PERLSYN(1) Perl Programmers Reference Guide PERLSYN(1)
NAME
perlsyn - Perl syntax
DESCRIPTION
A Perl program consists of a sequence of declarations and statements which run from the top to the bottom.
Loops, subroutines, and other control structures allow you to jump around within the code.
Perl is a free-form language: you can format and indent it however you like. Whitespace serves mostly to
separate tokens, unlike languages like Python where it is an important part of the syntax, or Fortran where it
is immaterial.
Many of Perl's syntactic elements are optional. Rather than requiring you to put parentheses around every
function call and declare every variable, you can often leave such explicit elements off and Perl will figure
out what you meant. This is known as Do What I Mean, abbreviated DWIM. It allows programmers to be lazy and
to code in a style with which they are comfortable.
Perl borrows syntax and concepts from many languages: awk, sed, C, Bourne Shell, Smalltalk, Lisp and even
English. Other languages have borrowed syntax from Perl, particularly its regular expression extensions. So
if you have programmed in another language you will see familiar pieces in Perl. They often work the same,
but see perltrap for information about how they differ.
Declarations
The only things you need to declare in Perl are report formats and subroutines (and sometimes not even
subroutines). A scalar variable holds the undefined value ("undef") until it has been assigned a defined
value, which is anything other than "undef". When used as a number, "undef" is treated as 0; when used as a
string, it is treated as the empty string, ""; and when used as a reference that isn't being assigned to, it
is treated as an error. If you enable warnings, you'll be notified of an uninitialized value whenever you
treat "undef" as a string or a number. Well, usually. Boolean contexts, such as:
if ($a) {}
are exempt from warnings (because they care about truth rather than definedness). Operators such as "++",
"--", "+=", "-=", and ".=", that operate on undefined variables such as:
undef $a;
$a++;
are also always exempt from such warnings.
A declaration can be put anywhere a statement can, but has no effect on the execution of the primary sequence
of statements: declarations all take effect at compile time. All declarations are typically put at the
beginning or the end of the script. However, if you're using lexically-scoped private variables created with
"my()", "state()", or "our()", you'll have to make sure your format or subroutine definition is within the
same block scope as the my if you expect to be able to access those private variables.
Declaring a subroutine allows a subroutine name to be used as if it were a list operator from that point
forward in the program. You can declare a subroutine without defining it by saying "sub name", thus:
sub myname;
$me = myname $0 or die "can't get myname";
A bare declaration like that declares the function to be a list operator, not a unary operator, so you have to
be careful to use parentheses (or "or" instead of "||".) The "||" operator binds too tightly to use after
list operators; it becomes part of the last element. You can always use parentheses around the list operators
arguments to turn the list operator back into something that behaves more like a function call.
Alternatively, you can use the prototype "($)" to turn the subroutine into a unary operator:
statements as if it were an ordinary statement. That means it actually has both compile-time and run-time
effects.
Comments
Text from a "#" character until the end of the line is a comment, and is ignored. Exceptions include "#"
inside a string or regular expression.
Simple Statements
The only kind of simple statement is an expression evaluated for its side-effects. Every simple statement
must be terminated with a semicolon, unless it is the final statement in a block, in which case the semicolon
is optional. But put the semicolon in anyway if the block takes up more than one line, because you may
eventually add another line. Note that there are operators like "eval {}", "sub {}", and "do {}" that look
like compound statements, but aren't--they're just TERMs in an expression--and thus need an explicit
termination when used as the last item in a statement.
Truth and Falsehood
The number 0, the strings '0' and "", the empty list "()", and "undef" are all false in a boolean context.
All other values are true. Negation of a true value by "!" or "not" returns a special false value. When
evaluated as a string it is treated as "", but as a number, it is treated as 0. Most Perl operators that
return true or false behave this way.
Statement Modifiers
Any simple statement may optionally be followed by a SINGLE modifier, just before the terminating semicolon
(or block ending). The possible modifiers are:
if EXPR
unless EXPR
while EXPR
until EXPR
for LIST
foreach LIST
when EXPR
The "EXPR" following the modifier is referred to as the "condition". Its truth or falsehood determines how
the modifier will behave.
