perl 3.0 patch #4 Patch #2 continued

[p5sagit/p5-mst-13.2.git] / perl.man.1

.rn '' }`
''' $Header: perl.man.1,v 3.0.1.1 89/11/11 04:41:22 lwall Locked $
''' 
''' $Log:       perl.man.1,v $
''' Revision 3.0.1.1  89/11/11  04:41:22  lwall
''' patch2: explained about sh and ${1+"$@"}
''' patch2: documented that space must separate word and '' string
''' 
''' Revision 3.0  89/10/18  15:21:29  lwall
''' 3.0 baseline
''' 
''' 
.de Sh
.br
.ne 5
.PP
\fB\\$1\fR
.PP
..
.de Sp
.if t .sp .5v
.if n .sp
..
.de Ip
.br
.ie \\n.$>=3 .ne \\$3
.el .ne 3
.IP "\\$1" \\$2
..
'''
'''     Set up \*(-- to give an unbreakable dash;
'''     string Tr holds user defined translation string.
'''     Bell System Logo is used as a dummy character.
'''
.tr \(*W-|\(bv\*(Tr
.ie n \{\
.ds -- \(*W-
.if (\n(.H=4u)&(1m=24u) .ds -- \(*W\h'-12u'\(*W\h'-12u'-\" diablo 10 pitch
.if (\n(.H=4u)&(1m=20u) .ds -- \(*W\h'-12u'\(*W\h'-8u'-\" diablo 12 pitch
.ds L" ""
.ds R" ""
.ds L' '
.ds R' '
'br\}
.el\{\
.ds -- \(em\|
.tr \*(Tr
.ds L" ``
.ds R" ''
.ds L' `
.ds R' '
'br\}
.TH PERL 1 "\*(RP"
.UC
.SH NAME
perl \- Practical Extraction and Report Language
.SH SYNOPSIS
.B perl
[options] filename args
.SH DESCRIPTION
.I Perl
is an interpreted language optimized for scanning arbitrary text files,
extracting information from those text files, and printing reports based
on that information.
It's also a good language for many system management tasks.
The language is intended to be practical (easy to use, efficient, complete)
rather than beautiful (tiny, elegant, minimal).
It combines (in the author's opinion, anyway) some of the best features of C,
\fIsed\fR, \fIawk\fR, and \fIsh\fR,
so people familiar with those languages should have little difficulty with it.
(Language historians will also note some vestiges of \fIcsh\fR, Pascal, and
even BASIC-PLUS.)
Expression syntax corresponds quite closely to C expression syntax.
Unlike most Unix utilities,
.I perl
does not arbitrarily limit the size of your data\*(--if you've got
the memory,
.I perl
can slurp in your whole file as a single string.
Recursion is of unlimited depth.
And the hash tables used by associative arrays grow as necessary to prevent
degraded performance.
.I Perl
uses sophisticated pattern matching techniques to scan large amounts of
data very quickly.
Although optimized for scanning text,
.I perl
can also deal with binary data, and can make dbm files look like associative
arrays (where dbm is available).
Setuid
.I perl
scripts are safer than C programs
through a dataflow tracing mechanism which prevents many stupid security holes.
If you have a problem that would ordinarily use \fIsed\fR
or \fIawk\fR or \fIsh\fR, but it
exceeds their capabilities or must run a little faster,
and you don't want to write the silly thing in C, then
.I perl
may be for you.
There are also translators to turn your
.I sed
and
.I awk
scripts into
.I perl
scripts.
OK, enough hype.
.PP
Upon startup,
.I perl
looks for your script in one of the following places:
.Ip 1. 4 2
Specified line by line via
.B \-e
switches on the command line.
.Ip 2. 4 2
Contained in the file specified by the first filename on the command line.
(Note that systems supporting the #! notation invoke interpreters this way.)
.Ip 3. 4 2
Passed in implicitly via standard input.
This only works if there are no filename arguments\*(--to pass
arguments to a
.I stdin
script you must explicitly specify a \- for the script name.
.PP
After locating your script,
.I perl
compiles it to an internal form.
If the script is syntactically correct, it is executed.
.Sh "Options"
Note: on first reading this section may not make much sense to you.  It's here
at the front for easy reference.
.PP
A single-character option may be combined with the following option, if any.
This is particularly useful when invoking a script using the #! construct which
only allows one argument.  Example:
.nf

.ne 2
        #!/usr/bin/perl \-spi.bak       # same as \-s \-p \-i.bak
        .\|.\|.

