=head1 NAME X perlsyn - Perl syntax =head1 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 B 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 B. 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 B, abbreviated B. It allows programmers to be B and to code in a style with which they are comfortable. Perl B 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 L for information about how they differ. =head2 Declarations X X X X 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 (C) until it has been assigned a defined value, which is anything other than C. When used as a number, C is treated as C; when used as a string, it is treated as the empty string, C; 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 C 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 C, C, C, C, and C<.>, 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 C, C, or C, 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 C_{, thus: X sub myname; $me = myname $0 or die "can't get myname"; A bare declaration like that declares the function to be a list operator, not a unary operator, so you have to be careful to use parentheses (or C instead of C.) The C operator binds too tightly to use after list operators; it becomes part of the last element. You can always use parentheses around the list operators arguments to turn the list operator back into something that behaves more like a function call. Alternatively, you can use the prototype C to turn the subroutine into a unary operator: sub myname ($); $me = myname $0 || die "can't get myname"; That now parses as you'd expect, but you still ought to get in the habit of using parentheses in that situation. For more on prototypes, see L. Subroutines declarations can also be loaded up with the C statement or both loaded and imported into your namespace with a C statement. See L 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. =head2 Comments X X Text from a C character until the end of the line is a comment, and is ignored. Exceptions include C inside a string or regular expression. =head2 Simple Statements X X X X 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 C, C, and C that I 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. =head2 Statement Modifiers X X X X X X X X X Any simple statement may optionally be followed by a I 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 C following the modifier is referred to as the "condition". Its truth or falsehood determines how the modifier will behave. C executes the statement once I and only if the condition is true. C is the opposite, it executes the statement I the condition is true (that is, if the condition is false). See L for definitions of true and false. print "Basset hounds got long ears" if length $ear >= 10; go_outside() and play() unless $is_raining; The C modifier is an iterator: it executes the statement once for each item in the LIST (with C aliased to each item in turn). There is no syntax to specify a C-style for loop or a lexically scoped iteration variable in this form. print "Hello $_!\n" for qw(world Dolly nurse); C repeats the statement I the condition is true. Postfix C has the same magic treatment of some kinds of condition that prefix C has. C does the opposite, it repeats the statement I the condition is true (or while the condition is false): # Both of these count from 0 to 10. print $i++ while $i 10; The C and C modifiers have the usual "C loop" semantics (conditional evaluated first), except when applied to a C-BLOCK (or to the Perl4 C-SUBROUTINE statement), in which case the block executes once before the conditional is evaluated. This is so that you can write loops like: do { $line = ; ... } until !defined($line) || $line eq ".\n" See L. Note also that the loop control statements described later will I work in this construct, because modifiers don't take loop labels. Sorry. You can always put another block inside of it (for C/C) or around it (for C) to do that sort of thing. X X X For C or C, just double the braces: do {{ next if $x == $y; # do something here }} until $x++ > $z; For C, you have to be more elaborate and put braces around it: X { do { last if $x == $y**2; # do something here } while $x++ and C, you have to do both and also use a loop label: LOOP: { do {{ next if $x == $y; last LOOP if $x == $y**2; # do something here }} until $x++ > $z; } B The behaviour of a C, C, or C modified with a statement modifier conditional or loop construct (for example, C) is B. The value of the C variable may be C, 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. X The C modifier is an experimental feature that first appeared in Perl 5.14. To use it, you should include a C declaration. (Technically, it requires only the C feature, but that aspect of it was not available before 5.14.) Operative only from within a C loop or a C block, it executes the statement only if the smartmatch C >> is true. If the statement executes, it is followed by a C from inside a C and C from inside a C. Under the current implementation, the C loop can be anywhere within the C modifier's dynamic scope, but must be within the C block's lexical scope. This restriction may be relaxed in a future release. See L"Switch Statements"> below. =head2 Compound Statements X X X X X X X X X X X X X X X 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. Because enclosing braces are also the syntax for hash reference constructor expressions (see L), you may occasionally need to disambiguate by placing a C immediately after an opening brace so that Perl realises the brace is the start of a block. You will more frequently need to disambiguate the other way, by placing a C immediately before an opening brace to force it to be interpreted as a hash reference constructor expression. It is considered good style to use these disambiguating mechanisms liberally, not only when Perl would otherwise guess incorrectly. 