"if" executes the statement once if and only if the condition is true. "unless" is the opposite, it executes
the statement unless the condition is true (that is, if the condition is false).
print "Basset hounds got long ears" if length $ear >= 10;
go_outside() and play() unless $is_raining;
The "for(each)" modifier is an iterator: it executes the statement once for each item in the LIST (with $_
aliased to each item in turn).
print "Hello $_!\n" for qw(world Dolly nurse);
"while" repeats the statement while the condition is true. "until" does the opposite, it repeats the
statement until the condition is true (or while the condition is false):
# Both of these count from 0 to 10.
print $i++ while $i <= 10;
print $j++ until $j > 10;
construct, because modifiers don't take loop labels. Sorry. You can always put another block inside of it
(for "next") or around it (for "last") to do that sort of thing. For "next", just double the braces:
do {{
next if $x == $y;
# do something here
}} until $x++ > $z;
For "last", you have to be more elaborate:
LOOP: {
do {
last if $x = $y**2;
# do something here
} while $x++ <= $z;
}
NOTE: The behaviour of a "my", "state", or "our" modified with a statement modifier conditional or loop
construct (for example, "my $x if ...") is undefined. The value of the "my" variable may be "undef", any
previously assigned value, or possibly anything else. Don't rely on it. Future versions of perl might do
something different from the version of perl you try it out on. Here be dragons.
The "when" modifier is an experimental feature that first appeared in Perl 5.14. To use it, you should
include a "use v5.14" declaration. (Technically, it requires only the "switch" feature, but that aspect of it
was not available before 5.14.) Operative only from within a "foreach" loop or a "given" block, it executes
the statement only if the smartmatch "$_ ~~ EXPR" is true. If the statement executes, it is followed by a
"next" from inside a "foreach" and "break" from inside a "given".
Under the current implementation, the "foreach" loop can be anywhere within the "when" modifier's dynamic
scope, but must be within the "given" block's lexical scope. This restricted may be relaxed in a future
release. See "Switch Statements" below.
Compound Statements
In Perl, a sequence of statements that defines a scope is called a block. Sometimes a block is delimited by
the file containing it (in the case of a required file, or the program as a whole), and sometimes a block is
delimited by the extent of a string (in the case of an eval).
But generally, a block is delimited by curly brackets, also known as braces. We will call this syntactic
construct a BLOCK.
The following compound statements may be used to control flow:
if (EXPR) BLOCK
if (EXPR) BLOCK else BLOCK
if (EXPR) BLOCK elsif (EXPR) BLOCK ...
if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
unless (EXPR) BLOCK
unless (EXPR) BLOCK else BLOCK
unless (EXPR) BLOCK elsif (EXPR) BLOCK ...
unless (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
given (EXPR) BLOCK
LABEL foreach VAR (LIST) BLOCK continue BLOCK
LABEL BLOCK
LABEL BLOCK continue BLOCK
PHASE BLOCK
The experimental "given" statement is not automatically enabled; see "Switch Statements" below for how to do
so, and the attendant caveats.
Unlike in C and Pascal, in Perl these are all defined in terms of BLOCKs, not statements. This means that the
curly brackets are required--no dangling statements allowed. If you want to write conditionals without curly
brackets, there are several other ways to do it. The following all do the same thing:
if (!open(FOO)) { die "Can't open $FOO: $!" }
die "Can't open $FOO: $!" unless open(FOO);
open(FOO) || die "Can't open $FOO: $!";
open(FOO) ? () : die "Can't open $FOO: $!";
# a bit exotic, that last one
The "if" statement is straightforward. Because BLOCKs are always bounded by curly brackets, there is never
any ambiguity about which "if" an "else" goes with. If you use "unless" in place of "if", the sense of the
test is reversed. Like "if", "unless" can be followed by "else". "unless" can even be followed by one or
more "elsif" statements, though you may want to think twice before using that particular language construct,
as everyone reading your code will have to think at least twice before they can understand what's going on.
The "while" statement executes the block as long as the expression is true. The "until" statement executes
the block as long as the expression is false. The LABEL is optional, and if present, consists of an
identifier followed by a colon. The LABEL identifies the loop for the loop control statements "next", "last",
and "redo". If the LABEL is omitted, the loop control statement refers to the innermost enclosing loop. This
may include dynamically looking back your call-stack at run time to find the LABEL. Such desperate behavior
triggers a warning if you use the "use warnings" pragma or the -w flag.
