=head1 DESCRIPTION
-A Perl script consists of a sequence of declarations and statements.
-The only things that need to be declared in Perl are report formats
-and subroutines. See the sections below for more information on those
-declarations. All uninitialized user-created objects are assumed to
-start with a null or 0 value until they are defined by some explicit
-operation such as assignment. (Though you can get warnings about the
-use of undefined values if you like.) The sequence of statements is
-executed just once, unlike in B<sed> and B<awk> scripts, where the
-sequence of statements is executed for each input line. While this means
-that you must explicitly loop over the lines of your input file (or
-files), it also means you have much more control over which files and
-which lines you look at. (Actually, I'm lying--it is possible to do an
-implicit loop with either the B<-n> or B<-p> switch. It's just not the
-mandatory default like it is in B<sed> and B<awk>.)
+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<free-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 are B<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 B<Do What I
+Mean>, abbreviated B<DWIM>. It allows programmers to be B<lazy> and to
+code in a style with which they are comfortable.
+
+Perl B<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 L<perltrap> for information about how they differ.
=head2 Declarations
-Perl is, for the most part, a free-form language. (The only
-exception to this is format declarations, for obvious reasons.) Comments
-are indicated by the "#" character, and extend to the end of the line. If
-you attempt to use C</* */> C-style comments, it will be interpreted
-either as division or pattern matching, depending on the context, and C++
-C<//> comments just look like a null regular expression, so don't do
-that.
+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 (C<undef>) until it has been assigned a defined
+value, which is anything other than C<undef>. When used as a number,
+C<undef> is treated as C<0>; 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<undef> 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 C<++>, C<-->, C<+=>,
+C<-=>, and C<.=>, 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 with my(), you'll have to make sure
+lexically-scoped private variables created with C<my()>, 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.
sub myname;
$me = myname $0 or die "can't get myname";
-Note that it functions as a list operator, not as a unary operator; so
-be careful to use C<or> instead of C<||> in this case. However, if
+Note that myname() functions as a list operator, not as a unary operator;
+so be careful to use C<or> instead of C<||> in this case. However, if
you were to declare the subroutine as C<sub myname ($)>, then
-C<myname> would functonion as a unary operator, so either C<or> or
+C<myname> would function as a unary operator, so either C<or> or
C<||> would work.
Subroutines declarations can also be loaded up with the C<require> statement
statements as if it were an ordinary statement. That means it actually
has both compile-time and run-time effects.
-=head2 Simple statements
+=head2 Comments
+
+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
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 there if the
-block takes up more than one line, because you may eventually add another line.)
-Note that there are some operators like C<eval {}> and C<do {}> 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 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 like C<eval {}> and
+C<do {}> 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.
+
+=head2 Truth and Falsehood
+
+The number 0, the strings C<'0'> and C<''>, the empty list C<()>, and
+C<undef> are all false in a boolean context. All other values are true.
+
+=head2 Statement Modifiers
Any simple statement may optionally be followed by a I<SINGLE> modifier,
just before the terminating semicolon (or block ending). The possible
unless EXPR
while EXPR
until EXPR
+ foreach LIST
+
+The C<EXPR> following the modifier is referred to as the "condition".
+Its truth or falsehood determines how the modifier will behave.
+
+C<if> executes the statement once I<if> and only if the condition is
+true. C<unless> is the opposite, it executes the statement I<unless>
+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 $is_raining;
-The C<if> and C<unless> modifiers have the expected semantics,
-presuming you're a speaker of English. The C<while> and C<until>
-modifiers also have the usual "while loop" semantics (conditional
-evaluated first), except when applied to a do-BLOCK (or to the
-now-deprecated do-SUBROUTINE statement), in which case the block
-executes once before the conditional is evaluated. This is so that you
-can write loops like:
+The C<foreach> modifier is an iterator: it executes the statement once
+for each item in the LIST (with C<$_> aliased to each item in turn).
+
+ print "Hello $_!\n" foreach qw(world Dolly nurse);
+
+C<while> repeats the statement I<while> the condition is true.