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 while (EXPR) BLOCK LABEL while (EXPR) BLOCK continue BLOCK LABEL until (EXPR) BLOCK LABEL until (EXPR) BLOCK continue BLOCK LABEL for (EXPR; EXPR; EXPR) BLOCK LABEL for VAR (LIST) BLOCK LABEL for VAR (LIST) BLOCK continue BLOCK LABEL foreach (EXPR; EXPR; EXPR) BLOCK LABEL foreach VAR (LIST) BLOCK LABEL foreach VAR (LIST) BLOCK continue BLOCK LABEL BLOCK LABEL BLOCK continue BLOCK PHASE BLOCK As of Perl 5.36, you can iterate over multiple values at a time by specifying a list of lexicals within parentheses: no warnings "experimental::for_list"; LABEL for my (VAR, VAR) (LIST) BLOCK LABEL for my (VAR, VAR) (LIST) BLOCK continue BLOCK LABEL foreach my (VAR, VAR) (LIST) BLOCK LABEL foreach my (VAR, VAR) (LIST) BLOCK continue BLOCK If enabled by the experimental C feature, the following may also be used try BLOCK catch (VAR) BLOCK try BLOCK catch (VAR) BLOCK finally BLOCK The experimental C statement is I; see L"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 I--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 C statement is straightforward. Because BLOCKs are always bounded by curly brackets, there is never any ambiguity about which C an C goes with. If you use C in place of C, the sense of the test is reversed. Like C, C can be followed by C. C can even be followed by one or more C 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 C statement executes the block as long as the expression is true. The C 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 C, C, and C. If the LABEL is omitted, the loop control statement refers to the innermost enclosing loop. This may include dynamically searching through your call-stack at run time to find the LABEL. Such desperate behavior triggers a warning if you use the C pragma or the B flag. If the condition expression of a C statement is based on any of a group of iterative expression types then it gets some magic treatment. The affected iterative expression types are L|perlfunc/readline EXPR>, the L >>|perlop/"I/O Operators"> input operator, L|perlfunc/readdir DIRHANDLE>, L|perlfunc/glob EXPR>, the L >>|perlop/"I/O Operators"> globbing operator, and L|perlfunc/each HASH>. If the condition expression is one of these expression types, then the value yielded by the iterative operator will be implicitly assigned to C. If the condition expression is one of these expression types or an explicit assignment of one of them to a scalar, then the condition actually tests for definedness of the expression's value, not for its regular truth value. If there is a C 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 C statement. When a block is preceded by a compilation phase keyword such as C, C, C, C, or C, then the block will run only during the corresponding phase of execution. See L 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 L for the mechanism. If you are using such a module, see the module's documentation for details of the syntax that it defines. =head2 Loop Control X X X X X X The C command starts the next iteration of the loop: LINE: while () { next LINE if /^#/; # discard comments ... } The C command immediately exits the loop in question. The C block, if any, is not executed: LINE: while () { last LINE if /^$/; # exit when done with header ... } The C command restarts the loop block without evaluating the conditional again. The C block, if any, is I 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 F. 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 $_ } which is Perl shorthand for the more explicitly written version: LINE: while (defined($line = )) { chomp($line); if ($line =~ s/\\$//) { $line .= ; redo LINE unless eof(); # not eof(ARGV)! } # now process $line } Note that if there were a C block on the above code, it would get executed only on lines discarded by the regex (since redo skips the continue block). A continue block is often used to reset line counters or C one-time matches: # inspired by :1,$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 $. reset if eof; # reset ?pat? } If the word C is replaced by the word C, the sense of the test is reversed, but the conditional is still tested before the first iteration. Loop control statements don't work in an C or C, since they aren't loops. You can double the braces to make them such, though. if (/pattern/) {{ last if /fred/; next if /barney/; # same effect as "last", # but doesn't document as well # do something here }} This is caused by the fact that a block by itself acts as a loop that executes once, see L"Basic BLOCKs">. The form C, available in Perl 4, is no longer available. Replace any occurrence of C by C. =head2 For Loops X X Perl's C-style C loop works like the corresponding C loop; that means that this: for ($i = 1; $i in the initialization section of the C, the lexical scope of those variables is exactly the C loop (the body of the loop and the control sections). To illustrate: X my $i = 'samba'; for (my $i = 1; $i loop (or the corresponding C loop) is empty, it is treated as true. That is, both for (;;) { ... } and while () { ... } are treated as infinite loops. Besides the normal array index looping, C 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. X X X $on_a_tty = -t STDIN && -t STDOUT; sub prompt { print "yes? " if $on_a_tty } for ( prompt(); ; prompt() ) { # do something } The condition expression of a C loop gets the same magic treatment of C et al that the condition expression of a C loop gets. =head2 Foreach Loops X X The C loop iterates over a normal list value and sets the scalar variable VAR to be each element of the list in turn. If the