perl 2.0 patch 1: removed redundant debugging code in regexp.c

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

.rn '' }`
''' $Header: perl.man.1,v 2.0.1.1 88/06/28 16:28:09 root Exp $
''' 
''' $Log:       perl.man.1,v $
''' Revision 2.0.1.1  88/06/28  16:28:09  root
''' patch1: fixed some quotes
''' patch1: clarified syntax of LIST
''' 
''' Revision 2.0  88/06/05  00:09:23  root
''' Baseline version 2.0.
''' 
''' 
.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 LOCAL
.SH NAME
perl - Practical Extraction and Report Language
.SH SYNOPSIS
.B perl [options] filename args
.SH DESCRIPTION
.I Perl
is a 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.
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 sed and awk scripts into 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 implicity via standard input.
This only works if there are no filename arguments\*(--to pass
arguments to a 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 \-n or \-p.
An implicit split command to the @F array
is done as the first thing inside the implicit while loop produced by
the \-n or \-p.
.nf

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

is equivalent to

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

.fi
.TP 5
.B \-D<number>
sets debugging flags.
To watch how it executes your script, use
.B \-D14.
(This only works if debugging is compiled into your
.IR perl .)
.TP 5
.B \-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
.B \-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 \-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 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
.B \-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 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
.I 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 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 perl script as a shell script.
The shell executes the second line as a normal shell command, and thus
starts up the perl interpreter.
On some systems $0 doesn't always contain the full pathname,
so the -S tells perl to search for the script if necessary.
After perl locates the script, it parses the lines and ignores them because
the variable $running_under_some_shell is never true.
.TP 5
.B \-U
allows perl to do unsafe operations.
Currently the only \*(L"unsafe\*(R" operation is the unlinking of directories while
running as superuser.
.TP 5
.B \-v
prints the version and patchlevel of your perl executable.
.TP 5
.B \-w
prints warnings about identifiers that are mentioned only once, and scalar
variables that are used before being set.
Also warns about redefined subroutines, and references to undefined
subroutines and filehandles.
.Sh "Data Types and Objects"
.PP
Perl has about two and a half data types: scalars, arrays of scalars, and
associative arrays.
Scalars and arrays of scalars are first class objects, for the most part,
in the sense that they can be used as a whole as values in an expression.
Associative arrays can only be accessed on an association by association basis;
they don't have a value as a whole (at least not yet).
.PP
Scalars are interpreted as strings or numbers as appropriate.
A scalar is interpreted as TRUE in the boolean sense if it is not the null
string or 0.
Booleans returned by operators are 1 for true and '0' or '' (the null
string) for false.
.PP
References to scalar variables always begin with \*(L'$\*(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 are denoted by \*(L'@\*(R':

    @days       \h'|2i'# ($days[0], $days[1],\|.\|.\|. $days[n])

.fi
.PP
Any of these five constructs may server as an lvalue,
that is, may be assigned to.
(You may also use an assignment to one of these lvalues as an lvalue in
certain contexts\*(--see s, tr and chop.)
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
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'$\*(R'
or \*(L'@\*(R', the \*(L"reserved\*(R" words aren't in fact reserved
with respect to variable names.
(They ARE reserved with respect to labels and filehandles, however, which
don't have an initial special character.
Hint: you could say open(LOG,'logfile') rather than open(log,'logfile').)
Case IS significant\*(--\*(L"FOO\*(R", \*(L"Foo\*(R" and \*(L"foo\*(R" are all
different names.
Names which start with a letter may also contain digits and underscores.
Names which do not start with a letter are limited to one character,
e.g. \*(L"$%\*(R" or \*(L"$$\*(R".
(Many one character names have a predefined significance to
.I perl.
More later.)
.PP
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.
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 perl finds another line containing the quote character, which
may be much further on in the script.
Variable substitution inside strings is limited (currently) to simple scalar variables.
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.
.PP
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);

.fi
Array assignment returns the number of elements assigned.
.PP
Numeric literals are specified in any of the usual floating point or
integer formats.
.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).
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 3
    while ($_ = <stdin>) {
    while (<stdin>) {
    for (\|;\|<stdin>;\|) {

.fi
The filehandles
.IR stdin ,
.I stdout
and
.I stderr
are predefined.
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 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.)
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 objects are assumed to start with a null or 0 value.
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.
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 ,
.I 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;
        }

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

.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
It's also nice for exiting subroutines early.
Note the double curly brackets:
.nf

.ne 8
        sub tokenize {{
                .\|.\|.
                if (/foo/) {
                        23;             # return value
                        last;
                }
                .\|.\|.
        }}