+C<until> does the opposite, it repeats the statement I<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;
+
+The C<while> and C<until> modifiers have the usual "C<while> loop"
+semantics (conditional evaluated first), except when applied to a
+C<do>-BLOCK (or to the deprecated C<do>-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 L<perlfunc/do>. Note also that the loop control
-statements described later will I<NOT> work in this construct, because
-modifiers don't take loop labels. Sorry. You can always wrap
-another block around it to do that sort of thing.
+See L<perlfunc/do>. Note also that the loop control statements described
+later will I<NOT> work in this construct, because modifiers don't take
+loop labels. Sorry. You can always put another block inside of it
+(for C<next>) or around it (for C<last>) to do that sort of thing.
+For C<next>, just double the braces:
+
+ do {{
+ next if $x == $y;
+ # do something here
+ }} until $x++ > $z;
-=head2 Compound statements
+For C<last>, you have to be more elaborate:
+
+ LOOP: {
+ do {
+ last if $x = $y**2;
+ # do something here
+ } while $x++ <= $z;
+ }
+
+B<NOTE:> The behaviour of a C<my> statement modified with a statement
+modifier conditional or loop construct (e.g. C<my $x if ...>) is
+B<undefined>. The value of the C<my> variable may be C<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.
+
+=head2 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
LABEL while (EXPR) BLOCK continue BLOCK
LABEL for (EXPR; EXPR; EXPR) BLOCK
LABEL foreach VAR (LIST) BLOCK
+ LABEL foreach VAR (LIST) BLOCK continue BLOCK
LABEL BLOCK continue BLOCK
Note that, unlike C and Pascal, these are defined in terms of BLOCKs,
the sense of the test is reversed.
The C<while> statement executes the block as long as the expression is
-true (does not evaluate to the null string or 0 or "0"). 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<next>,
-C<last>, and C<redo>. If the LABEL is omitted, the loop control statement
+true (does not evaluate to the null string C<""> or C<0> or C<"0">).
+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<next>, C<last>, and C<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 B<-w> flag.
+desperate behavior triggers a warning if you use the C<use warnings>
+pragma or the B<-w> flag.
If there is a C<continue> BLOCK, it is always executed just before the
-conditional is about to be evaluated again, just like the third part of a
-C<for> loop in C. Thus it can be used to increment a loop variable, even
-when the loop has been continued via the C<next> statement (which is
-similar to the C C<continue> statement).
+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<next> statement.
=head2 Loop Control
-The C<next> command is like the C<continue> statement in C; it starts
-the next iteration of the loop:
+The C<next> command starts the next iteration of the loop:
LINE: while (<STDIN>) {
next LINE if /^#/; # discard comments
...
}
-The C<last> command is like the C<break> statement in C (as used in
-loops); it immediately exits the loop in question. The
+The C<last> command immediately exits the loop in question. The
C<continue> block, if any, is not executed:
LINE: while (<STDIN>) {
# now process $line
}
-Or here's a simpleminded Pascal comment stripper (warning: assumes no
-{ or } in strings).
+Note that if there were a C<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 C<?pat?> one-time matches:
- LINE: while (<STDIN>) {
- while (s|({.*}.*){.*}|$1 |) {}
- s|{.*}| |;
- if (s|{.*| |) {
- $front = $_;
- while (<STDIN>) {
- if (/}/) { # end of comment?
- s|^|$front{|;
- redo LINE;
- }
- }
- }
- print;
+ # 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 $.
+ reset if eof(); # reset ?pat?
}
-Note that if there were a C<continue> block on the above code, it would get
-executed even on discarded lines.
-
If the word C<while> is replaced by the word C<until>, the sense of the
test is reversed, but the conditional is still tested before the first
iteration.
+The loop control statements don't work in an C<if> or C<unless>, since
+they aren't loops. You can double the braces to make them such, though.
+
+ if (/pattern/) {{
+ last if /fred/;
+ next if /barney/; # same effect as "last", but doesn't document as well
+ # do something here
+ }}
+
+This is caused by the fact that a block by itself acts as a loop that
+executes once, see L<"Basic BLOCKs and Switch Statements">.