variable is preceded with the keyword C, 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 C, it uses that variable instead of the global one, but it's still localized to the loop. This implicit localization occurs I for non C-style loops. X X The C keyword is actually a synonym for the C keyword, so you can use either. If VAR is omitted, C is set to each value. X 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 C loop index variable is an implicit alias for each item in the list that you're looping over. X If any part of LIST is an array, C will get very confused if you add or remove elements within the loop body, for example with C. So don't do that. X C probably won't do what you expect if VAR is a tied or other special variable. Don't do that either. As of Perl 5.22, there is an experimental variant of this loop that accepts a variable preceded by a backslash for VAR, in which case the items in the LIST must be references. The backslashed variable will become an alias to each referenced item in the LIST, which must be of the correct type. The variable needn't be a scalar in this case, and the backslash may be followed by C. To use this form, you must enable the C feature via C. (See L. See also L.) As of Perl 5.36, you can iterate over multiple values at a time. You can only iterate with lexical scalars as the iterator variables - unlike list assignment, it's not possible to use C to signify a value that isn't wanted. This is a limitation of the current implementation, and might be changed in the future. If the size of the LIST is not an exact multiple of the number of iterator variables, then on the last iteration the "excess" iterator variables are aliases to C, as if the LIST had C appended as many times as needed for its length to become an exact multiple. This happens whether LIST is a literal LIST or an array - ie arrays are not extended if their size is not a multiple of the iteration size, consistent with iterating an array one-at-a-time. As these padding elements are not lvalues, attempting to modify them will fail, consistent with the behaviour when iterating a list with literal Cs. If this is not the behaviour you desire, then before the loop starts either explicitly extend your array to be an exact multiple, or explicitly throw an exception. Examples: for (@ary) { s/foo/bar/ } for my $elem (@elements) { $elem *= 2; } for $count (reverse(1..10), "BOOM") { print $count, "\n"; sleep(1); } for (1..15) { print "Merry Christmas\n"; } foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) { print "Item: $item\n"; } use feature "refaliasing"; no warnings "experimental::refaliasing"; foreach \my %hash (@array_of_hash_references) { # do something with each %hash } foreach my ($foo, $bar, $baz) (@list) { # do something three-at-a-time } foreach my ($key, $value) (%hash) { # iterate over the hash # The hash is immediately copied to a flat list before the loop # starts. The list contains copies of keys but aliases of values. # This is the same behaviour as for $var (%hash) {...} } Here's how a C programmer might code up a particular algorithm in Perl: for (my $i = 0; $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 C explicitly iterates the other loop rather than merely terminating the inner one. And it's faster because Perl executes a C statement more rapidly than it would the equivalent C-style C loop. Perceptive Perl hackers may have noticed that a C loop has a return value, and that this value can be captured by wrapping the loop in a C block. The reward for this discovery is this cautionary advice: The return value of a C loop is unspecified and may change without notice. Do not rely on it. =head2 Try Catch Exception Handling X X X The C/C syntax provides control flow relating to exception handling. The C keyword introduces a block which will be executed when it is encountered, and the C block provides code to handle any exception that may be thrown by the first. try { my $x = call_a_function(); $x block (i.e. the C statement) will be executed if the initial block invokes the conditional C, or if either of the functions it invokes throws an uncaught exception. The C block can inspect the C lexical variable in this case to see what the exception was. If no exception was thrown then the C block does not happen. In either case, execution will then continue from the following statement - in this example the C. The C keyword must be immediately followed by a variable declaration in parentheses, which introduces a new variable visible to the body of the subsequent block. Inside the block this variable will contain the exception value that was thrown by the code in the C block. It is not necessary to use the C keyword to declare this variable; this is implied (similar as it is for subroutine signatures). Both the C and the C blocks are permitted to contain control-flow expressions, such as C, C, or C/C/C. In all cases they behave as expected without warnings. In particular, a C expression inside the C block will make its entire containing function return - this is in contrast to its behaviour inside an C block, where it would only make that block return. Like other control-flow syntax, C and C will yield the last evaluated value when placed as the final statement in a function or a C block. This permits the syntax to be used to create a value. In this case remember not to use the C expression, or that will cause the containing function to return. my $value = do { try { get_thing(@args); } catch ($e) { warn "Unable to get thing - $e"; $DEFAULT_THING; } }; As with other control-flow syntax, C blocks are not visible to C (just as for example, C or C loops are not). Successive levels of the C result can see subroutine calls and C blocks, because those affect the way that C}