Danny Amor's Online Library/Danny Amor's Online Library - Volume 1.iso/html/faqs/faq/perl-faq/part4

Text File | 1995-07-25 | 34.4 KB | 1,072 lines

Subject: comp.lang.perl FAQ 4/5 - General Programming
Newsgroups: comp.lang.perl,comp.answers,news.answers
From: [e-mail]
Date: 15 Nov 1994 10:09:33 GMT

Archive-name: perl-faq/part4
Version: $Id: part4,v 2.3 1994/11/07 18:06:47 spp Exp spp $
Posting-Frequency: bi-weekly

This posting contains answers to the following questions about General
Programming, Regular Expressions (Regexp) and Input/Output:


4.1) What are all these $@%*<> signs and how do I know when to use them?

    Those are type specifiers:
    $ for scalar values
    @ for indexed arrays
    % for hashed arrays (associative arrays)
    * for all types of that symbol name.  These are sometimes used like
        pointers
    <> are used for inputting a record from a filehandle.

    See the question on arrays of arrays for more about Perl pointers.

    While there are a few places where you don't actually need these type
    specifiers, except for files, you should always use them.  Note that
    <FILE> is NOT the type specifier for files; it's the equivalent of awk's
    getline function, that is, it reads a line from the handle FILE.  When
    doing open, close, and other operations besides the getline function on
    files, do NOT use the brackets.

    Beware of saying:
    $foo = BAR;
    Which wil be interpreted as
    $foo = 'BAR';
    and not as
    $foo = <BAR>;
    If you always quote your strings, you'll avoid this trap.

    Normally, files are manipulated something like this (with appropriate
    error checking added if it were production code):

    open (FILE, ">/tmp/foo.$$");
    print FILE "string\n";
    close FILE;

    If instead of a filehandle, you use a normal scalar variable with file
    manipulation functions, this is considered an indirect reference to a
    filehandle.  For example,

    $foo = "TEST01";
    open($foo, "file");

    After the open, these two while loops are equivalent:

    while (<$foo>) {}
    while (<TEST01>) {}

    as are these two statements:

    close $foo;
    close TEST01;

    but NOT to this:

    while (<$TEST01>) {} # error
        ^
        ^ note spurious dollar sign

    This is another common novice mistake; often it's assumed that

    open($foo, "output.$$");

    will fill in the value of $foo, which was previously undefined.  This
    just isn't so -- you must set $foo to be the name of a filehandle
    before you attempt to open it.


4.2) How come Perl operators have different precedence than C operators?

    Actually, they don't; all C operators have the same precedence in Perl as
    they do in C.  The problem is with a class of functions called list
    operators, e.g. print, chdir, exec, system, and so on.  These are somewhat
    bizarre in that they have different precedence depending on whether you
    look on the left or right of them.  Basically, they gobble up all things
    on their right.  For example,

    unlink $foo, "bar", @names, "others";

    will unlink all those file names.  A common mistake is to write:

    unlink "a_file" || die "snafu";

    The problem is that this gets interpreted as

    unlink("a_file" || die "snafu");

    To avoid this problem, you can always make them look like function calls
    or use an extra level of parentheses:

    unlink("a_file")  || die "snafu";
    (unlink "a_file") || die "snafu";

    In perl5, there are low precedence "and", "or", and "not" operators,
    which bind les tightly than comma.  This alllows you to write:

    unlink $foo, "bar", @names, "others"     or die "snafu";

    Sometimes you actually do care about the return value:

    unless ($io_ok = print("some", "list")) { }

    Yes, print() returns I/O success.  That means

    $io_ok = print(2+4) * 5;

    returns 5 times whether printing (2+4) succeeded, and
    print(2+4) * 5;
    returns the same 5*io_success value and tosses it.

    See the perlop(1) man page's section on Precedence for more gory details,
    and be sure to use the -w flag to catch things like this.


4.3) What's the difference between dynamic and static (lexical) scoping?
     What are my() and local()?