.fi
Options include:
.TP 5
.B \-a
turns on autosplit mode when used with a
.B \-n
or
.BR \-p .
An implicit split command to the @F array
is done as the first thing inside the implicit while loop produced by
the
.B \-n
or
.BR \-p .
.nf

        perl \-ane \'print pop(@F), "\en";\'

is equivalent to

        while (<>) {
                @F = split(\' \');
                print pop(@F), "\en";
        }

.fi
.TP 5
.BI \-d
runs the script under the perl debugger.
See the section on Debugging.
.TP 5
.BI \-D number
sets debugging flags.
To watch how it executes your script, use
.BR \-D14 .
(This only works if debugging is compiled into your
.IR perl .)
Another nice value is \-D1024, which lists your compiled syntax tree.
And \-D512 displays compiled regular expressions.
.TP 5
.BI \-e " commandline"
may be used to enter one line of script.
Multiple
.B \-e
commands may be given to build up a multi-line script.
If
.B \-e
is given,
.I perl
will not look for a script filename in the argument list.
.TP 5
.BI \-i extension
specifies that files processed by the <> construct are to be edited
in-place.
It does this by renaming the input file, opening the output file by the
same name, and selecting that output file as the default for print statements.
The extension, if supplied, is added to the name of the
old file to make a backup copy.
If no extension is supplied, no backup is made.
Saying \*(L"perl \-p \-i.bak \-e "s/foo/bar/;" .\|.\|. \*(R" is the same as using
the script:
.nf

.ne 2
        #!/usr/bin/perl \-pi.bak
        s/foo/bar/;

which is equivalent to

.ne 14
        #!/usr/bin/perl
        while (<>) {
                if ($ARGV ne $oldargv) {
                        rename($ARGV, $ARGV . \'.bak\');
                        open(ARGVOUT, ">$ARGV");
                        select(ARGVOUT);
                        $oldargv = $ARGV;
                }
                s/foo/bar/;
        }
        continue {
            print;      # this prints to original filename
        }
        select(STDOUT);

.fi
except that the
.B \-i
form doesn't need to compare $ARGV to $oldargv to know when
the filename has changed.
It does, however, use ARGVOUT for the selected filehandle.
Note that
.I STDOUT
is restored as the default output filehandle after the loop.
.Sp
You can use eof to locate the end of each input file, in case you want
to append to each file, or reset line numbering (see example under eof).
.TP 5
.BI \-I directory
may be used in conjunction with
.B \-P
to tell the C preprocessor where to look for include files.
By default /usr/include and /usr/lib/perl are searched.
.TP 5
.B \-n
causes
.I perl
to assume the following loop around your script, which makes it iterate
over filename arguments somewhat like \*(L"sed \-n\*(R" or \fIawk\fR:
.nf

.ne 3
        while (<>) {
                .\|.\|.         # your script goes here
        }

.fi
Note that the lines are not printed by default.
See
.B \-p
to have lines printed.
Here is an efficient way to delete all files older than a week:
.nf

        find . \-mtime +7 \-print | perl \-ne \'chop;unlink;\'

.fi
This is faster than using the \-exec switch of find because you don't have to
start a process on every filename found.
.TP 5
.B \-p
causes
.I perl
to assume the following loop around your script, which makes it iterate
over filename arguments somewhat like \fIsed\fR:
.nf

.ne 5
        while (<>) {
                .\|.\|.         # your script goes here
        } continue {
                print;
        }

.fi
Note that the lines are printed automatically.
To suppress printing use the
.B \-n
switch.
A
.B \-p
overrides a
.B \-n
switch.
.TP 5
.B \-P
causes your script to be run through the C preprocessor before
compilation by
.IR perl .
(Since both comments and cpp directives begin with the # character,
you should avoid starting comments with any words recognized
by the C preprocessor such as \*(L"if\*(R", \*(L"else\*(R" or \*(L"define\*(R".)
.TP 5
.B \-s
enables some rudimentary switch parsing for switches on the command line
after the script name but before any filename arguments (or before a \-\|\-).
Any switch found there is removed from @ARGV and sets the corresponding variable in the
.I perl
script.
The following script prints \*(L"true\*(R" if and only if the script is
invoked with a \-xyz switch.
.nf

.ne 2
        #!/usr/bin/perl \-s
        if ($xyz) { print "true\en"; }

.fi
.TP 5
.B \-S
makes
.I perl
use the PATH environment variable to search for the script
(unless the name of the script starts with a slash).
Typically this is used to emulate #! startup on machines that don't
support #!, in the following manner:
.nf

        #!/usr/bin/perl
        eval "exec /usr/bin/perl \-S $0 $*"
                if $running_under_some_shell;

