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PERLSYN
!!!PERLSYN
NAME
DESCRIPTION
----
!!NAME


perlsyn - Perl syntax
!!DESCRIPTION


A Perl script consists of a sequence of declarations and
statements. The sequence of statements is executed just
once, unlike in __sed__ and __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 __-n__ or __-p__ switch. It's
just not the mandatory default like it is in __sed__ and
__awk__.)


Perl is, for the most part, a free-form language. (The only
exception to this is format declarations, for obvious
reasons.) Text from a  character until
the end of the line is a comment, and is ignored. If you
attempt to use /* */ C-style comments, it will be
interpreted either as division or pattern matching,
depending on the context, and C ++
// comments just look like a null regular
expression, so don't do that.


__Declarations__


The only things you need to declare in Perl are report
formats and subroutines--and even undefined subroutines can
be handled through AUTOLOAD . A 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
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 (``don't-care'')
contexts and operators such as ++, --,
+=, -=, and .= are 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 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 $0             or die
Note that ''myname()'' functions as a list operator, not as a unary operator; so be careful to use or instead of in this case. However, if you were to declare the subroutine as sub myname ($), then myname would function as a unary operator, so either or or would work.


Subroutines declarations can also be loaded up with the
require statement or both loaded and imported into
your namespace with a use statement. See perlmod
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.


__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 eval {} and 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.


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
foreach EXPR
The if and unless modifiers have the expected semantics, presuming you're a speaker of English. The foreach modifier is an iterator: For each value in EXPR , it aliases $_ to the value and executes the statement. The while and until modifiers have the usual while loopdo-BLOCK (or to the 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:


    do {
$line =
See ``do'' in perlfunc. Note also that the loop control statements described later will ''NOT'' work in this 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++
For last, you have to be more elaborate:


    LOOP: {
do {
last if $x = $y**2;
# do something here
} while $x++


__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 ... else BLOCK
LABEL while (EXPR) BLOCK
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, 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
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.


The while statement executes the block as long as
the expression is true (does not evaluate to the null string
 or 0 or
). 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. Unlike a
foreach statement, a while statement never
implicitly localises any variables.


If there is a continue BLOCK , it is
always executed just before the conditional is about to be
evaluated again, just like the third part of a for
loop in C. Thus it can be used to increment a loop variable,
even when the loop has been continued via the next
statement (which is similar to the C continue
statement).


__Loop Control__


The next command is like the continue
statement in C; it starts the next iteration of the
loop:


    LINE: while (
The last command is like the break statement in C (as used in loops); it immediately exits the loop in question. The continue block, if any, is not executed:


    LINE: while (
The redo command restarts the loop block without evaluating the conditional again. The continue block, if any, is ''not'' 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 (
which is Perl short-hand for the more explicitly written version:


    LINE: while (defined($line =
Note that if there were a continue block on the above code, it would get executed even on discarded lines. This is often used to reset line counters or ?pat? one-time matches.


    # inspired by :1,$g/fred/s//WILMA/
while (
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.


The 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/) {{
next if /fred/;
next if /barney/;
# so something here
}}
The form while/if BLOCK BLOCK, available in Perl 4, is no longer available. Replace any occurrence of if BLOCK by if (do BLOCK).


__For Loops__


Perl's C-style for loop works like the
corresponding while loop; that means that
this:


    for ($i = 1; $i
is the same as this:


    $i = 1;
while ($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


__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.


The foreach keyword is actually a synonym for the
for keyword, so you can use foreach for
readability or for for brevity. (Or because the
Bourne shell is more familiar to you than ''csh'', so
writing for 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, 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;
}
    for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') {
print $count,
    for (1..15) { print
    foreach $item (split(/:[[\n:]*/, $ENV{TERMCAP})) {
print
Here's how a C programmer might code up a particular algorithm in Perl:


    for (my $i = 0; $i
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
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 and Switch Statements__


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 is particularly nice for
doing case structures.


    SWITCH: {
if (/^abc/) { $abc = 1; last SWITCH; }
if (/^def/) { $def = 1; last SWITCH; }
if (/^xyz/) { $xyz = 1; last SWITCH; }
$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


    SWITCH: {
$abc = 1, last SWITCH  if /^abc/;
$def = 1, last SWITCH  if /^def/;
$xyz = 1, last SWITCH  if /^xyz/;
$nothing = 1;
}
(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.)


or


    SWITCH: {
/^abc/
or formatted so it stands out more as a ``proper'' switch statement:


    SWITCH: {
/^abc/
        /^def/
        /^xyz/
or


    SWITCH: {
/^abc/ and $abc = 1, last SWITCH;
/^def/ and $def = 1, last SWITCH;
/^xyz/ and $xyz = 1, last SWITCH;
$nothing = 1;
}
or even, horrors,


    if (/^abc/)
{ $abc = 1 }
elsif (/^def/)
{ $def = 1 }
elsif (/^xyz/)
{ $xyz = 1 }
else
{ $nothing = 1 }
A common idiom for a switch statement is to use foreach's aliasing to make a temporary assignment to $_ for convenient matching:


    SWITCH: for ($where) {
/In Card Names/
Another interesting approach to a switch statement is arrange for a do block to return the proper value:


    $amode = do {
if     ($flag
Or


        print do {
($flags
Or if you are certainly that all the  clauses are true, you can use something like this, which ``switches'' on the value of the 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/
That kind of switch statement only works when you know the  clauses will be true. If you don't, the previous ?: example should be used.


You might also consider writing a hash of subroutine
references instead of synthesizing a switch
statement.


__Goto__


Although not for the faint of heart, Perl does support a
goto statement. There are three forms:
goto-LABEL, goto-EXPR, and
goto-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
gotos per FORTRAN , but isn't
necessarily recommended if you're optimizing for
maintainability:


    goto((
The goto-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.


__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
warn
You probably shouldn't rely upon the warn() 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.


__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 error or warning messages
(especially for strings that are processed with
eval()). The syntax for this mechanism is the same
as for most C preprocessors: it matches the regular
expression
/^#s*lines+(d+)s*(?:s
with $1 being the line number for the next line,
and $2 being the optional filename (specified
within 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
    % perl
# line 200
    % perl
eval qq[[n#line 200
    % perl
# line 345
----
One page links to perlsyn(1):
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Last edited on Monday, June 3, 2002 6:50:51 pm by "perry"
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