left terms and list operators (leftward)
left ->
nonassoc ++ --
right **
right ! ~ \ and unary + and -
left =~ !~
left * / % x
left + - .
left << >>
nonassoc named unary operators
nonassoc < > <= >= lt gt le ge
nonassoc == != <=> eq ne cmp
left &
left | ^
left &&
left ||
nonassoc ..
right ?:
right = += -= *= etc.
left , =>
nonassoc list operators (rightward)
right not
left and
left or xor
In the following sections, these operators are covered in precedence order.
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 parentheses:
# 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 Quote-like Operators toward the end of this section, as well as I/O Operators.
->'' is an infix dereference operator. If the right side is either a [...] or {...} subscript, then the left side must be either a hard or symbolic reference
to an array or hash (or a location capable of holding a hard reference, if
it's an lvalue (assignable)). See the perlref manpage.
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 auto-increment 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
been used in only 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 auto-decrement operator is not magical.
pow function, which actually works on
doubles internally.)
not for a lower precedence version of this.
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. (See also 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 Terms and List Operators (Leftward).)
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.
$_ 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 $_. The return value indicates the success of the
operation. (If the right argument is an expression rather than a search
pattern, substitution, or translation, it is interpreted as a search
pattern at run time. This can be is less efficient than an explicit search,
because the pattern must be compiled every time the expression is
evaluated.
Binary ``!~'' is just like ``=~'' except the return value is negated in the logical sense.
Binary ``/'' divides two numbers.
Binary ``%'' computes the modulus of the two numbers.
Binary ``x'' is the repetition operator. In a scalar context, it returns a string consisting of the left operand repeated the number of times specified by the right operand. In a list context, if the left operand is a list in parentheses, it repeats the list.
print '-' x 80; # print row of dashes
print "\t" x ($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. (See also Integer Arithmetic.)
-f, -M, etc. See the perlfunc manpage.
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 Terms and List Operators (Leftward).
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.
``lt'', ``le'', ``ge'', ``gt'' and ``cmp'' use the collation (sort) order
specified by the current locale if use locale is in effect. See the perllocale manpage.
Binary ``^'' returns its operators XORed together bit by bit. (See also Integer Arithmetic.)
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 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:
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 auto-increment algorithm if the operands 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.
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;
This is not necessarily 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;
In a list context, it's just the list argument separator, and inserts both its arguments into the list.
The => digraph is mostly just a synonym for the comma operator. It's useful for documenting arguments that come in pairs. As of release 5.001, it also forces any word to the left of it to be interpreted as a string.
open HANDLE, "filename" or die "Can't open: $!\n";
See also discussion of list operators in Terms and List Operators (Leftward).
Binary ``xor'' returns the exclusive-OR of the two surrounding expressions. It cannot short circuit, of course.
{} represents any pair of delimiters you choose. Non-bracketing delimiters use
the same character fore and aft, but the 4 sorts of brackets (round, angle,
square, curly) will all nest.
Customary Generic Meaning Interpolates
'' q{} Literal no
"" qq{} Literal yes
`` qx{} Command yes
qw{} Word list no
// m{} Pattern match yes
s{}{} Substitution yes
tr{}{} Translation no
For constructs that do interpolation, variables beginning with ``$'' or ``@'' are interpolated, as are the following sequences:
\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
\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
If use locale is in effect, the case map used by \l, \L, \u
and <
\U> is taken from the current locale. See the perllocale manpage.
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, back-quotes do NOT interpolate within double quotes, nor do single quotes impede evaluation of variables when used within double quotes.
/pattern/ search, except that it matches only once between calls to the
reset operator. This is a useful optimization when you want to
see only the first occurrence of something in each file of a set of files,
for instance. Only ??
patterns local to the current package are reset.
This usage is vaguely deprecated, and may be removed in some future version of Perl.