+
The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer
available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>.
=head2 For Loops
-Perl's C-style C<for> loop works exactly like the corresponding C<while> loop;
+Perl's C-style C<for> loop works like the corresponding C<while> loop;
that means that this:
for ($i = 1; $i < 10; $i++) {
$i++;
}
-(There is one minor difference: The first form implies a lexical scope
-for variables declared with C<my> in the initialization expression.)
+There is one minor difference: if variables are declared with C<my>
+in the initialization section of the C<for>, the lexical scope of
+those variables is exactly the C<for> loop (the body of the loop
+and the control sections).
Besides the normal array index looping, C<for> can lend itself
to many other interesting applications. Here's one that avoids the
# do something
}
+Using C<readline> (or the operator form, C<< <EXPR> >>) as the
+conditional of a C<for> loop is shorthand for the following. This
+behaviour is the same as a C<while> loop conditional.
+
+ for ( prompt(); defined( $_ = <STDIN> ); prompt() ) {
+ # do something
+ }
+
=head2 Foreach Loops
The C<foreach> loop iterates over a normal list value and sets the
implicitly local to the loop and regains its former value upon exiting
the loop. If the variable was previously declared with C<my>, it uses
that variable instead of the global one, but it's still localized to
-the loop. (Note that a lexically scoped variable can cause problems
-if you have subroutine or format declarations within the loop which
-refer to it.)
+the loop. This implicit localisation occurs I<only> in a C<foreach>
+loop.
The C<foreach> keyword is actually a synonym for the C<for> keyword, so
-you can use C<foreach> for readability or C<for> for brevity. If VAR is
-omitted, $_ is set to each value. If any element of LIST is an lvalue,
-you can modify it by modifying VAR inside the loop. That's because
+you can use C<foreach> for readability or C<for> for brevity. (Or because
+the Bourne shell is more familiar to you than I<csh>, so writing C<for>
+comes more naturally.) If VAR is omitted, C<$_> 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 C<foreach> loop index variable is an implicit alias for each item
in the list that you're looping over.
for (@ary) { s/foo/bar/ }
- foreach my $elem (@elements) {
+ for my $elem (@elements) {
$elem *= 2;
}
Whereas here's how a Perl programmer more comfortable with the idiom might
do it:
- OUTER: foreach my $wid (@ary1) {
- INNER: foreach my $jet (@ary2) {
+ OUTER: for my $wid (@ary1) {
+ INNER: for my $jet (@ary2) {
next OUTER if $wid > $jet;
$wid += $jet;
}
$nothing = 1;
}
-There is no official switch statement in Perl, because there are
-already several ways to write the equivalent. In addition to the
-above, you could write
+There is no official C<switch> statement in Perl, because there are
+already several ways to write the equivalent.
+
+However, starting from Perl 5.8 to get switch and case one can use
+the Switch extension and say:
+
+ use Switch;
+
+after which one has switch and case. It is not as fast as it could be
+because it's not really part of the language (it's done using source
+filters) but it is available, and it's very flexible.
+
+In addition to the above BLOCK construct, you could write
SWITCH: {
$abc = 1, last SWITCH if /^abc/;
}
(That's actually not as strange as it looks once you realize that you can
-use loop control "operators" within an expression, That's just the normal
-C comma operator.)
+use loop control "operators" within an expression. That's just the binary
+comma operator in scalar context. See L<perlop/"Comma Operator">.)
or
$nothing = 1;
}
-or formatted so it stands out more as a "proper" switch statement:
+or formatted so it stands out more as a "proper" C<switch> statement:
SWITCH: {
/^abc/ && do {
else
{ $nothing = 1 }
-
-A common idiom for a switch statement is to use C<foreach>'s aliasing to make
-a temporary assignment to $_ for convenient matching:
+A common idiom for a C<switch> statement is to use C<foreach>'s aliasing to make
+a temporary assignment to C<$_> for convenient matching:
SWITCH: for ($where) {
/In Card Names/ && do { push @flags, '-e'; last; };
for a C<do> block to return the proper value:
$amode = do {
- if ($flag & O_RDONLY) { "r" }
+ if ($flag & O_RDONLY) { "r" } # XXX: isn't this 0?
elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" }
elsif ($flag & O_RDWR) {
if ($flag & O_CREAT) { "w+" }
}
};
+Or
+
+ print do {
+ ($flags & O_WRONLY) ? "write-only" :
+ ($flags & O_RDWR) ? "read-write" :
+ "read-only";
+ };
+
+Or if you are certain that all the C<&&> clauses are true, you can use
+something like this, which "switches" on the value of the
+C<HTTP_USER_AGENT> environment variable.
+
+ #!/usr/bin/perl
+ # pick out jargon file page based on browser
+ $dir = 'http://www.wins.uva.nl/~mes/jargon';
+ for ($ENV{HTTP_USER_AGENT}) {
+ $page = /Mac/ && 'm/Macintrash.html'
+ || /Win(dows )?NT/ && 'e/evilandrude.html'
+ || /Win|MSIE|WebTV/ && 'm/MicroslothWindows.html'
+ || /Linux/ && 'l/Linux.html'
+ || /HP-UX/ && 'h/HP-SUX.html'
+ || /SunOS/ && 's/ScumOS.html'
+ || 'a/AppendixB.html';
+ }
+ print "Location: $dir/$page\015\012\015\012";
+
+That kind of switch statement only works when you know the C<&&> clauses
+will be true. If you don't, the previous C<?:> example should be used.
+
+You might also consider writing a hash of subroutine references
+instead of synthesizing a C<switch> statement.
+
=head2 Goto
-Although not for the faint of heart, Perl does support a C<goto> statement.
-A loop's LABEL is not actually a valid target for a C<goto>;
-it's just the name of the loop. There are three forms: goto-LABEL,
-goto-EXPR, and goto-&NAME.
+Although not for the faint of heart, Perl does support a C<goto>
+statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and
+C<goto>-&NAME. A loop's LABEL is not actually a valid target for
+a C<goto>; it's just the name of the loop.
-The goto-LABEL form finds the statement labeled with LABEL and resumes
+The C<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
+requires initialization, such as a subroutine or a C<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).
+construct such as C<last> or C<die>. The author of Perl has never felt the
+need to use this form of C<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 gotos per FORTRAN, but isn't
+The C<goto>-EXPR form expects a label name, whose scope will be resolved
+dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
necessarily recommended if you're optimizing for maintainability:
- goto ("FOO", "BAR", "GLARCH")[$i];
+ goto(("FOO", "BAR", "GLARCH")[$i]);
-The goto-&NAME form is highly magical, and substitutes a call to the
+The C<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
+C<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 C<goto>, not even caller()
+(except that any modifications to C<@_> in the current subroutine are
+propagated to the other subroutine.) After the C<goto>, not even C<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
=cut back
print "got $a\n";
-You probably shouldn't rely upon the warn() being podded out forever.
+You probably shouldn't rely upon the C<warn()> being podded out forever.
Not all pod translators are well-behaved in this regard, and perhaps
the compiler will become pickier.
=head2 Plain Old Comments (Not!)
-Much like the C preprocessor, perl can process line directives. Using
-this, one can control perl's idea of filenames and line numbers in
+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
-with eval()). The syntax for this mechanism is the same as for most
+with C<eval()>). The syntax for this mechanism is the same as for most
C preprocessors: it matches the regular expression
-C</^#\s*line\s+(\d+)\s*(?:\s"([^"]*)")?/> with C<$1> being the line
-number for the next line, and C<$2> being the optional filename
-(specified within quotes).
+
+ # example: '# line 42 "new_filename.plx"'
+ /^\# \s*
+ line \s+ (\d+) \s*
+ (?:\s("?)([^"]+)\2)? \s*
+ $/x
+
+with C<$1> being the line number for the next line, and C<$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:
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