.fi
The system ignores the first line and feeds the script to /bin/sh,
which proceeds to try to execute the
.I perl
script as a shell script.
The shell executes the second line as a normal shell command, and thus
starts up the
.I perl
interpreter.
On some systems $0 doesn't always contain the full pathname,
so the
.B \-S
tells
.I perl
to search for the script if necessary.
After
.I perl
locates the script, it parses the lines and ignores them because
the variable $running_under_some_shell is never true.
A better construct than $* would be ${1+"$@"}, which handles embedded spaces
and such in the filenames, but doesn't work if the script is being interpreted
by csh.
In order to start up sh rather than csh, some systems may have to replace the
#! line with a line containing just
a colon, which will be politely ignored by perl.
.TP 5
.B \-u
causes
.I perl
to dump core after compiling your script.
You can then take this core dump and turn it into an executable file
by using the undump program (not supplied).
This speeds startup at the expense of some disk space (which you can
minimize by stripping the executable).
(Still, a "hello world" executable comes out to about 200K on my machine.)
If you are going to run your executable as a set-id program then you
should probably compile it using taintperl rather than normal perl.
If you want to execute a portion of your script before dumping, use the
dump operator instead.
.TP 5
.B \-U
allows
.I perl
to do unsafe operations.
Currently the only \*(L"unsafe\*(R" operation is the unlinking of directories while
running as superuser.
.TP 5
.B \-v
prints the version and patchlevel of your
.I perl
executable.
.TP 5
.B \-w
prints warnings about identifiers that are mentioned only once, and scalar
variables that are used before being set.
Also warns about redefined subroutines, and references to undefined
filehandles or filehandles opened readonly that you are attempting to
write on.
Also warns you if you use == on values that don't look like numbers, and if
your subroutines recurse more than 100 deep.
.Sh "Data Types and Objects"
.PP
.I Perl
has three data types: scalars, arrays of scalars, and
associative arrays of scalars.
Normal arrays are indexed by number, and associative arrays by string.
.PP
The interpretation of operations and values in perl sometimes
depends on the requirements
of the context around the operation or value.
There are three major contexts: string, numeric and array.
Certain operations return array values
in contexts wanting an array, and scalar values otherwise.
(If this is true of an operation it will be mentioned in the documentation
for that operation.)
Operations which return scalars don't care whether the context is looking
for a string or a number, but
scalar variables and values are interpreted as strings or numbers
as appropriate to the context.
A scalar is interpreted as TRUE in the boolean sense if it is not the null
string or 0.
Booleans returned by operators are 1 for true and \'0\' or \'\' (the null
string) for false.
.PP
There are actually two varieties of null string: defined and undefined.
Undefined null strings are returned when there is no real value for something,
such as when there was an error, or at end of file, or when you refer
to an uninitialized variable or element of an array.
An undefined null string may become defined the first time you access it, but
prior to that you can use the defined() operator to determine whether the
value is defined or not.
.PP
References to scalar variables always begin with \*(L'$\*(R', even when referring
to a scalar that is part of an array.
Thus:
.nf

.ne 3
    $days       \h'|2i'# a simple scalar variable
    $days[28]   \h'|2i'# 29th element of array @days
    $days{\'Feb\'}\h'|2i'# one value from an associative array
    $#days      \h'|2i'# last index of array @days

but entire arrays or array slices are denoted by \*(L'@\*(R':

    @days       \h'|2i'# ($days[0], $days[1],\|.\|.\|. $days[n])
    @days[3,4,5]\h'|2i'# same as @days[3.\|.5]
    @days{'a','c'}\h'|2i'# same as ($days{'a'},$days{'c'})

and entire associative arrays are denoted by \*(L'%\*(R':

    %days       \h'|2i'# (key1, val1, key2, val2 .\|.\|.)
.fi
.PP
Any of these eight constructs may serve as an lvalue,
that is, may be assigned to.
(It also turns out that an assignment is itself an lvalue in
certain contexts\*(--see examples under s, tr and chop.)
Assignment to a scalar evaluates the righthand side in a scalar context,
while assignment to an array or array slice evaluates the righthand side
in an array context.
.PP
You may find the length of array @days by evaluating
\*(L"$#days\*(R", as in
.IR csh .
(Actually, it's not the length of the array, it's the subscript of the last element, since there is (ordinarily) a 0th element.)
Assigning to $#days changes the length of the array.
Shortening an array by this method does not actually destroy any values.
Lengthening an array that was previously shortened recovers the values that
were in those elements.
You can also gain some measure of efficiency by preextending an array that
is going to get big.
(You can also extend an array by assigning to an element that is off the
end of the array.
This differs from assigning to $#whatever in that intervening values
are set to null rather than recovered.)
You can truncate an array down to nothing by assigning the null list () to
it.
The following are exactly equivalent
.nf

        @whatever = ();
        $#whatever = $[ \- 1;

.fi
.PP
Multi-dimensional arrays are not directly supported, but see the discussion
of the $; variable later for a means of emulating multiple subscripts with
an associative array.
.PP
Every data type has its own namespace.
You can, without fear of conflict, use the same name for a scalar variable,
an array, an associative array, a filehandle, a subroutine name, and/or
a label.
Since variable and array references always start with \*(L'$\*(R', \*(L'@\*(R',
or \*(L'%\*(R', the \*(L"reserved\*(R" words aren't in fact reserved
with respect to variable names.
(They ARE reserved with respect to labels and filehandles, however, which
don't have an initial special character.
Hint: you could say open(LOG,\'logfile\') rather than open(log,\'logfile\').
Using uppercase filehandles also improves readability and protects you
from conflict with future reserved words.)
Case IS significant\*(--\*(L"FOO\*(R", \*(L"Foo\*(R" and \*(L"foo\*(R" are all
different names.
Names which start with a letter may also contain digits and underscores.
Names which do not start with a letter are limited to one character,
e.g. \*(L"$%\*(R" or \*(L"$$\*(R".
(Most of the one character names have a predefined significance to
.IR perl .
More later.)
.PP
Numeric literals are specified in any of the usual floating point or
integer formats:
.nf