=~ or
!~ operator, the $_ string is searched. (The string specified
with
=~ need not be an lvalue--it may be the result of an expression evaluation,
but remember the =~ binds rather tightly.) See also
the perlre manpage. See the perllocale manpage for discussion of additional considerations which apply when use locale is in effect.
Options are:
g Match globally, i.e., find all occurrences.
i Do case-insensitive pattern matching.
m Treat string as multiple lines.
o Compile pattern only once.
s Treat string as single line.
x Use extended regular expressions.
If ``/'' is the delimiter then the initial m is optional. With the 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).
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 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 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 $Etc. The conditional is true if any variables were assigned, i.e., if the pattern matched.
The /g modifier specifies global pattern matching--that is, matching as many times
as possible within the string. How it behaves depends on the context. In a
list context, it returns a list of all the substrings matched by all the
parentheses in the regular expression. If there are no parentheses, it
returns a list of all the matched strings, as if there were parentheses
around the whole pattern.
In a scalar context, m//g iterates through the string, returning
TRUE each time it matches, and
FALSE when it eventually runs out of matches. (In other words, it remembers where it left off last time and restarts the search at that point. You can actually find the current match position of a string or set it using the pos function--see
pos.) Note that you can use this feature to stack m//g
matches or intermix m//g matches with m/\G.../.
If you modify the string in any way, the match position is reset to the beginning. Examples:
# list context
($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
# scalar context
$/ = ""; $* = 1; # $* deprecated in modern perls
while ($paragraph = <>) {
while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
$sentences++;
}
}
print "$sentences\n";
# using m//g with \G
$_ = "ppooqppq";
while ($i++ < 2) {
print "1: '";
print $1 while /(o)/g; print "', pos=", pos, "\n";
print "2: '";
print $1 if /\G(q)/; print "', pos=", pos, "\n";
print "3: '";
print $1 while /(p)/g; print "', pos=", pos, "\n";
}
The last example should print:
1: 'oo', pos=4
2: 'q', pos=4
3: 'pp', pos=7
1: '', pos=7
2: 'q', pos=7
3: '', pos=7
Note how m//g matches change the value reported by pos, but the non-global match doesn't.
A useful idiom for lex-like scanners is /\G.../g. You can combine several regexps like this to process a string
part-by-part, doing different actions depending on which regexp matched.
The next regexp would step in at the place the previous one left off.
$_ = <<'EOL';
$url = new URI::URL "http://www/";; die if $url eq "xXx";
EOL
LOOP:
{
print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/g;
print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/g;
print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/g;
print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/g;
print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/g;
print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/g;
print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/g;
print ". 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!
$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 for more discussion.
split(' ', q/STRING/);
Some frequently seen examples:
use POSIX qw( setlocale localeconv )
@EXPORT = qw( foo bar baz );
If no string is specified via the =~ or !~ operator, the $_
variable is searched and modified. (The string specified with =~ must be a scalar variable, an array element, a hash element, or an
assignment to one of those, i.e., an lvalue.)
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 want the pattern compiled only once the first time the variable is interpolated, use the
/o option. If the pattern evaluates to a null string, the last successfully
executed regular expression is used instead. See the perlre manpage for further explanation on these. See the perllocale manpage for discussion of additional considerations which apply when use locale is in effect.
Options are:
e Evaluate the right side as an expression.
g Replace globally, i.e., all occurrences.
i Do case-insensitive pattern matching.
m Treat string as multiple lines.
o Compile pattern only once.
s Treat string as single line.
x Use extended regular expressions.
Any non-alphanumeric, non-whitespace delimiter may replace the slashes. If
single quotes are used, no interpretation is done on the replacement string
(the /e modifier overrides this, however). Unlike Perl 4, Perl 5 treats back-ticks 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.,
s or s<foo>/bar/.
A /e will cause the replacement portion to be interpreter as a full-fledged Perl
expression and evaled right then and there. It is, however,
syntax checked at compile-time.