If there is a "continue" BLOCK, it is always executed just before the conditional is about to be evaluated
again. Thus it can be used to increment a loop variable, even when the loop has been continued via the "next"
statement.
When a block is preceding by a compilation phase keyword such as "BEGIN", "END", "INIT", "CHECK", or
"UNITCHECK", then the block will run only during the corresponding phase of execution. See perlmod for more
details.
Extension modules can also hook into the Perl parser to define new kinds of compound statements. These are
introduced by a keyword which the extension recognizes, and the syntax following the keyword is defined
entirely by the extension. If you are an implementor, see "PL_keyword_plugin" in perlapi for the mechanism.
If you are using such a module, see the module's documentation for details of the syntax that it defines.
Loop Control
The "next" command starts the next iteration of the loop:
LINE: while (<STDIN>) {
next LINE if /^#/; # discard comments
...
}
indicate continuation, you want to skip ahead and get the next record.
while (<>) {
chomp;
if (s/\\$//) {
$_ .= <>;
redo unless eof();
}
# now process $_
}
which is Perl shorthand for the more explicitly written version:
LINE: while (defined($line = <ARGV>)) {
chomp($line);
if ($line =~ s/\\$//) {
$line .= <ARGV>;
redo LINE unless eof(); # not eof(ARGV)!
}
# now process $line
}
Note that if there were a "continue" block on the above code, it would get executed only on lines discarded by
the regex (since redo skips the continue block). A continue block is often used to reset line counters or
"m?pat?" one-time matches:
# inspired by :1,$g/fred/s//WILMA/
while (<>) {
m?(fred)? && s//WILMA $1 WILMA/;
m?(barney)? && s//BETTY $1 BETTY/;
m?(homer)? && s//MARGE $1 MARGE/;
} continue {
print "$ARGV $.: $_";
close ARGV if eof; # reset $.
reset if eof; # reset ?pat?
}
If the word "while" is replaced by the word "until", the sense of the test is reversed, but the conditional is
still tested before the first iteration.
Loop control statements don't work in an "if" or "unless", since they aren't loops. You can double the braces
to make them such, though.
if (/pattern/) {{
last if /fred/;
next if /barney/; # same effect as "last",
# but doesn't document as well
# do something here
}}
This is caused by the fact that a block by itself acts as a loop that executes once, see "Basic BLOCKs".
The form "while/if BLOCK BLOCK", available in Perl 4, is no longer available. Replace any occurrence of "if
BLOCK" by "if (do BLOCK)".
...
} continue {
$i++;
}
There is one minor difference: if variables are declared with "my" in the initialization section of the "for",
the lexical scope of those variables is exactly the "for" loop (the body of the loop and the control
sections).
Besides the normal array index looping, "for" can lend itself to many other interesting applications. Here's
one that avoids the problem you get into if you explicitly test for end-of-file on an interactive file
descriptor causing your program to appear to hang.
$on_a_tty = -t STDIN && -t STDOUT;
sub prompt { print "yes? " if $on_a_tty }
for ( prompt(); <STDIN>; prompt() ) {
# do something
}
Using "readline" (or the operator form, "<EXPR>") as the conditional of a "for" loop is shorthand for the
following. This behaviour is the same as a "while" loop conditional.
for ( prompt(); defined( $_ = <STDIN> ); prompt() ) {
# do something
}
Foreach Loops
The "foreach" loop iterates over a normal list value and sets the variable VAR to be each element of the list
in turn. If the variable is preceded with the keyword "my", then it is lexically scoped, and is therefore
visible only within the loop. Otherwise, the variable is implicitly local to the loop and regains its former
value upon exiting the loop. If the variable was previously declared with "my", it uses that variable instead
of the global one, but it's still localized to the loop. This implicit localization occurs only in a
"foreach" loop.
The "foreach" keyword is actually a synonym for the "for" keyword, so you can use either. If VAR is omitted,
$_ is set to each value.
If any element of LIST is an lvalue, you can modify it by modifying VAR inside the loop. Conversely, if any
element of LIST is NOT an lvalue, any attempt to modify that element will fail. In other words, the "foreach"
loop index variable is an implicit alias for each item in the list that you're looping over.
If any part of LIST is an array, "foreach" will get very confused if you add or remove elements within the
loop body, for example with "splice". So don't do that.