.ne 5
    12345
    12345.67
    .23E-10
    0xffff      # hex
    0377        # octal

.fi
String literals are delimited by either single or double quotes.
They work much like shell quotes:
double-quoted string literals are subject to backslash and variable
substitution; single-quoted strings are not (except for \e\' and \e\e).
The usual backslash rules apply for making characters such as newline, tab, etc.
You can also embed newlines directly in your strings, i.e. they can end on
a different line than they begin.
This is nice, but if you forget your trailing quote, the error will not be
reported until
.I perl
finds another line containing the quote character, which
may be much further on in the script.
Variable substitution inside strings is limited to scalar variables, normal
array values, and array slices.
(In other words, identifiers beginning with $ or @, followed by an optional
bracketed expression as a subscript.)
The following code segment prints out \*(L"The price is $100.\*(R"
.nf

.ne 2
    $Price = \'$100\';\h'|3.5i'# not interpreted
    print "The price is $Price.\e\|n";\h'|3.5i'# interpreted

.fi
Note that you can put curly brackets around the identifier to delimit it
from following alphanumerics.
Also note that a single quoted string must be separated from a preceding
word by a space, since single quote is a valid character in an identifier
(see Packages).
.PP
Array values are interpolated into double-quoted strings by joining all the
elements of the array with the delimiter specified in the $" variable,
space by default.
(Since in versions of perl prior to 3.0 the @ character was not a metacharacter
in double-quoted strings, the interpolation of @array, $array[EXPR],
@array[LIST], $array{EXPR}, or @array{LIST} only happens if array is
referenced elsewhere in the program or is predefined.)
The following are equivalent:
.nf

.ne 4
        $temp = join($",@ARGV);
        system "echo $temp";

        system "echo @ARGV";

.fi
Within search patterns (which also undergo double-quotish substitution)
there is a bad ambiguity:  Is /$foo[bar]/ to be
interpreted as /${foo}[bar]/ (where [bar] is a character class for the
regular expression) or as /${foo[bar]}/ (where [bar] is the subscript to
array @foo)?
If @foo doesn't otherwise exist, then it's obviously a character class.
If @foo exists, perl takes a good guess about [bar], and is almost always right.
If it does guess wrong, or if you're just plain paranoid,
you can force the correct interpretation with curly brackets as above.
.PP
A line-oriented form of quoting is based on the shell here-is syntax.
Following a << you specify a string to terminate the quoted material, and all lines
following the current line down to the terminating string are the value
of the item.
The terminating string may be either an identifier (a word), or some
quoted text.
If quoted, the type of quotes you use determines the treatment of the text,
just as in regular quoting.
An unquoted identifier works like double quotes.
There must be no space between the << and the identifier.
(If you put a space it will be treated as a null identifier, which is
valid, and matches the first blank line\*(--see Merry Christmas example below.)
The terminating string must appear by itself (unquoted and with no surrounding
whitespace) on the terminating line.
.nf

        print <<EOF;            # same as above
The price is $Price.
EOF

        print <<"EOF";          # same as above
The price is $Price.
EOF

        print << x 10;          # null identifier is delimiter
Merry Christmas!

        print <<`EOC`;          # execute commands
echo hi there
echo lo there
EOC

        print <<foo, <<bar;     # you can stack them
I said foo.
foo
I said bar.
bar

.fi
Array literals are denoted by separating individual values by commas, and
enclosing the list in parentheses.
In a context not requiring an array value, the value of the array literal
is the value of the final element, as in the C comma operator.
For example,
.nf

.ne 4
    @foo = (\'cc\', \'\-E\', $bar);

assigns the entire array value to array foo, but

    $foo = (\'cc\', \'\-E\', $bar);

.fi
assigns the value of variable bar to variable foo.
Array lists may be assigned to if and only if each element of the list
is an lvalue:
.nf

    ($a, $b, $c) = (1, 2, 3);

    ($map{\'red\'}, $map{\'blue\'}, $map{\'green\'}) = (0x00f, 0x0f0, 0xf00);

The final element may be an array or an associative array:

    ($a, $b, @rest) = split;
    local($a, $b, %rest) = @_;

.fi
You can actually put an array anywhere in the list, but the first array
in the list will soak up all the values, and anything after it will get
a null value.
This may be useful in a local().
.PP
An associative array literal contains pairs of values to be interpreted
as a key and a value:
.nf

.ne 2
    # same as map assignment above
    %map = ('red',0x00f,'blue',0x0f0,'green',0xf00);

.fi
Array assignment in a scalar context returns the number of elements
produced by the expression on the right side of the assignment:
.nf