    [NOTE: This question refers to perl5 only.  There is no my() in perl4]
    Scoping refers to visibility of variables.  A dynamic variable is
    created via local() and is just a local value for a global variable,
    whereas a lexical variable created via my() is more what you're
    expecting from a C auto.  (See also "What's the difference between
    deep and shallow binding.")  In general, we suggest you use lexical
    variables wherever possible, as they're faster to access and easier to
    understand.   The "use strict vars" pragma will enforce that all
    variables are either lexical, or full classified by package name.  We
    strongly suggest that you develop your code with "use strict;" and the
    -w flag.  (When using formats, however, you will still have to use
    dynamic variables.)  Here's an example of the difference:

    $scount = 1; $lcount = 2;
        sub foo {
            my($i,$j) = @_;
            my    $scount = 10;
            local $lcount = 20;
            &bar();
        }
        sub bar {
            print "scount is $scount\en";
            print "lcount is $lcount\en";
        }

    This prints:

        scount is 1
        lcount is 20

    Notice that the variables declared with my() are visible only within
    the scope of the block which names them.  They are not visible outside
    of this block, not even in routines or blocks that it calls.  local()
    variables, on the other hand, are visible to routines that are called
    from the block where they are declared.  Neither is visible after the
    end (the final closing curly brace) of the block at all.

    Oh, lexical variables are only available in perl5.  Have we
    mentioned yet that you might consider upgrading? :-)


4.4) What's the difference between deep and shallow binding?

    This only matters when you're making subroutines yourself, at least
    so far.   This will give you shallow binding:

    {
          my $x = time;
          $coderef = sub { $x };
        }

    When you call &$coderef(), it will get whatever dynamic $x happens
    to be around when invoked.  However, you can get the other behaviour
    this way:

    {
          my $x = time;
          $coderef = eval "sub { \$x }";
        }

    Now you'll access the lexical variable $x which is set to the
    time the subroutine was created.  Note that the difference in these
    two behaviours can be considered a bug, not a feature, so you should
    in particular not rely upon shallow binding, as it will likely go
    away in the future.  See perlref(1).


4.5) How can I manipulate fixed-record-length files?

    The most efficient way is using pack and unpack.  This is faster than
    using substr.  Here is a sample chunk of code to break up and put back
    together again some fixed-format input lines, in this case, from ps.

    # sample input line:
    #   15158 p5  T      0:00 perl /mnt/tchrist/scripts/now-what
    $ps_t = 'A6 A4 A7 A5 A*';
    open(PS, "ps|");
    $_ = <PS>; print;
    while (<PS>) {
        ($pid, $tt, $stat, $time, $command) = unpack($ps_t, $_);
        for $var ('pid', 'tt', 'stat', 'time', 'command' ) {
        print "$var: <", eval "\$$var", ">\n";
        }
        print 'line=', pack($ps_t, $pid, $tt, $stat, $time, $command),  "\n";
    }


4.6) How can I make a file handle local to a subroutine?

    You must use the type-globbing *VAR notation.  Here is some code to
    cat an include file, calling itself recursively on nested local
    include files (i.e. those with #include "file", not #include <file>):

    sub cat_include {
        local($name) = @_;
        local(*FILE);
        local($_);

        warn "<INCLUDING $name>\n";
        if (!open (FILE, $name)) {
        warn "can't open $name: $!\n";
        return;
        }
        while (<FILE>) {
        if (/^#\s*include "([^"]*)"/) {
            &cat_include($1);
        } else {
            print;
        }
        }
        close FILE;
    }


4.7) How can I call alarm() or usleep() from Perl?

    If you want finer granularity than 1 second (as usleep() provides) and
    have itimers and syscall() on your system, you can use the following.
    You could also use select().

    It takes a floating-point number representing how long to delay until
    you get the SIGALRM, and returns a floating- point number representing
    how much time was left in the old timer, if any.  Note that the C
    function uses integers, but this one doesn't mind fractional numbers.

    # alarm; send me a SIGALRM in this many seconds (fractions ok)
    # tom christiansen <[e-mail]>
    sub alarm {
    require 'syscall.ph';
    require 'sys/time.ph';

    local($ticks) = @_;
    local($in_timer,$out_timer);
    local($isecs, $iusecs, $secs, $usecs);

    local($itimer_t) = 'L4'; # should be &itimer'typedef()

    $secs = int($ticks);
    $usecs = ($ticks - $secs) * 1e6;

    $out_timer = pack($itimer_t,0,0,0,0);
    $in_timer  = pack($itimer_t,0,0,$secs,$usecs);

    syscall(&SYS_setitimer, &ITIMER_REAL, $in_timer, $out_timer)
        && die "alarm: setitimer syscall failed: $!";