"foreach" probably won't do what you expect if VAR is a tied or other special variable. Don't do that
either.
Examples:
for (@ary) { s/foo/bar/ }
for my $elem (@elements) {
$elem *= 2;
}
Here's how a C programmer might code up a particular algorithm in Perl:
for (my $i = 0; $i < @ary1; $i++) {
for (my $j = 0; $j < @ary2; $j++) {
if ($ary1[$i] > $ary2[$j]) {
last; # can't go to outer :-(
}
$ary1[$i] += $ary2[$j];
}
# this is where that last takes me
}
Whereas here's how a Perl programmer more comfortable with the idiom might do it:
OUTER: for my $wid (@ary1) {
INNER: for my $jet (@ary2) {
next OUTER if $wid > $jet;
$wid += $jet;
}
}
See how much easier this is? It's cleaner, safer, and faster. It's cleaner because it's less noisy. It's
safer because if code gets added between the inner and outer loops later on, the new code won't be
accidentally executed. The "next" explicitly iterates the other loop rather than merely terminating the inner
one. And it's faster because Perl executes a "foreach" statement more rapidly than it would the equivalent
"for" loop.
Basic BLOCKs
A BLOCK by itself (labeled or not) is semantically equivalent to a loop that executes once. Thus you can use
any of the loop control statements in it to leave or restart the block. (Note that this is NOT true in
"eval{}", "sub{}", or contrary to popular belief "do{}" blocks, which do NOT count as loops.) The "continue"
block is optional.
The BLOCK construct can be used to emulate case structures.
SWITCH: {
if (/^abc/) { $abc = 1; last SWITCH; }
if (/^def/) { $def = 1; last SWITCH; }
if (/^xyz/) { $xyz = 1; last SWITCH; }
$nothing = 1;
}
You'll also find that "foreach" loop used to create a topicalizer and a switch:
SWITCH:
for ($var) {
if (/^abc/) { $abc = 1; last SWITCH; }
if (/^def/) { $def = 1; last SWITCH; }
if (/^xyz/) { $xyz = 1; last SWITCH; }
$nothing = 1;
}
Such constructs are quite frequently used, both because older versions of Perl had no official "switch"
statement, and also because the new version described immediately below remains experimental and can sometimes
Under the "switch" feature, Perl gains the experimental keywords "given", "when", "default", "continue", and
"break". Starting from Perl 5.16, one can prefix the switch keywords with "CORE::" to access the feature
without a "use feature" statement. The keywords "given" and "when" are analogous to "switch" and "case" in
other languages, so the code in the previous section could be rewritten as
use v5.10.1;
for ($var) {
when (/^abc/) { $abc = 1 }
when (/^def/) { $def = 1 }
when (/^xyz/) { $xyz = 1 }
default { $nothing = 1 }
}
The "foreach" is the non-experimental way to set a topicalizer. If you wish to use the highly experimental
"given", that could be written like this:
use v5.10.1;
given ($var) {
when (/^abc/) { $abc = 1 }
when (/^def/) { $def = 1 }
when (/^xyz/) { $xyz = 1 }
default { $nothing = 1 }
}
As of 5.14, that can also be written this way:
use v5.14;
for ($var) {
$abc = 1 when /^abc/;
$def = 1 when /^def/;
$xyz = 1 when /^xyz/;
default { $nothing = 1 }
}
Or if you don't care to play it safe, like this:
use v5.14;
given ($var) {
$abc = 1 when /^abc/;
$def = 1 when /^def/;
$xyz = 1 when /^xyz/;
default { $nothing = 1 }
}
The arguments to "given" and "when" are in scalar context, and "given" assigns the $_ variable its topic
value.
Exactly what the EXPR argument to "when" does is hard to describe precisely, but in general, it tries to guess
what you want done. Sometimes it is interpreted as "$_ ~~ EXPR", and sometimes it does not. It also behaves
differently when lexically enclosed by a "given" block than it does when dynamically enclosed by a "foreach"
loop. The rules are far too difficult to understand to be described here. See "Experimental Details on given
and when" later on.
"goto"-LABEL, "goto"-EXPR, and "goto"-&NAME. A loop's LABEL is not actually a valid target for a "goto"; it's
just the name of the loop.
