perlsyn - Perl syntax
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 afree-form language, you can format and indent it however you like. Whitespace mostly serves to separate tokens, unlike languages like Python where it is an important part of the syntax.
Many of Perl's syntactic elements areoptional. 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 asDo What I Mean, abbreviatedDWIM. It allows programmers to belazy and to code in a style with which they are comfortable.
Perlborrows 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 seeperltrap for information about how they differ.
The only things you need to declare in Perl are report formats and subroutines (and sometimes not even subroutines). A variable holds the undefined value (undef) until it has been assigned a defined value, which is anything other thanundef. When used as a number,undef is treated as0; 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 treatundef as a string or a number. Well, usually. Boolean contexts, such as:
my $a;if ($a) {}are exempt from warnings (because they care about truth rather than definedness). Operators such as++,--,+=,-=, and.=, that operate on undefined left values such as:
my $a;$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. Typically all the declarations are put at the beginning or the end of the script. However, if you're using lexically-scoped private variables created withmy(), 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 sayingsub name, thus:
sub myname;$me = myname $0 or die "can't get myname";Note that myname() functions as a list operator, not as a unary operator; so be careful to useor instead of|| in this case. However, if you were to declare the subroutine assub myname ($), thenmyname would function as a unary operator, so eitheror or|| would work.
Subroutines declarations can also be loaded up with therequire statement or both loaded and imported into your namespace with ause statement. Seeperlmod for details on this.
A statement sequence may contain declarations of lexically-scoped variables, but apart from declaring a variable name, the declaration acts like an ordinary statement, and is elaborated within the sequence of statements as if it were an ordinary statement. That means it actually has both compile-time and run-time effects.
Text from a"#" character until the end of the line is a comment, and is ignored. Exceptions include"#" inside a string or regular expression.
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. (A semicolon is still encouraged if the block takes up more than one line, because you may eventually add another line.) Note that there are some operators likeeval {} anddo {} that look like compound statements, but aren't (they're just TERMs in an expression), and thus need an explicit termination if used as the last item in a statement.
The number 0, the strings'0' and'', the empty list(), andundef are all false in a boolean context. All other values are true. Negation of a true value by! ornot returns a special false value. When evaluated as a string it is treated as'', but as a number, it is treated as 0.
Any simple statement may optionally be followed by aSINGLE modifier, just before the terminating semicolon (or block ending). The possible modifiers are:
if EXPRunless EXPRwhile EXPRuntil EXPRforeach LISTTheEXPR following the modifier is referred to as the "condition". Its truth or falsehood determines how the modifier will behave.
if executes the statement onceif and only if the condition is true.unless is the opposite, it executes the statementunless the condition is true (i.e., if the condition is false).
print "Basset hounds got long ears" if length $ear >= 10;go_outside() and play() unless $is_raining;Theforeach modifier is an iterator: it executes the statement once for each item in the LIST (with$_ aliased to each item in turn).
print "Hello $_!\n" foreach qw(world Dolly nurse);while repeats the statementwhile the condition is true.until does the opposite, it repeats the statementuntil 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 $j > 10;Thewhile anduntil modifiers have the usual "while loop" semantics (conditional evaluated first), except when applied to ado-BLOCK (or to the deprecateddo-SUBROUTINE statement), in which case the block executes once before the conditional is evaluated. This is so that you can write loops like:
do {$line = <STDIN>;... } until $line eq ".\n";See"do" in perlfunc. Note also that the loop control statements described later willNOT work in this construct, because modifiers don't take loop labels. Sorry. You can always put another block inside of it (fornext) or around it (forlast) to do that sort of thing. Fornext, just double the braces:
do {{next if $x == $y;# do something here }} until $x++ > $z;Forlast, you have to be more elaborate:
LOOP: { do {last if $x = $y**2;# do something here } while $x++ <= $z; }NOTE: The behaviour of amy statement modified with a statement modifier conditional or loop construct (e.g.my $x if ...) isundefined. The value of themy variable may beundef, 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.
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) BLOCKif (EXPR) BLOCK else BLOCKif (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCKLABEL while (EXPR) BLOCKLABEL while (EXPR) BLOCK continue BLOCKLABEL until (EXPR) BLOCKLABEL until (EXPR) BLOCK continue BLOCKLABEL for (EXPR; EXPR; EXPR) BLOCKLABEL foreach VAR (LIST) BLOCKLABEL foreach VAR (LIST) BLOCK continue BLOCKLABEL BLOCK continue BLOCKNote that, unlike C and Pascal, these are defined in terms of BLOCKs, not statements. This means that the curly brackets arerequired--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) or die "Can't open $FOO: $!";# FOO or bust! open(FOO) ? 'hi mom' : die "Can't open $FOO: $!";# a bit exotic, that last oneTheif statement is straightforward. Because BLOCKs are always bounded by curly brackets, there is never any ambiguity about whichif anelse goes with. If you useunless in place ofif, the sense of the test is reversed.