    ($isecs, $iusecs, $secs, $usecs) = unpack($itimer_t,$out_timer);
    return $secs + ($usecs/1e6);
    }


4.8) How can I do an atexit() or setjmp()/longjmp() in Perl?  (Exception handling)

    Perl's exception-handling mechanism is its eval operator.  You
    can use eval as setjmp and die as longjmp.  Here's an example
    of Larry's for timed-out input, which in C is often implemented
    using setjmp and longjmp:

      $SIG{ALRM} = TIMEOUT;
      sub TIMEOUT { die "restart input\n" }

      do { eval { &realcode } } while $@ =~ /^restart input/;

      sub realcode {
          alarm 15;
          $ans = <STDIN>;
          alarm 0;
      }

   Here's an example of Tom's for doing atexit() handling:

    sub atexit { push(@_exit_subs, @_) }

    sub _cleanup { unlink $tmp }

    &atexit('_cleanup');

    eval <<'End_Of_Eval';  $here = __LINE__;
    # as much code here as you want
    End_Of_Eval

    $oops = $@;  # save error message

    # now call his stuff
    for (@_exit_subs) { &$_() }

    $oops && ($oops =~ s/\(eval\) line (\d+)/$0 .
        " line " . ($1+$here)/e, die $oops);

    You can register your own routines via the &atexit function now.  You
    might also want to use the &realcode method of Larry's rather than
    embedding all your code in the here-is document.  Make sure to leave
    via die rather than exit, or write your own &exit routine and call
    that instead.   In general, it's better for nested routines to exit
    via die rather than exit for just this reason.

    In Perl5, it is easy to set this up because of the automatic processing
    of per-package END functions.

    Eval is also quite useful for testing for system dependent features,
    like symlinks, or using a user-input regexp that might otherwise
    blowup on you.


4.9) How do I catch signals in perl?

    Perl allows you to trap signals using the %SIG associative array.
    Using the signals you want to trap as the key, you can assign a
    subroutine to that signal.  The %SIG array will only contain those
    values which the programmer defines.  Therefore, you do not have to
    assign all signals.  For example, to exit cleanly from a ^C:

    $SIG{'INT'} = 'CLEANUP';
    sub CLEANUP {
        print "\n\nCaught Interrupt (^C), Aborting\n";
        exit(1);
    }

    There are two special "routines" for signals called DEFAULT and IGNORE.
    DEFAULT erases the current assignment, restoring the default value of
    the signal.  IGNORE causes the signal to be ignored.  In general, you
    don't need to remember these as you can emulate their functionality
    with standard programming features.  DEFAULT can be emulated by
    deleting the signal from the array and IGNORE can be emulated by any
    undeclared subroutine.

4.10) Why doesn't Perl interpret my octal data octally?

    Perl only understands octal and hex numbers as such when they occur
    as literals in your program.  If they are read in from somewhere and
    assigned, then no automatic conversion takes place.  You must
    explicitly use oct() or hex() if you want this kind of thing to happen.
    Actually, oct() knows to interpret both hex and octal numbers, while
    hex only converts hexadecimal ones.  For example:

    {
        print "What mode would you like? ";
        $mode = <STDIN>;
        $mode = oct($mode);
        unless ($mode) {
        print "You can't really want mode 0!\n";
        redo;
        }
        chmod $mode, $file;
    }

    Without the octal conversion, a requested mode of 755 would turn
    into 01363, yielding bizarre file permissions of --wxrw--wt.

    If you want something that handles decimal, octal and hex input,
    you could follow the suggestion in the man page and use:

    $val = oct($val) if $val =~ /^0/;


4.11) How can I compare two date strings?

    If the dates are in an easily parsed, predetermined format, then you
    can break them up into their component parts and call &timelocal from
    the distributed perl library.  If the date strings are in arbitrary
    formats, however, it's probably easier to use the getdate program from
    the Cnews distribution, since it accepts a wide variety of dates.  Note
    that in either case the return values you will really be comparing will
    be the total time in seconds as returned by time().

    Here's a getdate function for perl that's not very efficient; you can
    do better than this by sending it many dates at once or modifying
    getdate to behave better on a pipe.  Beware the hardcoded pathname.

    sub getdate {
        local($_) = shift;

        s/-(\d{4})$/+$1/ || s/\+(\d{4})$/-$1/;
        # getdate has broken timezone sign reversal!