The "goto"-LABEL form finds the statement labeled with LABEL and resumes execution there. It may not be used
to go into any construct that requires initialization, such as a subroutine or a "foreach" loop. It also
can't be used to go into a construct that is optimized away. It can be used to go almost anywhere else within
the dynamic scope, including out of subroutines, but it's usually better to use some other construct such as
"last" or "die". The author of Perl has never felt the need to use this form of "goto" (in Perl, that is--C
is another matter).
The "goto"-EXPR form expects a label name, whose scope will be resolved dynamically. This allows for computed
"goto"s per FORTRAN, but isn't necessarily recommended if you're optimizing for maintainability:
goto(("FOO", "BAR", "GLARCH")[$i]);
The "goto"-&NAME form is highly magical, and substitutes a call to the named subroutine for the currently
running subroutine. This is used by "AUTOLOAD()" subroutines that wish to load another subroutine and then
pretend that the other subroutine had been called in the first place (except that any modifications to @_ in
the current subroutine are propagated to the other subroutine.) After the "goto", not even "caller()" will be
able to tell that this routine was called first.
In almost all cases like this, it's usually a far, far better idea to use the structured control flow
mechanisms of "next", "last", or "redo" instead of resorting to a "goto". For certain applications, the catch
and throw pair of "eval{}" and die() for exception processing can also be a prudent approach.
The Ellipsis Statement
Beginning in Perl 5.12, Perl accepts an ellipsis, ""..."", as a placeholder for code that you haven't
implemented yet. This form of ellipsis, the unimplemented statement, should not be confused with the binary
flip-flop "..." operator. One is a statement and the other an operator. (Perl doesn't usually confuse them
because usually Perl can tell whether it wants an operator or a statement, but see below for exceptions.)
When Perl 5.12 or later encounters an ellipsis statement, it parses this without error, but if and when you
should actually try to execute it, Perl throws an exception with the text "Unimplemented":
use v5.12;
sub unimplemented { ... }
eval { unimplemented() };
if ($@ =~ /^Unimplemented at /) {
say "I found an ellipsis!";
}
You can only use the elliptical statement to stand in for a complete statement. These examples of how the
ellipsis works:
use v5.12;
{ ... }
sub foo { ... }
...;
eval { ... };
sub somemeth {
my $self = shift;
...;
}
$x = do {
print ...;
open(my $fh, ">", "/dev/passwd") or ...;
if ($condition && ... ) { say "Howdy" };
There are some cases where Perl can't immediately tell the difference between an expression and a statement.
For instance, the syntax for a block and an anonymous hash reference constructor look the same unless there's
something in the braces to give Perl a hint. The ellipsis is a syntax error if Perl doesn't guess that the "{
... }" is a block. In that case, it doesn't think the "..." is an ellipsis because it's expecting an
expression instead of a statement:
@transformed = map { ... } @input; # syntax error
You can use a ";" inside your block to denote that the "{ ... }" is a block and not a hash reference
constructor. Now the ellipsis works:
@transformed = map {; ... } @input; # ; disambiguates
@transformed = map { ...; } @input; # ; disambiguates
Note: Some folks colloquially refer to this bit of punctuation as a "yada-yada" or "triple-dot", but its true
name is actually an ellipsis. Perl does not yet accept the Unicode version, U+2026 HORIZONTAL ELLIPSIS, as an
alias for "...", but someday it may.
PODs: Embedded Documentation
Perl has a mechanism for intermixing documentation with source code. While it's expecting the beginning of a
new statement, if the compiler encounters a line that begins with an equal sign and a word, like this
=head1 Here There Be Pods!
Then that text and all remaining text up through and including a line beginning with "=cut" will be ignored.
The format of the intervening text is described in perlpod.
This allows you to intermix your source code and your documentation text freely, as in
=item snazzle($)
The snazzle() function will behave in the most spectacular
form that you can possibly imagine, not even excepting
cybernetic pyrotechnics.
=cut back to the compiler, nuff of this pod stuff!
sub snazzle($) {
my $thingie = shift;
.........
}
Note that pod translators should look at only paragraphs beginning with a pod directive (it makes parsing
easier), whereas the compiler actually knows to look for pod escapes even in the middle of a paragraph. This
means that the following secret stuff will be ignored by both the compiler and the translators.
