Skunkware 5/Skunkware 5.iso/man/cat.1/perlsyn.1

Text File | 1995-07-25 | 15KB | 397 lines

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     NNNNAAAAMMMMEEEE
          perlsyn - Perl syntax

     DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN
          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 sssseeeedddd
          and aaaawwwwkkkk 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 ----nnnn or ----pppp
          switch.  It's just not the mandatory default like it is in
          sssseeeedddd and aaaawwwwkkkk.)

          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.

          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.

          As of Perl 5, 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 just

              sub myname;
              $me = myname $0             or die "can't get myname";

          Note that it functions as a list operator though, not a
          unary operator, so be careful to use or instead of || there.

          Subroutines declarations can also be imported by a use
          statement.

          Also as of Perl 5, a statement sequence may contain



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          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.

          SSSSiiiimmmmpppplllleeee ssssttttaaaatttteeeemmmmeeeennnnttttssss

          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, since you may 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 _S_I_N_G_L_E
          modifier, just before the terminating semicolon (or block
          ending).  The possible modifiers are:

              if EXPR
              unless EXPR
              while EXPR
              until EXPR

          The if and unless modifiers have the expected semantics,
          presuming you're a speaker of English.  The while and 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:

              do {
                  $_ = <STDIN>;
                  ...
              } until $_ eq ".\n";

          See the do entry in the _p_e_r_l_f_u_n_c manpage.  Note also that
          the loop control statements described later will _N_O_T work in
          this construct, since modifiers don't take loop labels.
          Sorry.  You can always wrap another block around it to do
          that sort of thing.)

          CCCCoooommmmppppoooouuuunnnndddd ssssttttaaaatttteeeemmmmeeeennnnttttssss

          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



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          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 _r_e_q_u_i_r_e_d--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.



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          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



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          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;
                                 };






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                  /^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 }





























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