        $_ = `/usr/local/lib/news/newsbin/getdate '$_'`;
        chop;
        $_;
    }

    Richard Ohnemus <[e-mail]> actually has a getdate.y for
    use with the Perl yacc.  You can get this from ftp.sterling.com
    [192.124.9.1] in /local/perl-byacc1.8.1.tar.Z, or send the author mail
    for details.

    You might also consider using these:

    date.pl        - print dates how you want with the sysv +FORMAT method
    date.shar      - routines to manipulate and calculate dates
    ftp-chat2.shar - updated version of ftpget. includes library and demo
             programs
    getdate.shar   - returns number of seconds since epoch for any given
             date
    ptime.shar     - print dates how you want with the sysv +FORMAT method

    You probably want 'getdate.shar'... these and other files can be ftp'd
    from the /pub/perl/scripts directory on ftp.cis.ufl.edu. See the README
    file in the /pub/perl directory for time and the European mirror site
    details.


4.12) How can I find the Julian Day?

    Here's an example of a Julian Date function provided by Thomas R.
    Kimpton*.

    #!/usr/local/bin/perl

    @theJulianDate = ( 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334 );

    #************************************************************************
    #****   Return 1 if we are after the leap day in a leap year.       *****
    #************************************************************************

    sub leapDay
    {
        my($year,$month,$day) = @_;

        if (year % 4) {
        return(0);
        }

        if (!(year % 100)) {             # years that are multiples of 100
                                     # are not leap years
        if (year % 400) {            # unless they are multiples of 400
            return(0);
        }
        }
        if (month < 2) {
            return(0);
        } elsif ((month == 2) && (day < 29)) {
        return(0);
        } else {
        return(1);
        }
    }

    #************************************************************************
    #****   Pass in the date, in seconds, of the day you want the       *****
    #****   julian date for.  If your localtime() returns the year day  *****
    #****   return that, otherwise figure out the julian date.          *****
    #************************************************************************

    sub julianDate
    {
        my($dateInSeconds) = @_;
        my($sec, $min, $hour, $mday, $mon, $year, $wday, $yday);

        ($sec, $min, $hour, $mday, $mon, $year, $wday, $yday) =
        localtime($dateInSeconds);
        if (defined($yday)) {
        return($yday+1);
        } else {
        return($theJulianDate[$mon] + $mday + &leapDay($year,$mon,$mday));
        }

    }

    print "Today's julian date is: ",&julianDate(time),"\n";


4.13) What's the fastest way to code up a given task in perl?

    Post it to comp.lang.perl and ask Tom or Randal a question about it.
    ;)

    Because Perl so lends itself to a variety of different approaches for
    any given task, a common question is which is the fastest way to code a
    given task.  Since some approaches can be dramatically more efficient
    that others, it's sometimes worth knowing which is best.
    Unfortunately, the implementation that first comes to mind, perhaps as
    a direct translation from C or the shell, often yields suboptimal
    performance.  Not all approaches have the same results across different
    hardware and software platforms.  Furthermore, legibility must
    sometimes be sacrificed for speed.

    While an experienced perl programmer can sometimes eye-ball the code
    and make an educated guess regarding which way would be fastest,
    surprises can still occur.  So, in the spirit of perl programming
    being an empirical science, the best way to find out which of several
    different methods runs the fastest is simply to code them all up and
    time them. For example:

    $COUNT = 10_000; $| = 1;

    print "method 1: ";

        ($u, $s) = times;
        for ($i = 0; $i < $COUNT; $i++) {
        # code for method 1
        }
        ($nu, $ns) = times;
        printf "%8.4fu %8.4fs\n", ($nu - $u), ($ns - $s);

    print "method 2: ";

        ($u, $s) = times;
        for ($i = 0; $i < $COUNT; $i++) {
        # code for method 2
        }
        ($nu, $ns) = times;
        printf "%8.4fu %8.4fs\n", ($nu - $u), ($ns - $s);

    Perl5 includes a new module called Benchmark.pm.  You can now simplify
    the code to use the Benchmarking, like so:

        use Benchmark;

            timethese($count, {
                Name1 => '...code for method 1...',
                Name2 => '...code for method 2...',
                ... });