$a=3;
=secret stuff
"eval()"). The syntax for this mechanism is almost the same as for most C preprocessors: it matches the
regular expression
# example: '# line 42 "new_filename.plx"'
/^\# \s*
line \s+ (\d+) \s*
(?:\s("?)([^"]+)\g2)? \s*
$/x
with $1 being the line number for the next line, and $3 being the optional filename (specified with or without
quotes). Note that no whitespace may precede the "#", unlike modern C preprocessors.
There is a fairly obvious gotcha included with the line directive: Debuggers and profilers will only show the
last source line to appear at a particular line number in a given file. Care should be taken not to cause
line number collisions in code you'd like to debug later.
Here are some examples that you should be able to type into your command shell:
% perl
# line 200 "bzzzt"
# the '#' on the previous line must be the first char on line
die 'foo';
__END__
foo at bzzzt line 201.
% perl
# line 200 "bzzzt"
eval qq[\n#line 2001 ""\ndie 'foo']; print $@;
__END__
foo at - line 2001.
% perl
eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
__END__
foo at foo bar line 200.
% perl
# line 345 "goop"
eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'";
print $@;
__END__
foo at goop line 345.
Experimental Details on given and when
As previously mentioned, the "switch" feature is considered highly experimental; it is subject to change with
little notice. In particular, both "given" and "when" have tricky behaviours that are expected to change to
become less tricky in the future. Do not rely upon their current (mis)implementations.
Here is a longer example of "given":
use feature ":5.10";
given ($foo) {
when (undef) {
say '$foo is undefined';
say 'a complicated check for $foo is true';
}
default {
die q(I don't know what to do with $foo);
}
}
As currently implemented, "given(EXPR)" assign the value of EXPR to merely a lexically scoped copy (!) of $_,
not a dynamically scoped alias the way "foreach" does. That makes it similar to
do { my $_ = EXPR; ... }
except that the block is automatically broken out of by a successful "when" or an explicit "break". Because
it is only a copy, and because it is only lexically scoped, not dynamically scoped, you cannot do the things
with it that you are used to in a "foreach" loop. In particular, you probably cannot use arbitrary function
calls. Best stick to "foreach" for that.
Most of the power comes from the implicit smartmatching that can sometimes apply. Most of the time,
"when(EXPR)" is treated as an implicit smartmatch of $_, that is, "$_ ~~ EXPR". (See "Smartmatch Operator" in
perlop for more information on smartmatching.) But when EXPR is one of the 10 exceptional cases (or things
like them) listed below, it is used directly as a boolean.
1. A user-defined subroutine call or a method invocation.
2. A regular expression match in the form of "/REGEX/", "$foo =~ /REGEX/", or "$foo =~ EXPR". Also, a
negated regular expression match in the form "!/REGEX/", "$foo !~ /REGEX/", or "$foo !~ EXPR".
3. A smart match that uses an explicit "~~" operator, such as "EXPR ~~ EXPR".
4. A boolean comparison operator such as "$_ < 10" or "$x eq "abc"" The relational operators that this
applies to are the six numeric comparisons ("<", ">", "<=", ">=", "==", and "!="), and the six string
comparisons ("lt", "gt", "le", "ge", "eq", and "ne").
NOTE: You will often have to use "$c ~~ $_" because the default case uses "$_ ~~ $c" , which is frequently
the opposite of what you want.
5. At least the three builtin functions "defined(...)", "exists(...)", and "eof(...)". We might someday add
more of these later if we think of them.
6. A negated expression, whether "!(EXPR)" or "not(EXPR)", or a logical exclusive-or, "(EXPR1) xor (EXPR2)".
The bitwise versions ("~" and "^") are not included.
7. A filetest operator, with exactly 4 exceptions: "-s", "-M", "-A", and "-C", as these return numerical
values, not boolean ones. The "-z" filetest operator is not included in the exception list.
8. The ".." and "..." flip-flop operators. Note that the "..." flip-flop operator is completely different
from the "..." elliptical statement just described.
In those 8 cases above, the value of EXPR is used directly as a boolean, so no smartmatching is done. You may
think of "when" as a smartsmartmatch.
Furthermore, Perl inspects the operands of logical operators to decide whether to use smartmatching for each
one by applying the above test to the operands:
when (/^\d+$/ && $_ < 75) { ... }
will be treated as a boolean match because the rules say both a regex match and an explicit test on $_ will be
treated as boolean.