        $x = (($foo,$bar) = (3,2,1));   # set $x to 3, not 2

.fi
.PP
There are several other pseudo-literals that you should know about.
If a string is enclosed by backticks (grave accents), it first undergoes
variable substitution just like a double quoted string.
It is then interpreted as a command, and the output of that command
is the value of the pseudo-literal, like in a shell.
The command is executed each time the pseudo-literal is evaluated.
The status value of the command is returned in $? (see Predefined Names
for the interpretation of $?).
Unlike in \f2csh\f1, no translation is done on the return
data\*(--newlines remain newlines.
Unlike in any of the shells, single quotes do not hide variable names
in the command from interpretation.
To pass a $ through to the shell you need to hide it with a backslash.
.PP
Evaluating a filehandle in angle brackets yields the next line
from that file (newline included, so it's never false until EOF, at
which time an undefined value is returned).
Ordinarily you must assign that value to a variable,
but there is one situation where in which an automatic assignment happens.
If (and only if) the input symbol is the only thing inside the conditional of a
.I while
loop, the value is
automatically assigned to the variable \*(L"$_\*(R".
(This may seem like an odd thing to you, but you'll use the construct
in almost every
.I perl
script you write.)
Anyway, the following lines are equivalent to each other:
.nf

.ne 5
    while ($_ = <STDIN>) { print; }
    while (<STDIN>) { print; }
    for (\|;\|<STDIN>;\|) { print; }
    print while $_ = <STDIN>;
    print while <STDIN>;

.fi
The filehandles
.IR STDIN ,
.I STDOUT
and
.I STDERR
are predefined.
(The filehandles
.IR stdin ,
.I stdout
and
.I stderr
will also work except in packages, where they would be interpreted as
local identifiers rather than global.)
Additional filehandles may be created with the
.I open
function.
.PP
If a <FILEHANDLE> is used in a context that is looking for an array, an array
consisting of all the input lines is returned, one line per array element.
It's easy to make a LARGE data space this way, so use with care.
.PP
The null filehandle <> is special and can be used to emulate the behavior of
\fIsed\fR and \fIawk\fR.
Input from <> comes either from standard input, or from each file listed on
the command line.
Here's how it works: the first time <> is evaluated, the ARGV array is checked,
and if it is null, $ARGV[0] is set to \'-\', which when opened gives you standard
input.
The ARGV array is then processed as a list of filenames.
The loop
.nf

.ne 3
        while (<>) {
                .\|.\|.                 # code for each line
        }

.ne 10
is equivalent to

        unshift(@ARGV, \'\-\') \|if \|$#ARGV < $[;
        while ($ARGV = shift) {
                open(ARGV, $ARGV);
                while (<ARGV>) {
                        .\|.\|.         # code for each line
                }
        }

.fi
except that it isn't as cumbersome to say.
It really does shift array ARGV and put the current filename into
variable ARGV.
It also uses filehandle ARGV internally.
You can modify @ARGV before the first <> as long as you leave the first
filename at the beginning of the array.
Line numbers ($.) continue as if the input was one big happy file.
(But see example under eof for how to reset line numbers on each file.)
.PP
.ne 5
If you want to set @ARGV to your own list of files, go right ahead.
If you want to pass switches into your script, you can
put a loop on the front like this:
.nf

.ne 10
        while ($_ = $ARGV[0], /\|^\-/\|) {
                shift;
            last if /\|^\-\|\-$\|/\|;
                /\|^\-D\|(.*\|)/ \|&& \|($debug = $1);
                /\|^\-v\|/ \|&& \|$verbose++;
                .\|.\|.         # other switches
        }
        while (<>) {
                .\|.\|.         # code for each line
        }

.fi
The <> symbol will return FALSE only once.
If you call it again after this it will assume you are processing another
@ARGV list, and if you haven't set @ARGV, will input from
.IR STDIN .
.PP
If the string inside the angle brackets is a reference to a scalar variable
(e.g. <$foo>),
then that variable contains the name of the filehandle to input from.
.PP
If the string inside angle brackets is not a filehandle, it is interpreted
as a filename pattern to be globbed, and either an array of filenames or the
next filename in the list is returned, depending on context.
One level of $ interpretation is done first, but you can't say <$foo>
because that's an indirect filehandle as explained in the previous
paragraph.
You could insert curly brackets to force interpretation as a
filename glob: <${foo}>.
Example:
.nf

.ne 3
        while (<*.c>) {
                chmod 0644, $_;
        }

is equivalent to

.ne 5
        open(foo, "echo *.c | tr \-s \' \et\er\ef\' \'\e\e012\e\e012\e\e012\e\e012\'|");
        while (<foo>) {
                chop;
                chmod 0644, $_;
        }

.fi
In fact, it's currently implemented that way.
(Which means it will not work on filenames with spaces in them unless
you have /bin/csh on your machine.)
Of course, the shortest way to do the above is:
.nf

        chmod 0644, <*.c>;