    It will output something that looks similar to this:

        Benchmark: timing 100 iterations of Name1, Name2...
                Name1:  2 secs (0.50 usr 0.00 sys = 0.50 cpu)
                Name2:  1 secs (0.48 usr 0.00 sys = 0.48 cpu)


    For example, the following code will show the time difference between
    three different ways of assigning the first character of a string to
    a variable:

    use Benchmark;
    timethese(100000, {
        'regex1' => '$str="ABCD"; $str =~ s/^(.)//; $ch = $1',
        'regex2' => '$str="ABCD"; $str =~ s/^.//; $ch = $&',
        'substr' => '$str="ABCD"; $ch=substr($str,0,1); substr($str,0,1)="",
    });

    The results will be returned like this:

    Benchmark: timing 100000 iterations of regex1, regex2, substr...
       regex1: 11 secs (10.80 usr   0.00 sys =  10.80 cpu)
       regex2: 10 secs (10.23 usr   0.00 sys =  10.23 cpu)
       substr:  7 secs ( 5.62 usr    0.00 sys =   5.62 cpu)

    For more specific tips, see the section on Efficiency in the
    ``Other Oddments'' chapter at the end of the Camel Book.


4.14) Do I always/never have to quote my strings or use semicolons?

    You don't have to quote strings that can't mean anything else in the
    language, like identifiers with any upper-case letters in them.
    Therefore, it's fine to do this:

    $SIG{INT} = Timeout_Routine;
    or

    @Days = (Sun, Mon, Tue, Wed, Thu, Fri, Sat, Sun);

    but you can't get away with this:

    $foo{while} = until;

    in place of

    $foo{'while'} = 'until';

    The requirements on semicolons have been increasingly relaxed.  You no
    longer need one at the end of a block, but stylistically, you're better
    to use them if you don't put the curly brace on the same line:

    for (1..10) { print }

    is ok, as is

    @nlist = sort { $a <=> $b } @olist;

    but you probably shouldn't do this:

    for ($i = 0; $i < @a; $i++) {
        print "i is $i\n"  # <-- oops!
    }

    because you might want to add lines later, and anyway, it looks
    funny. :-)


4.15) What is variable suicide and how can I prevent it?

    Variable suicide is a nasty side effect of dynamic scoping and the way
    variables are passed by reference.  If you say

    $x = 17;
    &munge($x);
    sub munge {
        local($x);
        local($myvar) = $_[0];
        ...
    }

    Then you have just clobbered $_[0]!  Why this is occurring is pretty
    heavy wizardry: the reference to $x stored in $_[0] was temporarily
    occluded by the previous local($x) statement (which, you're recall,
    occurs at run-time, not compile-time).  The work around is simple,
    however: declare your formal parameters first:

    sub munge {
        local($myvar) = $_[0];
        local($x);
        ...
    }

    That doesn't help you if you're going to be trying to access @_
    directly after the local()s.  In this case, careful use of the package
    facility is your only recourse.

    Another manifestation of this problem occurs due to the magical nature
    of the index variable in a foreach() loop.

    @num = 0 .. 4;
    print "num begin  @num\n";
    foreach $m (@num) { &ug }
    print "num finish @num\n";
    sub ug {
        local($m) = 42;
        print "m=$m  $num[0],$num[1],$num[2],$num[3]\n";
    }

    Which prints out the mysterious:

    num begin  0 1 2 3 4
    m=42  42,1,2,3
    m=42  0,42,2,3
    m=42  0,1,42,3
    m=42  0,1,2,42
    m=42  0,1,2,3
    num finish 0 1 2 3 4

    What's happening here is that $m is an alias for each element of @num.
    Inside &ug, you temporarily change $m.  Well, that means that you've
    also temporarily changed whatever $m is an alias to!!  The only
    workaround is to be careful with global variables, using packages,
    and/or just be aware of this potential in foreach() loops.

    The perl5 static autos via "my" will not have this problem.


4.16) What does "Malformed command links" mean?

    This is a bug in 4.035.  While in general it's merely a cosmetic
    problem, it often comanifests with a highly undesirable coredumping
    problem.  Programs known to be affected by the fatal coredump include
    plum and pcops.  This bug has been fixed since 4.036.  It did not
    resurface in 5.000.


