PERL Syntax

PERL Syntax

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 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.) Comments are indicated by the "#" character, and extend to the end of the line. 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.

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 (ARRAY) 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 "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 one
The if statement is straightforward. Since 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 "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 next , last , and redo (see below). 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).

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.

In either the if or the while statement, you may replace "(EXPR)" with a BLOCK, and the conditional is true if the value of the last statement in that block is true. (This feature continues to work in Perl 5 but is deprecated. Please change any occurrences of "if BLOCK" to "if (do BLOCK)".)

The C-style for loop works exactly like the corresponding while loop:

 

        for ($i = 1; $i < 10; $i++) {
            ...
        }
is the same as

 

        $i = 1;
        while ($i < 10) {
            ...
        } continue {
            $i++;
        }
The foreach loop iterates over a normal list value and sets the variable VAR to be each element of the list in turn. The variable is implicitly local to the loop (unless declared previously with my ), and regains its former value upon exiting the loop. The foreach keyword is actually a synonym for the for keyword, so you can use foreach for readability or for for brevity. If VAR is omitted, $_ is set to each value. If ARRAY is an actual array (as opposed to an expression returning a list value), you can modify each element of the array by modifying VAR inside the loop. Examples:

 

        for (@ary) { s/foo/bar/; }
        foreach $elem (@elements) {
            $elem *= 2;
        }
        for ((10,9,8,7,6,5,4,3,2,1,'BOOM')) {
            print $_, "\n"; sleep(1);
        }
        for (1..15) { print "Merry Christmas\n"; }
        foreach $item (split(/:[\\n:]*/, $ENV{'TERMCAP'})) {
            print "Item: $item\n";
        }
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. The continue block is optional. This 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 one you realize that you can use loop control "operators" within an expression, That's just the normal C comma operator.)

or

 

        SWITCH: {
            /^abc/ && do { $abc = 1; last SWITCH; };
            /^def/ && do { $def = 1; last SWITCH; };
            /^xyz/ && do { $xyz = 1; last SWITCH; };
            $nothing = 1;
        }
or formatted so it stands out more as a "proper" switch statement:

 

        SWITCH: {
            /^abc/             && do { 
                                $abc = 1; 
                                last SWITCH; 
                           };
            /^def/             && do { 
                                $def = 1; 
                                last SWITCH; 
                           };
            /^xyz/             && do { 
                                $xyz = 1; 
                                last SWITCH; 
                            };
            $nothing = 1;
        }
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 }