.fi
.Sh "Syntax"
.PP
A
.I perl
script consists of a sequence of declarations and commands.
The only things that need to be declared in
.I perl
are report formats and subroutines.
See the sections below for more information on those declarations.
All uninitialized objects user-created objects are assumed to
start with a null or 0 value until they
are defined by some explicit operation such as assignment.
The sequence of commands is executed just once, unlike in
.I sed
and
.I awk
scripts, where the sequence of commands 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.)
.PP
A declaration can be put anywhere a command can, but has no effect on the
execution of the primary sequence of commands--declarations all take effect
at compile time.
Typically all the declarations are put at the beginning or the end of the script.
.PP
.I Perl
is, for the most part, a free-form language.
(The only exception to this is format declarations, for fairly obvious reasons.)
Comments are indicated by the # character, and extend to the end of the line.
If you attempt to use /* */ C comments, it will be interpreted either as
division or pattern matching, depending on the context.
So don't do that.
.Sh "Compound statements"
In
.IR perl ,
a sequence of commands may be treated as one command by enclosing it
in curly brackets.
We will call this a BLOCK.
.PP
The following compound commands may be used to control flow:
.nf

.ne 4
        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

.fi
Note that, unlike C and Pascal, these are defined in terms of BLOCKs, not
statements.
This means that the curly brackets are \fIrequired\fR\*(--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:
.nf

.ne 5
        if (!open(foo)) { die "Can't open $foo: $!"; }
        die "Can't open $foo: $!" unless open(foo);
        open(foo) || die "Can't open $foo: $!"; # foo or bust!
        open(foo) ? die "Can't open $foo: $!" : \'hi mom\';
                                # a bit exotic, that last one

.fi
.PP
The
.I if
statement is straightforward.
Since BLOCKs are always bounded by curly brackets, there is never any
ambiguity about which
.I if
an
.I else
goes with.
If you use
.I unless
in place of
.IR if ,
the sense of the test is reversed.
.PP
The
.I while
statement executes the block as long as the expression is true
(does not evaluate to the null string 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
.IR next ,
.IR last ,
and
.I redo
(see below).
If there is a
.I continue
BLOCK, it is always executed just before
the conditional is about to be evaluated again, similarly to the third part
of a
.I for
loop in C.
Thus it can be used to increment a loop variable, even when the loop has
been continued via the
.I next
statement (similar to the C \*(L"continue\*(R" statement).
.PP
If the word
.I while
is replaced by the word
.IR until ,
the sense of the test is reversed, but the conditional is still tested before
the first iteration.
.PP
In either the
.I if
or the
.I while
statement, you may replace \*(L"(EXPR)\*(R" with a BLOCK, and the conditional
is true if the value of the last command in that block is true.
.PP
The
.I for
loop works exactly like the corresponding
.I while
loop:
.nf

.ne 12
        for ($i = 1; $i < 10; $i++) {
                .\|.\|.
        }

is the same as

        $i = 1;
        while ($i < 10) {
                .\|.\|.
        } continue {
                $i++;
        }
.fi
.PP
The foreach loop iterates over a normal array value and sets the variable
VAR to be each element of the array in turn.
The \*(L"foreach\*(R" keyword is actually identical to the \*(L"for\*(R" keyword,
so you can use \*(L"foreach\*(R" for readability or \*(L"for\*(R" for brevity.
If VAR is omitted, $_ is set to each value.
If ARRAY is an actual array (as opposed to an expression returning an array
value), you can modify each element of the array
by modifying VAR inside the loop.
Examples:
.nf

.ne 5
        for (@ary) { s/foo/bar/; }

        foreach $elem (@elements) {
                $elem *= 2;
        }

.ne 3
        for ((10,9,8,7,6,5,4,3,2,1,\'BOOM\')) {
                print $_, "\en"; sleep(1);
        }

        for (1..15) { print "Merry Christmas\en"; }

.ne 3
        foreach $item (split(/:[\e\e\en:]*/, $ENV{\'TERMCAP\'}) {
                print "Item: $item\en";
        }

.fi
.PP
The BLOCK by itself (labeled or not) is 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
.I continue
block is optional.
This construct is particularly nice for doing case structures.
.nf

.ne 6
        foo: {
                if (/^abc/) { $abc = 1; last foo; }
                if (/^def/) { $def = 1; last foo; }
                if (/^xyz/) { $xyz = 1; last foo; }
                $nothing = 1;
        }

.fi
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
.nf

.ne 6
        foo: {
                $abc = 1, last foo      if /^abc/;
                $def = 1, last foo      if /^def/;
                $xyz = 1, last foo      if /^xyz/;
                $nothing = 1;
        }

or

.ne 6
        foo: {
                /^abc/ && do { $abc = 1; last foo; }
                /^def/ && do { $def = 1; last foo; }
                /^xyz/ && do { $xyz = 1; last foo; }
                $nothing = 1;
        }

or

.ne 6
        foo: {
                /^abc/ && ($abc = 1, last foo);
                /^def/ && ($def = 1, last foo);
                /^xyz/ && ($xyz = 1, last foo);
                $nothing = 1;
        }

or even

.ne 8
        if (/^abc/)
                { $abc = 1; last foo; }
        elsif (/^def/)
                { $def = 1; last foo; }
        elsif (/^xyz/)
                { $xyz = 1; last foo; }
        else
                {$nothing = 1;}