4.17) How can I set up a footer format to be used with write()?

    While the $^ variable contains the name of the current header format,
    there is no corresponding mechanism to automatically do the same thing
    for a footer.  Not knowing how big a format is going to be until you
    evaluate it is one of the major problems.

    If you have a fixed-size footer, you can get footers by checking for
    line left on page ($-) before each write, and printing the footer
    yourself if necessary.

    Another strategy is to open a pipe to yourself, using open(KID, "|-")
    and always write()ing to the KID, who then postprocesses its STDIN to
    rearrange headers and footers however you like.  Not very convenient,
    but doable.


4.18) Why does my Perl program keep growing in size?

    This is caused by a strange occurance that Larry has dubbed "feeping
    creaturism".  Larry is always adding one more feature, always getting
    Perl to handle one more problem.  Hence, it keeps growing.  Once you've
    worked with perl long enough, you will probably start to do the same
    thing.  You will then notice this problem as you see your scripts
    becoming larger and larger.

    Oh, wait... you meant a currently running program and it's stack size.
    Mea culpa, I misunderstood you.  ;)  While there may be a real memory
    leak in the Perl source code or even whichever malloc() you're using,
    common causes are incomplete eval()s or local()s in loops.

    An eval() which terminates in error due to a failed parsing will leave
    a bit of memory unusable.

    A local() inside a loop:

    for (1..100) {
        local(@array);
    }

    will build up 100 versions of @array before the loop is done.  The
    work-around is:

    local(@array);
    for (1..100) {
        undef @array;
    }

    Larry reports that this behavior is fixed for perl5.


4.19) Can I do RPC in Perl?

    Yes, you can, since Perl has access to sockets.  An example of the rup
    program written in Perl can be found in the script ruptime.pl at the
    scripts archive on ftp.cis.ufl.edu.  I warn you, however, that it's not
    a pretty sight, as it's used nothing from h2ph or c2ph, so everything is
    utterly hard-wired.


4.20) How can I quote a variable to use in a regexp?

    From the manual:

    $pattern =~ s/(\W)/\\$1/g;

    Now you can freely use /$pattern/ without fear of any unexpected meta-
    characters in it throwing off the search.  If you don't know whether a
    pattern is valid or not, enclose it in an eval to avoid a fatal run-
    time error.

    Perl5 provides a vastly improved way of doing this.  Simply use the
    new quotemeta character (\Q) within your variable.

4.21) How can I change the first N letters of a string?

    Remember that the substr() function produces an lvalue, that is, it may
    be assigned to.  Therefore, to change the first character to an S, you
    could do this:

    substr($var,0,1) = 'S';

    This assumes that $[ is 0;  for a library routine where you can't know
    $[, you should use this instead:

    substr($var,$[,1) = 'S';

    While it would be slower, you could in this case use a substitute:

    $var =~ s/^./S/;

    But this won't work if the string is empty or its first character is a
    newline, which "." will never match.  So you could use this instead:

    $var =~ s/^[^\0]?/S/;

    To do things like translation of the first part of a string, use
    substr, as in:

    substr($var, $[, 10) =~ tr/a-z/A-Z/;

    If you don't know the length of what to translate, something like this
    works:

    /^(\S+)/ && substr($_,$[,length($1)) =~ tr/a-z/A-Z/;

    For some things it's convenient to use the /e switch of the substitute
    operator:

    s/^(\S+)/($tmp = $1) =~ tr#a-z#A-Z#, $tmp/e

    although in this case, it runs more slowly than does the previous
    example.


4.22) Can I use Perl regular expressions to match balanced text?

    No, or at least, not by the themselves.

    Regexps just aren't powerful enough.  Although Perl's patterns aren't
    strictly regular because they do backreferencing (the \1 notation), you
    still can't do it.  You need to employ auxiliary logic.  A simple
    approach would involve keeping a bit of state around, something
    vaguely like this (although we don't handle patterns on the same line):

    while(<>) {
        if (/pat1/) {
        if ($inpat++ > 0) { warn "already saw pat1" }
        redo;
        }
        if (/pat2/) {
        if (--$inpat < 0) { warn "never saw pat1" }
        redo;
        }
    }

    A rather more elaborate subroutine to pull out balanced and possibly
    nested single chars, like ` and ', { and }, or ( and ) can be found
    on convex.com in /pub/perl/scripts/pull_quotes.