Also:
when ([qw(foo bar)] && /baz/) { ... }
will use smartmatching because only one of the operands is a boolean: the other uses smartmatching, and that
wins.
Further:
when ([qw(foo bar)] || /^baz/) { ... }
will use smart matching (only the first operand is considered), whereas
when (/^baz/ || [qw(foo bar)]) { ... }
will test only the regex, which causes both operands to be treated as boolean. Watch out for this one, then,
because an arrayref is always a true value, which makes it effectively redundant. Not a good idea.
Tautologous boolean operators are still going to be optimized away. Don't be tempted to write
when ("foo" or "bar") { ... }
This will optimize down to "foo", so "bar" will never be considered (even though the rules say to use a
smartmatch on "foo"). For an alternation like this, an array ref will work, because this will instigate
smartmatching:
when ([qw(foo bar)] { ... }
This is somewhat equivalent to the C-style switch statement's fallthrough functionality (not to be confused
with Perl's fallthrough functionality--see below), wherein the same block is used for several "case"
statements.
Another useful shortcut is that, if you use a literal array or hash as the argument to "given", it is turned
into a reference. So "given(@foo)" is the same as "given(\@foo)", for example.
"default" behaves exactly like "when(1 == 1)", which is to say that it always matches.
Breaking out
You can use the "break" keyword to break out of the enclosing "given" block. Every "when" block is implicitly
ended with a "break".
Fall-through
You can use the "continue" keyword to fall through from one case to the next:
given($foo) {
when (/x/) { say '$foo contains an x'; continue }
be one.
ยท The value of the last evaluated expression of the "given" block if no condition is true.
In both last cases, the last expression is evaluated in the context that was applied to the "given" block.
Note that, unlike "if" and "unless", failed "when" statements always evaluate to an empty list.
my $price = do {
given ($item) {
when (["pear", "apple"]) { 1 }
break when "vote"; # My vote cannot be bought
1e10 when /Mona Lisa/;
"unknown";
}
};
Currently, "given" blocks can't always be used as proper expressions. This may be addressed in a future
version of Perl.
Switching in a loop
Instead of using "given()", you can use a "foreach()" loop. For example, here's one way to count how many
times a particular string occurs in an array:
use v5.10.1;
my $count = 0;
for (@array) {
when ("foo") { ++$count }
}
print "\@array contains $count copies of 'foo'\n";
Or in a more recent version:
use v5.14;
my $count = 0;
for (@array) {
++$count when "foo";
}
print "\@array contains $count copies of 'foo'\n";
At the end of all "when" blocks, there is an implicit "next". You can override that with an explicit "last"
if you're interested in only the first match alone.
This doesn't work if you explicitly specify a loop variable, as in "for $item (@array)". You have to use the
default variable $_.
Differences from Perl 6
The Perl 5 smartmatch and "given"/"when" constructs are not compatible with their Perl 6 analogues. The most
visible difference and least important difference is that, in Perl 5, parentheses are required around the
argument to "given()" and "when()" (except when this last one is used as a statement modifier). Parentheses
in Perl 6 are always optional in a control construct such as "if()", "while()", or "when()"; they can't be
made optional in Perl 5 without a great deal of potential confusion, because Perl 5 would parse the expression
if (@primary ~~ "red") {
say "primary smartmatches red";
}
if ("red" ~~ @primary) {
say "red smartmatches primary";
}
say "that's all, folks!";
But it doesn't work at all in Perl 6. Instead, you should use the (parallelizable) "any" operator instead:
if any(@primary) eq "red" {
say "primary smartmatches red";
}
if "red" eq any(@primary) {
say "red smartmatches primary";
}
The table of smartmatches in "Smartmatch Operator" in perlop is not identical to that proposed by the Perl 6
specification, mainly due to differences between Perl 6's and Perl 5's data models, but also because the Perl
6 spec has changed since Perl 5 rushed into early adoption.
In Perl 6, "when()" will always do an implicit smartmatch with its argument, while in Perl 5 it is convenient
(albeit potentially confusing) to suppress this implicit smartmatch in various rather loosely-defined
situations, as roughly outlined above. (The difference is largely because Perl 5 does not have, even
internally, a boolean type.)
perl v5.16.3 2013-03-04 PERLSYN(1)
Back to main site | Back to man page index