.fi
As it happens, these are all optimized internally to a switch structure,
so perl jumps directly to the desired statement, and you needn't worry
about perl executing a lot of unnecessary statements when you have a string
of 50 elsifs, as long as you are testing the same simple scalar variable
using ==, eq, or pattern matching as above.
(If you're curious as to whether the optimizer has done this for a particular
case statement, you can use the \-D1024 switch to list the syntax tree
before execution.)
.Sh "Simple statements"
The only kind of simple statement is an expression evaluated for its side
effects.
Every expression (simple statement) must be terminated with a semicolon.
Note that this is like C, but unlike Pascal (and
.IR awk ).
.PP
Any simple statement may optionally be followed by a
single modifier, just before the terminating semicolon.
The possible modifiers are:
.nf

.ne 4
        if EXPR
        unless EXPR
        while EXPR
        until EXPR

.fi
The
.I if
and
.I unless
modifiers have the expected semantics.
The
.I while
and
.I until
modifiers also have the expected semantics (conditional evaluated first),
except when applied to a do-BLOCK command,
in which case the block executes once before the conditional is evaluated.
This is so that you can write loops like:
.nf

.ne 4
        do {
                $_ = <STDIN>;
                .\|.\|.
        } until $_ \|eq \|".\|\e\|n";

.fi
(See the
.I do
operator below.  Note also that the loop control commands described later will
NOT work in this construct, since modifiers don't take loop labels.
Sorry.)
.Sh "Expressions"
Since
.I perl
expressions work almost exactly like C expressions, only the differences
will be mentioned here.
.PP
Here's what
.I perl
has that C doesn't:
.Ip ** 8 2
The exponentiation operator.
.Ip **= 8
The exponentiation assignment operator.
.Ip (\|) 8 3
The null list, used to initialize an array to null.
.Ip . 8
Concatenation of two strings.
.Ip .= 8
The concatenation assignment operator.
.Ip eq 8
String equality (== is numeric equality).
For a mnemonic just think of \*(L"eq\*(R" as a string.
(If you are used to the
.I awk
behavior of using == for either string or numeric equality
based on the current form of the comparands, beware!
You must be explicit here.)
.Ip ne 8
String inequality (!= is numeric inequality).
.Ip lt 8
String less than.
.Ip gt 8
String greater than.
.Ip le 8
String less than or equal.
.Ip ge 8
String greater than or equal.
.Ip =~ 8 2
Certain operations search or modify the string \*(L"$_\*(R" by default.
This operator makes that kind of operation work on some other string.
The right argument is a search pattern, substitution, or translation.
The left argument is what is supposed to be searched, substituted, or
translated instead of the default \*(L"$_\*(R".
The return value indicates the success of the operation.
(If the right argument is an expression other than a search pattern,
substitution, or translation, it is interpreted as a search pattern
at run time.
This is less efficient than an explicit search, since the pattern must
be compiled every time the expression is evaluated.)
The precedence of this operator is lower than unary minus and autoincrement/decrement, but higher than everything else.
.Ip !~ 8
Just like =~ except the return value is negated.
.Ip x 8
The repetition operator.
Returns a string consisting of the left operand repeated the
number of times specified by the right operand.
.nf

        print \'\-\' x 80;              # print row of dashes
        print \'\-\' x80;               # illegal, x80 is identifier

        print "\et" x ($tab/8), \' \' x ($tab%8);       # tab over

.fi
.Ip x= 8
The repetition assignment operator.
.Ip .\|. 8
The range operator, which is really two different operators depending
on the context.
In an array context, returns an array of values counting (by ones)
from the left value to the right value.
This is useful for writing \*(L"for (1..10)\*(R" loops and for doing
slice operations on arrays.
.Sp
In a scalar context, .\|. returns a boolean value.
The operator is bistable, like a flip-flop..
Each .\|. operator maintains its own boolean state.
It is false as long as its left operand is false.
Once the left operand is true, the range operator stays true
until the right operand is true,
AFTER which the range operator becomes false again.
(It doesn't become false till the next time the range operator is evaluated.
It can become false on the same evaluation it became true, but it still returns
true once.)
The right operand is not evaluated while the operator is in the \*(L"false\*(R" state,
and the left operand is not evaluated while the operator is in the \*(L"true\*(R" state.
The scalar .\|. operator is primarily intended for doing line number ranges
after
the fashion of \fIsed\fR or \fIawk\fR.
The precedence is a little lower than || and &&.
The value returned is either the null string for false, or a sequence number
(beginning with 1) for true.
The sequence number is reset for each range encountered.
The final sequence number in a range has the string \'E0\' appended to it, which
doesn't affect its numeric value, but gives you something to search for if you
want to exclude the endpoint.
You can exclude the beginning point by waiting for the sequence number to be
greater than 1.
If either operand of scalar .\|. is static, that operand is implicitly compared
to the $. variable, the current line number.
Examples:
.nf

.ne 6
As a scalar operator:
    if (101 .\|. 200) { print; }        # print 2nd hundred lines

    next line if (1 .\|. /^$/); # skip header lines

    s/^/> / if (/^$/ .\|. eof());       # quote body

.ne 4
As an array operator:
    for (101 .\|. 200) { print; }       # print $_ 100 times