.fi
.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 3
The null list, used to initialize an array to null.
.Ip . 8
Concatenation of two strings.
.Ip .= 8
The corresponding 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 corresponding assignment operator.
.Ip .. 8
The range operator, which is bistable.
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 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 .. 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 .. is static, that operand is implicitly compared to
the $. variable, the current line number.
Examples:
.nf

.ne 5
    if (101 .. 200) { print; }  # print 2nd hundred lines

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

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

.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 stdin.
It returns 1 for true and '' for false.
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 writeable 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 writeable 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.
Since input doesn't work well on binary files you should probably test a
filehandle before doing any input if you're unsure of the nature of the
filehandle you've been handed (usually via stdin).
Both \-T and \-B return TRUE on a null file, or a file at EOF when testing
a filehandle.
.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 ++($foo = '99');  # prints '100'
        print ++($foo = 'a0');  # prints 'a1'
        print ++($foo = 'Az');  # prints 'Ba'
        print ++($foo = 'zz');  # prints 'aaa'

.fi
The autodecrement is not magical.
.PP
Along with the literals and variables mentioned earlier,
the following operations can serve as terms in an expression.
Some of these operations take a LIST as an argument.
Such a list can consist of any combination of scalar arguments or arrays;
the arrays will be included in the list as if each individual element were
interpolated at that point in the list.
Elements of the LIST should be separated by commas.
.Ip "/PATTERN/i" 8 4
Searches a string for a pattern, and returns true (1) or false ('').
If no string is specified via the =~ or !~ operator,
the $_ string is searched.
(The string specified with =~ need not be an lvalue\*(--it may be the result of an expression evaluation, but remember the =~ binds rather tightly.)
See also the section on regular expressions.
.Sp
If you prepend an `m' you can use any pair of characters as delimiters.
This is particularly useful for matching Unix path names that contain `/'.
If the final delimiter is followed by the optional letter `i', the matching is
done in a case-insensitive manner.
.Sp
If used in a context that requires an array value, a pattern match returns an
array consisting of the subexpressions matched by the parens in pattern,
i.e. ($1, $2, $3.\|.\|.).
.Sp
Examples:
.nf

.ne 4
    open(tty, '/dev/tty');
    <tty> \|=~ \|/\|^y\|/i \|&& \|do foo(\|);   # do foo if desired

    if (/Version: \|*\|([0-9.]*\|)\|/\|) { $version = $1; }

    next if m#^/usr/spool/uucp#;

    if (($F1,$F2,$Etc) = ($foo =~ /^(\eS+)\es+(\eS+)\es*(.*)/))

.fi
This last example splits $foo into the first two words and the remainder
of the line, and assigns those three fields to $F1, $F2 and $Etc.
The conditional is true if any variables were assigned, i.e. if the pattern
matched.
.Ip "?PATTERN?" 8 4
This is just like the /pattern/ search, except that it matches only once between
calls to the
.I reset
operator.
This is a useful optimization when you only want to see the first occurence of
something in each file of a set of files, for instance.
.Ip "chdir EXPR" 8 2
Changes the working directory to EXPR, if possible.
Returns 1 upon success, 0 otherwise.
See example under die().
.Ip "chmod LIST" 8 2
Changes the permissions of a list of files.
The first element of the list must be the numerical mode.
Returns the number of files successfully changed.
.nf

.ne 2
        $cnt = chmod 0755,'foo','bar';
        chmod 0755,@executables;

.fi
.Ip "chop(VARIABLE)" 8 5
.Ip "chop" 8
Chops off the last character of a string and returns it.
It's used primarily to remove the newline from the end of an input record,
but is much more efficient than s/\en// because it neither scans nor copies
the string.
If VARIABLE is omitted, chops $_.
Example:
.nf

.ne 5
        while (<>) {
                chop;   # avoid \en on last field
                @array = split(/:/);
                .\|.\|.
        }

.fi
You can actually chop anything that's an lvalue, including an assignment:
.nf

        chop($cwd = `pwd`);

.fi
.Ip "chown LIST" 8 2
Changes the owner (and group) of a list of files.
The first two elements of the list must be the NUMERICAL uid and gid,
in that order.
Returns the number of files successfully changed.
.nf

.ne 2
        $cnt = chown $uid,$gid,'foo','bar';
        chown $uid,$gid,@filenames;

.fi
.ne 23
Here's an example of looking up non-numeric uids:
.nf

        print "User: ";
        $user = <stdin>;
        chop($user);
        print "Files: "
        $pattern = <stdin>;
        chop($pattern);
        open(pass,'/etc/passwd') || die "Can't open passwd";
        while (<pass>) {
                ($login,$pass,$uid,$gid) = split(/:/);
                $uid{$login} = $uid;
                $gid{$login} = $gid;
        }
        @ary = <$pattern>;      # get filenames
        if ($uid{$user} eq '') {
                die "$user not in passwd file";
        }
        else {
                unshift(@ary,$uid{$user},$gid{$user});
                chown @ary;
        }