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 use the \<digit> form in only the left hand side. Anywhere else it's $<digit>.
Occasionally, you can't use just 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;
$_ 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 sed devotees,
y is provided as a synonym for 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.,
tr[A-Z][a-z]
or tr/ABCD/.
Options:
c Complement the SEARCHLIST.
d Delete found but unreplaced characters.
s Squash duplicate replaced characters.
If the /c modifier is specified, the
SEARCHLIST character set is complemented. If the /d modifier is specified, any characters specified by
SEARCHLIST not found in
REPLACEMENTLIST are deleted. (Note that this is slightly more flexible than the behavior of some
tr
programs, which delete anything they find in the
SEARCHLIST, period.) If the /s modifier is specified, sequences of characters that were translated to the
same character are squashed down to a single instance of the character.
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 = $host) =~ tr/a-z/A-Z/;
tr/a-zA-Z/ /cs; # change non-alphas to single space
tr [\200-\377]
[\000-\177]; # delete 8th bit
If multiple translations are given for a character, only the first one is used:
tr/AAA/XYZ/
will translate any A to X.
Note that because the translation table is built at compile time, neither the
SEARCHLIST nor the
REPLACEMENTLIST are subjected to double quote interpolation. That means that if you want to use variables, you must use an eval:
eval "tr/$oldlist/$newlist/";
die $@ if $@;
eval "tr/$oldlist/$newlist/, 1" or die $@;
$/ to use a different line terminator.) The command is executed each time the
pseudo-literal is evaluated. The status value of the command is returned in $? (see the perlvar manpage for the interpretation of $?). Unlike in csh, no translation is done on the return data--newlines remain newlines.
Unlike in any of the shells, single quotes do not hide variable names in
the command from interpretation. To pass a $ through to the shell you need
to hide it with a backslash. The generalized form of back-ticks is qx//. (Because back-ticks always undergo shell expansion as well, see the perlsec manpage for security concerns.)
Evaluating a filehandle in angle brackets yields the next line from that
file (newline, if any, included), or undef at end of file. 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 or
for loop, the value is automatically assigned to the variable
$_. The assigned value is then tested to see if it is defined. (This may seem
like an odd thing to you, but you'll use the construct in almost every Perl
script you write.) Anyway, the following lines are equivalent to each
other:
while (defined($_ = <STDIN>)) { print; }
while (<STDIN>) { print; }
for (;<STDIN>;) { print; }
print while defined($_ = <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. See open() for details on this.
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, 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 <$foo>
because that's an indirect filehandle as explained in the previous
paragraph. (In older versions of Perl, programmers would insert curly
brackets to force interpretation as a filename glob: <${foo}>. These days, it's considered cleaner to call the internal function
directly as glob, 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, $_;
}
In fact, it's currently implemented that way. (Which means it will not work
on filenames with spaces in them unless you have csh on your
machine.) Of course, the shortest way to do the above is:
chmod 0644, <*.c>;
Because globbing invokes a shell, it's often faster to call
readdir yourself and do your own grep on the
filenames. Furthermore, due to its current implementation of using a shell,
the glob routine may get ``Arg list too long'' errors (unless
you've installed tcsh as /bin/csh).
A glob evaluates its (embedded) argument only when it is starting a new list. All values must be read before it will start over. In a list context this isn't important, because you automatically get them all anyway. In a scalar context, however, the operator returns the next value each time it is called, or a FALSE value if you've just run out. Again, FALSE is returned only once. So if you're expecting a single value from a glob, it is much better to say
($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]);
'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.
The bitwise operators (``&'', ``|'', ``^'', ``~'', ``<<``, and ''>>``) always produce integral results. However, use integer still has meaning for them. By default, their results are interpreted as
unsigned integers. However, if use integer is in effect, their results are interpreted as signed integers. For
example, ~0 usually evaluates to a large integral value. However, use integer; ~0 is -1.