Thewhile statement executes the block as long as the expression istrue. Theuntil statement executes the block as long as the expression is false. The LABEL is optional, and if present, consists of an identifier followed by a colon. The LABEL identifies the loop for the loop control statementsnext,last, andredo. If the LABEL is omitted, the loop control statement refers to the innermost enclosing loop. This may include dynamically looking back your call-stack at run time to find the LABEL. Such desperate behavior triggers a warning if you use theuse warnings pragma or the-w flag.
If there is acontinue BLOCK, it is always executed just before the conditional is about to be evaluated again. Thus it can be used to increment a loop variable, even when the loop has been continued via thenext statement.
Thenext command starts the next iteration of the loop:
LINE: while (<STDIN>) {next LINE if /^#/;# discard comments... }Thelast command immediately exits the loop in question. Thecontinue block, if any, is not executed:
LINE: while (<STDIN>) {last LINE if /^$/;# exit when done with header... }Theredo command restarts the loop block without evaluating the conditional again. Thecontinue block, if any, isnot executed. This command is normally used by programs that want to lie to themselves about what was just input.
For example, when processing a file like/etc/termcap. If your input lines might end in backslashes to indicate continuation, you want to skip ahead and get the next record.
while (<>) {chomp;if (s/\\$//) { $_ .= <>; redo unless eof();}# now process $_ }which is Perl short-hand for the more explicitly written version:
LINE: while (defined($line = <ARGV>)) {chomp($line);if ($line =~ s/\\$//) { $line .= <ARGV>; redo LINE unless eof(); # not eof(ARGV)!}# now process $line }Note that if there were acontinue block on the above code, it would get executed only on lines discarded by the regex (since redo skips the continue block). A continue block is often used to reset line counters or?pat? one-time matches:
# inspired by :1,$g/fred/s//WILMA/ while (<>) {?(fred)? && s//WILMA $1 WILMA/;?(barney)? && s//BETTY $1 BETTY/;?(homer)? && s//MARGE $1 MARGE/; } continue {print "$ARGV $.: $_";close ARGV if eof();# reset $.reset if eof();# reset ?pat? }If the wordwhile is replaced by the worduntil, the sense of the test is reversed, but the conditional is still tested before the first iteration.
The loop control statements don't work in anif orunless, since they aren't loops. You can double the braces to make them such, though.
if (/pattern/) {{last if /fred/;next if /barney/; # same effect as "last", but doesn't document as well# do something here }}This is caused by the fact that a block by itself acts as a loop that executes once, see"Basic BLOCKs".
The formwhile/if BLOCK BLOCK, available in Perl 4, is no longer available. Replace any occurrence ofif BLOCK byif (do BLOCK).
Perl's C-stylefor loop works like the correspondingwhile loop; that means that this:
for ($i = 1; $i < 10; $i++) {... }is the same as this:
$i = 1; while ($i < 10) {... } continue {$i++; }There is one minor difference: if variables are declared withmy in the initialization section of thefor, the lexical scope of those variables is exactly thefor 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 }Usingreadline (or the operator form,<EXPR>) as the conditional of afor loop is shorthand for the following. This behaviour is the same as awhile loop conditional.
for ( prompt(); defined( $_ = <STDIN> ); prompt() ) { # do something}Theforeach loop iterates over a normal list value and sets the variable VAR to be each element of the list in turn. If the variable is preceded with the keywordmy, then it is lexically scoped, and is therefore visible only within the loop. Otherwise, the variable is implicitly local to the loop and regains its former value upon exiting the loop. If the variable was previously declared withmy, it uses that variable instead of the global one, but it's still localized to the loop. This implicit localisation occursonly in aforeach loop.
Theforeach keyword is actually a synonym for thefor keyword, so you can useforeach for readability orfor for brevity. (Or because the Bourne shell is more familiar to you thancsh, so writingfor comes more naturally.) If VAR is omitted,$_ 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, theforeach 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 withsplice. 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; } for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') {print $count, "\n"; sleep(1); } for (1..15) { print "Merry Christmas\n"; } foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {print "Item: $item\n"; }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. Thenext explicitly iterates the other loop rather than merely terminating the inner one. And it's faster because Perl executes aforeach statement more rapidly than it would the equivalentfor loop.