4.23) What does it mean that regexps are greedy?  How can I get around it?

    The basic idea behind regexps being greedy is that they will match the
    maximum amount of data that they can, sometimes resulting in incorrect
    or strange answers.

    For example, I recently came across something like this:

    $_="this (is) an (example) of multiple parens";
    while ( m#\((.*)\)#g ) {
        print "$1\n";
    }

    This code was supposed to match everything between a set of
    parentheses.  The expected output was:

    is
    example

    However, the backreference ($1) ended up containing "is) an (example",
    clearly not what was intended.

    In perl4, the way to stop this from happening is to use a negated
    group.  If the above example is rewritten as follows, the results are
    correct:

    while ( m#\(([^)]*)\)#g ) {

    In perl5 there is a new minimal matching metacharacter, '?'.  This
    character is added to the normal metacharacters to modify their
    behaviour, such as "*?", "+?", or even "??".  The example would now be
    written in the following style:

    while (m#\((.*?)\)#g )

    Hint: This new operator leads to a very elegant method of stripping
    comments from C code:

    s:/\*.*?\*/::gs


4.24) How do I use a regular expression to strip C style comments from a
      file?

    Since we're talking about how to strip comments under perl5, now is a
    good time to talk about doing it in perl4.  The easiest way to strip
    comments in perl4 is to transform the comment close (*/) into something
    that can't be in the string, or is at least extremely unlikely to be in
    the string.  I find \256 (the registered or reserved sign, an R inside
    a circle) is fairly unlikely to be used and is easy to remember.  So,
    our code looks something like this:

    s:\*/:\256:g;        # Change all */ to circled R
    s:/\*[^\256]*\256::g;   # Remove everything from \* to circled R
    print;

    To ensure that you correctly handle multi-line comments, don't forget
    to set $* to 1, informing perl that it should do multi-line pattern
    matching.

    [Untested changes.  If it's wrong or you don't understand it, check
        with Jeff.  If it's wrong, let me know so I can change it. ]

    Jeff Friedl* suggests that the above solution is incorrect.  He says it
    will fail on imbedded comments and function proto-typing as well as on
    comments that are part of strings.  The following regexp should handle
    everything:

        $/ = undef;
        $_ = <>;

        s#/\*[^*]*\*+([^/*][^*]*\*+)*/|([^/"']*("[^"\\]*(\\[\d\D][^"\\]*)*"[^/"']*|'[^'\\]*(\\[\d\D][^'\\]*)*'[^/"']*|/+[^*/][^/"']*)*)#$2#g;
        print;


4.25) Why doesn't "local($foo) = <FILE>;" work right?

    Well, it does.  The thing to remember is that local() provides an array
    context, and that the <FILE> syntax in an array context will read all the
    lines in a file.  To work around this, use:

    local($foo);
    $foo = <FILE>;

    You can use the scalar() operator to cast the expression into a scalar
    context:

    local($foo) = scalar(<FILE>);


4.26) How can I detect keyboard input without reading it?

    You should check out the Frequently Asked Questions list in
    comp.unix.* for things like this: the answer is essentially the same.
    It's very system dependent.  Here's one solution that works on BSD
    systems:

    sub key_ready {
        local($rin, $nfd);
        vec($rin, fileno(STDIN), 1) = 1;
        return $nfd = select($rin,undef,undef,0);
    }


4.27) How can I read a single character from the keyboard under UNIX and DOS?

    A closely related question to the no-echo question below is how to
    input a single character from the keyboard.  Again, this is a system
    dependent operation.  The following code may or may not help you.  It
    should work on both SysV and BSD flavors of UNIX:

    $BSD = -f '/vmunix';
    if ($BSD) {
        system "stty cbreak </dev/tty >/dev/tty 2>&1";
    }
    else {
        system "stty", '-icanon',
        system "stty", 'eol', "\001";
    }

    $key = getc(STDIN);

    if ($BSD) {
        system "stty -cbreak </dev/tty >/dev/tty 2>&1";
    }
    else {
        system "stty", 'icanon';
        system "stty", 'eol', '^@'; # ascii null
    }
    print "\n";