    @foo = @foo[$[ .\|. $#foo]; # an expensive no-op
    @foo = @foo[$#foo-4 .\|. $#foo];    # slice last 5 items

.fi
.Ip \-x 8
A file test.
This unary operator takes one argument, either a filename or a filehandle,
and tests the associated file to see if something is true about it.
If the argument is omitted, tests $_, except for \-t, which tests
.IR STDIN .
It returns 1 for true and \'\' for false, or the undefined value if the
file doesn't exist.
Precedence is higher than logical and relational operators, but lower than
arithmetic operators.
The operator may be any of:
.nf
        \-r     File is readable by effective uid.
        \-w     File is writable by effective uid.
        \-x     File is executable by effective uid.
        \-o     File is owned by effective uid.
        \-R     File is readable by real uid.
        \-W     File is writable by real uid.
        \-X     File is executable by real uid.
        \-O     File is owned by real uid.
        \-e     File exists.
        \-z     File has zero size.
        \-s     File has non-zero size.
        \-f     File is a plain file.
        \-d     File is a directory.
        \-l     File is a symbolic link.
        \-p     File is a named pipe (FIFO).
        \-S     File is a socket.
        \-b     File is a block special file.
        \-c     File is a character special file.
        \-u     File has setuid bit set.
        \-g     File has setgid bit set.
        \-k     File has sticky bit set.
        \-t     Filehandle is opened to a tty.
        \-T     File is a text file.
        \-B     File is a binary file (opposite of \-T).

.fi
The interpretation of the file permission operators \-r, \-R, \-w, \-W, \-x and \-X
is based solely on the mode of the file and the uids and gids of the user.
There may be other reasons you can't actually read, write or execute the file.
Also note that, for the superuser, \-r, \-R, \-w and \-W always return 1, and 
\-x and \-X return 1 if any execute bit is set in the mode.
Scripts run by the superuser may thus need to do a stat() in order to determine
the actual mode of the file, or temporarily set the uid to something else.
.Sp
Example:
.nf
.ne 7
        
        while (<>) {
                chop;
                next unless \-f $_;     # ignore specials
                .\|.\|.
        }

.fi
Note that \-s/a/b/ does not do a negated substitution.
Saying \-exp($foo) still works as expected, however\*(--only single letters
following a minus are interpreted as file tests.
.Sp
The \-T and \-B switches work as follows.
The first block or so of the file is examined for odd characters such as
strange control codes or metacharacters.
If too many odd characters (>10%) are found, it's a \-B file, otherwise it's a \-T file.
Also, any file containing null in the first block is considered a binary file.
If \-T or \-B is used on a filehandle, the current stdio buffer is examined
rather than the first block.
Both \-T and \-B return TRUE on a null file, or a file at EOF when testing
a filehandle.
.PP
If any of the file tests (or either stat operator) are given the special
filehandle consisting of a solitary underline, then the stat structure
of the previous file test (or stat operator) is used, saving a system
call.
(This doesn't work with \-t, and you need to remember that lstat and -l
will leave values in the stat structure for the symbolic link, not the
real file.)
Example:
.nf

        print "Can do.\en" if -r $a || -w _ || -x _;

.ne 9
        stat($filename);
        print "Readable\en" if -r _;
        print "Writable\en" if -w _;
        print "Executable\en" if -x _;
        print "Setuid\en" if -u _;
        print "Setgid\en" if -g _;
        print "Sticky\en" if -k _;
        print "Text\en" if -T _;
        print "Binary\en" if -B _;

.fi
.PP
Here is what C has that
.I perl
doesn't:
.Ip "unary &" 12
Address-of operator.
.Ip "unary *" 12
Dereference-address operator.
.Ip "(TYPE)" 12
Type casting operator.
.PP
Like C,
.I perl
does a certain amount of expression evaluation at compile time, whenever
it determines that all of the arguments to an operator are static and have
no side effects.
In particular, string concatenation happens at compile time between literals that don't do variable substitution.
Backslash interpretation also happens at compile time.
You can say
.nf

.ne 2
        \'Now is the time for all\' . "\|\e\|n" .
        \'good men to come to.\'

.fi
and this all reduces to one string internally.
.PP
The autoincrement operator has a little extra built-in magic to it.
If you increment a variable that is numeric, or that has ever been used in
a numeric context, you get a normal increment.
If, however, the variable has only been used in string contexts since it
was set, and has a value that is not null and matches the
pattern /^[a\-zA\-Z]*[0\-9]*$/, the increment is done
as a string, preserving each character within its range, with carry:
.nf

        print ++($foo = \'99\');        # prints \*(L'100\*(R'
        print ++($foo = \'a0\');        # prints \*(L'a1\*(R'
        print ++($foo = \'Az\');        # prints \*(L'Ba\*(R'
        print ++($foo = \'zz\');        # prints \*(L'aaa\*(R'

.fi
The autodecrement is not magical.