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 isNOT true ineval{},sub{}, or contrary to popular beliefdo{} blocks, which doNOT count as loops.) Thecontinue 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; }Such constructs are quite frequently used, because older versions of Perl had no officialswitch statement.
Starting from Perl 5.10, you can say
use feature "switch";which enables a switch feature that is closely based on the Perl 6 proposal.
The keywordsgiven andwhen are analogous toswitch andcase in other languages, so the code above could be written as
given($_) {when (/^abc/) { $abc = 1; }when (/^def/) { $def = 1; }when (/^xyz/) { $xyz = 1; }default { $nothing = 1; } }This construct is very flexible and powerful. For example:
use feature ":5.10"; given($foo) {when (undef) { say '$foo is undefined';}when ("foo") { say '$foo is the string "foo"';}when ([1,3,5,7,9]) { say '$foo is an odd digit'; continue; # Fall through}when ($_ < 100) { say '$foo is numerically less than 100';}when (\&complicated_check) { say 'a complicated check for $foo is true';}default { die q(I don't know what to do with $foo);} }given(EXPR) will assign the value of EXPR to$_ within the lexical scope of the block, so it's similar to
do { my $_ = EXPR; ... }except that the block is automatically broken out of by a successfulwhen or an explicitbreak.
Most of the power comes from implicit smart matching:
when($foo)is exactly equivalent to
when($_ ~~ $foo)Most of the time,when(EXPR) is treated as an implicit smart match of$_, i.e.$_ ~~ EXPR. (See"Smart matching in detail" for more information on smart matching.) But when EXPR is one of the below exceptional cases, it is used directly as a boolean:
a subroutine or method call
a regular expression match, i.e./REGEX/ or$foo =~ /REGEX/, or a negated regular expression match (!/REGEX/ or$foo !~ /REGEX/).
a comparison such as$_ < 10 or$x eq "abc" (or of course$_ ~~ $c)
defined(...),exists(...), oreof(...)
a negated expression!(...) ornot (...), or a logical exclusive-or(...) xor (...).
a filetest operator, with the exception of-s,-M,-A, and-C, that return numerical values, not boolean ones.
the.. and... flip-flop operators.
In those cases the value of EXPR is used directly as a boolean.
Furthermore:
If EXPR is... && ... or... and ..., the test is applied recursively to both arguments. Ifboth arguments pass the test, then the argument is treated as boolean.
If EXPR is... || ...,... // ... or... or ..., the test is applied recursively to the first argument.
These rules look complicated, but usually they will do what you want. For example you could write:
when (/^\d+$/ && $_ < 75) { ... }Another useful shortcut is that, if you use a literal array or hash as the argument togiven, it is turned into a reference. Sogiven(@foo) is the same asgiven(\@foo), for example.
default behaves exactly likewhen(1 == 1), which is to say that it always matches.
You can use thebreak keyword to break out of the enclosinggiven block. Everywhen block is implicitly ended with abreak.
You can use thecontinue keyword to fall through from one case to the next:
given($foo) {when (/x/) { say '$foo contains an x'; continue }when (/y/) { say '$foo contains a y' }default { say '$foo does not contain a y' } }Instead of usinggiven(), you can use aforeach() loop. For example, here's one way to count how many times a particular string occurs in an array:
my $count = 0; for (@array) {when ("foo") { ++$count } } print "\@array contains $count copies of 'foo'\n";On exit from thewhen block, there is an implicitnext. You can override that with an explicitlast if you're only interested in the first match.
This doesn't work if you explicitly specify a loop variable, as infor $item (@array). You have to use the default variable$_. (You can usefor my $_ (@array).)
The behaviour of a smart match depends on what type of thing its arguments are. The behaviour is determined by the following table: the first row that applies determines the match behaviour (which is thus mostly determined by the type of the right operand). Note that the smart match implicitly dereferences any non-blessed hash or array ref, so the "Hash" and "Array" entries apply in those cases. (For blessed references, the "Object" entries apply.)
Note that the "Matching Code" column is not always an exact rendition. For example, the smart match operator short-circuits whenever possible, butgrep does not.
