because * is higher precedence than ||:
chdir $foo * 20; # chdir ($foo * 20)
chdir($foo) * 20; # (chdir $foo) * 20
chdir ($foo) * 20; # (chdir $foo) * 20
chdir +($foo) * 20; # chdir ($foo * 20)
rand 10 * 20; # rand (10 * 20)
rand(10) * 20; # (rand 10) * 20
rand (10) * 20; # (rand 10) * 20
rand +(10) * 20; # rand (10 * 20)
See also "List Operators".
Binary ">" returns true if the left argument is numerically greater than the right argument.
Binary "<=" 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 than or equal to the right argument.
Binary "lt" returns true if the left argument is stringwise less than the right argument.
Binary "gt" returns true if the left argument is stringwise greater than the right argument.
Binary "le" returns true if the left argument is stringwise less than or equal to the right argument.
Binary "ge" returns true if the left argument is stringwise greater than or equal to the right argument.
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 "eq" returns true if the left argument is stringwise equal to the right argument.
Binary "ne" returns true if the left argument is stringwise not equal to the right argument.
Binary "cmp" returns -1, 0, or 1 depending on whether the left argument is stringwise less than, equal to, or greater than the right argument.
Binary "^" returns its operators XORed together bit by bit.
The || and && operators differ from C's in that, rather than returning 0 or 1, they return the last value evaluated. Thus, a reasonably portable way to find out the home directory (assuming it's not "0") might be:
$home = $ENV{'HOME'} || $ENV{'LOGDIR'} ||
(getpwuid($<))[7] || die "You're homeless!\n";
As more readable alternatives to && and ||, Perl provides "and" and
"or" operators (see below). The short-circuit behavior is identical. 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;
With the C-style operators that would have been written like this:
unlink("alpha", "beta", "gamma")
|| (gripe(), next LINE);
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
of sed, 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, AFTER which the range operator becomes false
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 awk), but it still returns true once.
If you don't want it to test the right operand till the next evaluation
(as in sed), use three dots ("...") instead of two.) The 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
$.
variable, the current
line number. Examples:
As a scalar operator:
if (101 .. 200) { print; } # print 2nd hundred lines
next line if (1 .. /^$/); # skip header lines
s/^/> / if (/^$/ .. eof()); # quote body
As a list operator:
for (101 .. 200) { print; } # print $_ 100 times
@foo = @foo[$[ .. $#foo]; # an expensive no-op
@foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
The range operator (in a list context) makes use of the magical
autoincrement algorithm if the operaands are strings. You
can say
@alphabet = ('A' .. 'Z');
to get all the letters of the alphabet, or
$hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
to get a hexadecimal digit, or
@z2 = ('01' .. '31'); print $z2[$mday];
to get dates with leading zeros. If the final value specified is not
in the sequence that the magical increment would produce, the sequence
goes until the next value would be longer than the final value
specified.
($a_or_b ? $a : $b) = $c;
Note that this is not guaranteed to contribute to the readability of
your program.
Assignment operators work as in C. That is,
$a += 2;
is equivalent to
$a = $a + 2;
although without duplicating any side effects that dereferencing the lvalue
might trigger, such as from tie(). Other assignment operators work similarly.
The following are recognized:
**= += *= &= <<= &&=
-= /= |= >>= ||=
.= %= ^=
x=
Note that while these are grouped by family, they all have the precedence
of assignment.
Unlike in C, the assignment operator produces a valid lvalue. Modifying an assignment is equivalent to doing the assignment and then modifying the variable that was assigned to. This is useful for modifying a copy of something, like this:
($tmp = $global) =~ tr [A-Z] [a-z];
Likewise,
($a += 2) *= 3;
is equivalent to
$a += 2;
$a *= 3;
Binary "," is the comma operator. In a 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 both its arguments into the list.
open HANDLE, "filename"
or die "Can't open: $!\n";
See also discussion of list operators in List Operators (Leftward).