    You could also handle the stty operations yourself for speed if you're
    going to be doing a lot of them.  This code works to toggle cbreak
    and echo modes on a BSD system:

    sub set_cbreak { # &set_cbreak(1) or &set_cbreak(0)
    local($on) = $_[0];
    local($sgttyb,@ary);
    require 'sys/ioctl.ph';
    $sgttyb_t   = 'C4 S' unless $sgttyb_t;  # c2ph: &sgttyb'typedef()

    ioctl(STDIN,&TIOCGETP,$sgttyb) || die "Can't ioctl TIOCGETP: $!";

    @ary = unpack($sgttyb_t,$sgttyb);
    if ($on) {
        $ary[4] |= &CBREAK;
        $ary[4] &= ~&ECHO;
    } else {
        $ary[4] &= ~&CBREAK;
        $ary[4] |= &ECHO;
    }
    $sgttyb = pack($sgttyb_t,@ary);

    ioctl(STDIN,&TIOCSETP,$sgttyb) || die "Can't ioctl TIOCSETP: $!";
    }

    Note that this is one of the few times you actually want to use the
    getc() function; it's in general way too expensive to call for normal
    I/O.  Normally, you just use the <FILE> syntax, or perhaps the read()
    or sysread() functions.

    For perspectives on more portable solutions, use anon ftp to retrieve
    the file /pub/perl/info/keypress from convex.com.

    For DOS systems, Dan Carson <[e-mail]> reports:

    To put the PC in "raw" mode, use ioctl with some magic numbers gleaned
    from msdos.c (Perl source file) and Ralf Brown's interrupt list (comes
    across the net every so often):

    $old_ioctl = ioctl(STDIN,0,0);     # Gets device info
    $old_ioctl &= 0xff;
    ioctl(STDIN,1,$old_ioctl | 32);    # Writes it back, setting bit 5

    Then to read a single character:

    sysread(STDIN,$c,1);               # Read a single character

    And to put the PC back to "cooked" mode:

    ioctl(STDIN,1,$old_ioctl);         # Sets it back to cooked mode.


    So now you have $c.  If ord($c) == 0, you have a two byte code, which
    means you hit a special key.  Read another byte (sysread(STDIN,$c,1)),
    and that value tells you what combination it was according to this
    table:

    # PC 2-byte keycodes = ^@ + the following:

    # HEX     KEYS
    # ---     ----
    # 0F      SHF TAB
    # 10-19   ALT QWERTYUIOP
    # 1E-26   ALT ASDFGHJKL
    # 2C-32   ALT ZXCVBNM
    # 3B-44   F1-F10
    # 47-49   HOME,UP,PgUp
    # 4B      LEFT
    # 4D      RIGHT
    # 4F-53   END,DOWN,PgDn,Ins,Del
    # 54-5D   SHF F1-F10
    # 5E-67   CTR F1-F10
    # 68-71   ALT F1-F10
    # 73-77   CTR LEFT,RIGHT,END,PgDn,HOME
    # 78-83   ALT 1234567890-=
    # 84      CTR PgUp

    This is all trial and error I did a long time ago, I hope I'm reading the
    file that worked.


4.28) How can I get input from the keyboard without it echoing to the
      screen?

    Terminal echoing is generally handled directly by the shell.
    Therefore, there is no direct way in perl to turn echoing on and off.
    However, you can call the command "stty [-]echo".  The following will
    allow you to accept input without it being echoed to the screen, for
    example as a way to accept passwords (error checking deleted for
    brevity):

    print "Please enter your password: ";
        system("stty -echo");
    chop($password=<STDIN>);
    print "\n";
    system("stty echo");

4.29) Is there any easy way to strip blank space from the beginning/end of
    a string?

    Yes, there is.  Using the substitution command, you can match the
    blanks and replace it with nothing.  For example, if you have the
    string "     String     " you can use this:

        $_ = "     String     ";
    print ":$_:\n";        # OUTPUT: ":     String     :"
    s/^\s*//;
    print ":$_:\n";        # OUTPUT: ":String     :"
    s/\s*$//;
    print ":$_:\n";        # OUTPUT: ":String:"

    Unfortunately, there is no simple single statement that will strip
    whitespace from both the front and the back in perl4.  However, in
    perl5 you should be able to say:

    s/\s*(.*?)\s*$/$1/;
Stephen P Potter        [e-mail]        Varimetrix Corporation
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