$a $b Type of Match Implied Matching Code ====== ===== ===================== ============= Any undef undefined !defined $a Any Object invokes ~~ overloading on $object, or dies Hash CodeRef sub truth for each key[1] !grep { !$b->($_) } keys %$a Array CodeRef sub truth for each elt[1] !grep { !$b->($_) } @$a Any CodeRef scalar sub truth $b->($a) Hash Hash hash keys identical (every key is found in both hashes) Array Hash hash slice existence grep { exists $b->{$_} } @$a Regex Hash hash key grep grep /$a/, keys %$b undef Hash always false (undef can't be a key) Any Hash hash entry existence exists $b->{$a} Hash Array hash slice existence grep { exists $a->{$_} } @$b Array Array arrays are comparable[2] Regex Array array grep grep /$a/, @$b undef Array array contains undef grep !defined, @$b Any Array match against an array element[3] grep $a ~~ $_, @$b Hash Regex hash key grep grep /$b/, keys %$a Array Regex array grep grep /$b/, @$a Any Regex pattern match $a =~ /$b/ Object Any invokes ~~ overloading on $object, or falls back: Any Num numeric equality $a == $b Num numish[4] numeric equality $a == $b undef Any undefined !defined($b) Any Any string equality $a eq $b1 - empty hashes or arrays will match.2 - that is, each element smart-matches the element of same index in the other array. [3]3 - If a circular reference is found, we fall back to referential equality.4 - either a real number, or a string that looks like a numberYou can change the way that an object is matched by overloading the~~ operator. This may alter the usual smart match semantics.
It should be noted that~~ will refuse to work on objects that don't overload it (in order to avoid relying on the object's underlying structure).
Note also that smart match's matching rules take precedence over overloading, so if$obj has smart match overloading, then
$obj ~~ Xwill not automatically invoke the overload method with X as an argument; instead the table above is consulted as normal, and based in the type of X, overloading may or may not be invoked.
Seeoverload.
The Perl 5 smart match andgiven/when constructs are not absolutely identical to their Perl 6 analogues. The most visible difference is that, in Perl 5, parentheses are required around the argument togiven() andwhen(). Parentheses in Perl 6 are always optional in a control construct such asif(),while(), orwhen(); they can't be made optional in Perl 5 without a great deal of potential confusion, because Perl 5 would parse the expression
given $foo { ...}as though the argument togiven were an element of the hash%foo, interpreting the braces as hash-element syntax.
The table of smart matches is not identical to that proposed by the Perl 6 specification, mainly due to the differences between Perl 6's and Perl 5's data models.
In Perl 6,when() will always do an implicit smart match with its argument, whilst it is convenient in Perl 5 to suppress this implicit smart match in certain situations, as documented above. (The difference is largely because Perl 5 does not, even internally, have a boolean type.)
Although not for the faint of heart, Perl does support agoto statement. There are three forms:goto-LABEL,goto-EXPR, andgoto-&NAME. A loop's LABEL is not actually a valid target for agoto; it's just the name of the loop.
Thegoto-LABEL form finds the statement labeled with LABEL and resumes execution there. It may not be used to go into any construct that requires initialization, such as a subroutine or aforeach loop. It also can't be used to go into a construct that is optimized away. It can be used to go almost anywhere else within the dynamic scope, including out of subroutines, but it's usually better to use some other construct such aslast ordie. The author of Perl has never felt the need to use this form ofgoto (in Perl, that is--C is another matter).
Thegoto-EXPR form expects a label name, whose scope will be resolved dynamically. This allows for computedgotos per FORTRAN, but isn't necessarily recommended if you're optimizing for maintainability:
goto(("FOO", "BAR", "GLARCH")[$i]);Thegoto-&NAME form is highly magical, and substitutes a call to the named subroutine for the currently running subroutine. This is used byAUTOLOAD() 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 thegoto, not evencaller() 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 ofnext,last, orredo instead of resorting to agoto. For certain applications, the catch and throw pair ofeval{} and die() for exception processing can also be a prudent approach.
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 inperlpod.
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 warn "Neither POD nor CODE!?"=cut backprint "got $a\n";You probably shouldn't rely upon thewarn() being podded out forever. Not all pod translators are well-behaved in this regard, and perhaps the compiler will become pickier.
One may also use pod directives to quickly comment out a section of code.
Perl can process line directives, much like the C preprocessor. Using this, one can control Perl's idea of filenames and line numbers in error or warning messages (especially for strings that are processed witheval()). The syntax for this mechanism is the same as for most C preprocessors: it matches the regular expression
# example: '# line 42 "new_filename.plx"'/^\# \s* line \s+ (\d+) \s* (?:\s("?)([^"]+)\2)? \s* $/xwith$1 being the line number for the next line, and$3 being the optional filename (specified with or without quotes).
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 linedie 'foo';__END__foo at bzzzt line 201.% perl# line 200 "bzzzt"eval qq[\n#line 2001 ""\ndie 'foo']; print $@;__END__foo at - line 2001.% perleval 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.Perldoc Browser is maintained by Dan Book (DBOOK). Please contact him via theGitHub issue tracker oremail regarding any issues with the site itself, search, or rendering of documentation.
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