Binary "xor" returns the exclusive-OR of the two surrounding expressions. It cannot short circuit, of course.
Customary Generic Meaning Interpolates
'' q{} Literal no
"" qq{} Literal yes
`` qx{} Command yes
qw{} Word list no
// m{} Pattern match yes
s{}{} Substitution yes
tr{}{} Translation no
For constructs that do interpolation, variables beginning with "$ or "@"
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
\033 octal char
\x1b hex char
\c[ control char
\l lowercase next char
\u uppercase next char
\L lowercase till \E
\U uppercase till \E
\E end case modification
\Q quote regexp metacharacters till \E
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
pattern from the variables. If this is not what you want, use \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, backquotes do NOT interpolate within double quotes, nor do single quotes impede evaluation of variables when used within double quotes.
PATTERN may contain variables, which will be interpolated (and the pattern recompiled) every time the pattern search is evaluated. (Note that $) and $| 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 /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 /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 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 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.
Examples:
open(TTY, '/dev/tty');
<TTY> =~ /^y/i && foo(); # do foo if desired
if (/Version: *([0-9.]*)/) { $version = $1; }
next if m#^/usr/spool/uucp#;
# poor man's grep
$arg = shift;
while (<>) {
print if /$arg/o; # compile only once
}
if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
This last example splits $foo into the first two words and the
remainder of the line, and assigns those three fields to $F1, $F2 and
$Etc. The conditional is true if any variables were assigned, i.e. if
the pattern matched.
The /g modifier specifies global pattern matching--that is, matching as many times as possible within the string. How it behaves depends on the context. In a list context, it returns a list of all the substrings matched by all the parentheses in the regular expression. If there are no parentheses, it returns a list of all the matched strings, as if there were parentheses around the whole pattern.
In a scalar context, m//g iterates through the string, returning TRUE each time it matches, and FALSE when it eventually runs out of matches. (In other words, it remembers where it left off last time and restarts the search at that point. You can actually find the current match position of a string using the pos() function--see the perlfunc manpage .) If you modify the string in any way, the match position is reset to the beginning. Examples:
# list context
($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
# scalar context
$/ = ""; $* = 1; # $* deprecated in Perl 5
while ($paragraph = <>) {
while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
$sentences++;
}
}
print "$sentences\n";
$foo = q!I said, "You said, 'She said it.'"!;
$bar = q('This is it.');
$_ .= qq
(*** The previous line contains the naughty word "$1".\n)
if /(tcl|rexx|python)/; # :-)
$today = qx{ date };
See
I/O Operators
for more discussion.
split(' ', q/STRING/);
Some frequently seen examples:
use POSIX qw( setlocale localeconv )
@EXPORT = qw( foo bar baz );
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 the variable is interpolated, use the /o option. If the pattern evaluates to a null string, the most recently executed (and successfully compiled) regular expression is used instead. See the perlre manpage for further explanation on these.
Options are:
Examples:
s/\bgreen\b/mauve/g; # don't change wintergreen
$path =~ s|/usr/bin|/usr/local/bin|;
s/Login: $foo/Login: $bar/; # run-time pattern
($foo = $bar) =~ s/this/that/;
$count = ($paragraph =~ s/Mister\b/Mr./g);
$_ = 'abc123xyz';
s/\d+/$&*2/e; # yields 'abc246xyz'
s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
s/%(.)/$percent{$1}/g; # change percent escapes; no /e
s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
s/^=(\w+)/&pod($1)/ge; # use function call
# /e's can even nest; this will expand
# simple embedded variables in $_
s/(\$\w+)/$1/eeg;
# Delete C comments.
$program =~ s {
/\* (?# Match the opening delimiter.)
.*? (?# Match a minimal number of characters.)
\*/ (?# Match the closing delimiter.)
} []gsx;
s/^\s*(.*?)\s*$/$1/; # trim white space
s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
Note the use of $ instead of \ in the last example. Unlike
sed, we only use the \<digit> form in the left hand side.
Anywhere else it's $<digit>.
Occasionally, you can't just use a /g to get all the changes to occur. Here are two common cases:
# put commas in the right places in an integer
1 while s/(.*\d)(\d\d\d)/$1,$2/g; # perl4
1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; # perl5
# expand tabs to 8-column spacing
1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
If the /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. This latter is useful for counting characters in a class or for squashing character sequences in a class.
Examples:
$ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
$cnt = tr/*/*/; # count the stars in $_
$cnt = $sky =~ tr/*/*/; # count the stars in $sky
$cnt = tr/0-9//; # count the digits in $_
tr/a-zA-Z//s; # bookkeeper -> bokeper
($HOST = $host) =~ tr/a-z/A-Z/;
tr/a-zA-Z/ /cs; # change non-alphas to single space
tr [\200-\377]
[\000-\177]; # delete 8th bit
Note that because the translation table is built at compile time, neither
the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
interpolation. That means that if you want to use variables, you must use
an eval():
eval "tr/$oldlist/$newlist/";
die $@ if $@;
eval "tr/$oldlist/$newlist/, 1" or die $@;
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. If and ONLY if the input symbol is the only thing inside the conditional of a while loop, the value is automatically assigned to the variable $_ . (This may seem like an odd thing to you, but you'll use the construct in almost every Perl script you write.) Anyway, the following lines are equivalent to each other:
while ($_ = <STDIN>) { print; }
while (<STDIN>) { print; }
for (;<STDIN>;) { print; }
print while $_ = <STDIN>;
print while <STDIN>;
The filehandles STDIN, STDOUT and STDERR are predefined. (The
filehandles stdin, stdout and 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.
If a <FILEHANDLE> 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 LARGE data space this way, so use with care.
The null filehandle <> is special and can be used to emulate the behavior of sed and awk. Input from <> 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 checked, and if it is null, $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
while (<>) {
... # code for each line
}
is equivalent to the following Perl-like pseudo code:
unshift(@ARGV, '-') if $#ARGV < $[;
while ($ARGV = shift) {
open(ARGV, $ARGV);
while (<ARGV>) {
... # code for each line
}
}
except that it isn't so cumbersome to say, and will actually work. It
really does shift array @ARGV and put the current filename into variable
$ARGV. It also uses filehandle 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.)
You can modify @ARGV before the first <> as long as the array ends up containing the list of filenames you really want. Line numbers ( $. ) continue as if the input were one big happy file. (But see example under eof() for how to reset line numbers on each file.)
If you want to set @ARGV to your own list of files, go right ahead. If you want to pass switches into your script, you can use one of the Getopts modules or put a loop on the front like this:
while ($_ = $ARGV[0], /^-/) {
shift;
last if /^--$/;
if (/^-D(.*)/) { $debug = $1 }
if (/^-v/) { $verbose++ }
... # other switches
}
while (<>) {
... # code for each line
}
The <> symbol will return FALSE only once. If you call it again after
this it will assume you are processing another @ARGV list, and if you
haven't set @ARGV, will input from STDIN.
If the string inside the angle brackets is a reference to a scalar variable (e.g. <$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 <$foo> because that's an indirect filehandle as explained in the previous paragraph. You could insert curly brackets to force interpretation as a filename glob: <${foo}>. (Alternately, you can call the internal function directly as glob($foo), which is probably the right way to have done it in the first place.) Example:
while (<*.c>) {
chmod 0644, $_;
}
is equivalent to
open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
while (<FOO>) {
chop;
chmod 0644, $_;
}
In fact, it's currently implemented that way. (Which means it will 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 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
long" errors (unless you've installed tcsh(1L) as /bin/csh).
'Now is the time for all' . "\n" .
'good men to come to.'
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
expression represents so that the interpreter
won't have to.
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
no integer;
which lasts until the end of that BLOCK.