.fi
.Ip "close(FILEHANDLE)" 8 5
.Ip "close FILEHANDLE" 8
Closes the file or pipe associated with the file handle.
You don't have to close FILEHANDLE if you are immediately going to
do another open on it, since open will close it for you.
(See
.IR open .)
However, an explicit close on an input file resets the line counter ($.), while
the implicit close done by
.I open
does not.
Also, closing a pipe will wait for the process executing on the pipe to complete,
in case you want to look at the output of the pipe afterwards.
Example:
.nf

.ne 4
        open(output,'|sort >foo');      # pipe to sort
        .\|.\|. # print stuff to output
        close(output);          # wait for sort to finish
        open(input,'foo');      # get sort's results

.fi
FILEHANDLE may be an expression whose value gives the real filehandle name.
.Ip "crypt(PLAINTEXT,SALT)" 8 6
Encrypts a string exactly like the crypt() function in the C library.
Useful for checking the password file for lousy passwords.
Only the guys wearing white hats should do this.
.Ip "delete $ASSOC{KEY}" 8 6
Deletes the specified value from the specified associative array.
Returns the deleted value;
The following deletes all the values of an associative array:
.nf

.ne 3
        foreach $key (keys(ARRAY)) {
                delete $ARRAY{$key};
        }

.fi
(But it would be faster to use the reset command.)
.Ip "die EXPR" 8 6
Prints the value of EXPR to stderr and exits with the current value of $!
(errno).
If $! is 0, exits with the value of ($? >> 8) (`command` status).
If ($? >> 8) is 0, exits with 255.
Equivalent examples:
.nf

.ne 3
        die "Can't cd to spool.\en" unless chdir '/usr/spool/news';

        chdir '/usr/spool/news' || die "Can't cd to spool.\en" 

.fi
.Sp
If the value of EXPR does not end in a newline, the current script line
number and input line number (if any) are also printed, and a newline is
supplied.
Hint: sometimes appending \*(L", stopped\*(R" to your message will cause it to make
better sense when the string \*(L"at foo line 123\*(R" is appended.
Suppose you are running script \*(L"canasta\*(R".
.nf

.ne 7
        die "/etc/games is no good";
        die "/etc/games is no good, stopped";

produce, respectively

        /etc/games is no good at canasta line 123.
        /etc/games is no good, stopped at canasta line 123.

.fi
See also
.IR exit .
.Ip "do BLOCK" 8 4
Returns the value of the last command in the sequence of commands indicated
by BLOCK.
When modified by a loop modifier, executes the BLOCK once before testing the
loop condition.
(On other statements the loop modifiers test the conditional first.)
.Ip "do SUBROUTINE (LIST)" 8 3
Executes a SUBROUTINE declared by a
.I sub
declaration, and returns the value
of the last expression evaluated in SUBROUTINE.
If you pass arrays as part of LIST you may wish to pass the length
of the array in front of each array.
(See the section on subroutines later on.)
SUBROUTINE may be a scalar variable, in which case the variable contains
the name of the subroutine to execute.
The parentheses are required to avoid confusion with the next form of \*(L"do\*(R".
.Ip "do EXPR" 8 3
Uses the value of EXPR as a filename and executes the contents of the file
as a perl script.
It's primary use is to include subroutines from a perl subroutine library.
.nf
        do 'stat.pl';

is just like

        eval `cat stat.pl`;

.fi
except that it's more efficient, more concise, keeps track of the current
filename for error messages, and searches all the -I libraries if the file
isn't in the current directory (see also the @INC array in Predefined Names).
It's the same, however, in that it does reparse the file every time you
call it, so if you are going to use the file inside a loop you might prefer
to use #include, at the expense of a little more startup time.
(The main problem with #include is that cpp doesn't grok # comments--a
workaround is to use \*(L";#\*(R" for standalone comments.)
Note that the following are NOT equivalent:
.nf

.ne 2
        do $foo;        # eval a file
        do $foo();      # call a subroutine

.fi
.Ip "each(ASSOC_ARRAY)" 8 6
Returns a 2 element array consisting of the key and value for the next
value of an associative array, so that you can iterate over it.
Entries are returned in an apparently random order.
When the array is entirely read, a null array is returned (which when
assigned produces a FALSE (0) value).
The next call to each() after that will start iterating again.
The iterator can be reset only by reading all the elements from the array.
You must not modify the array while iterating over it.
There is a single iterator for each associative array, shared by all
each(), keys() and values() function calls in the program.
The following prints out your environment like the printenv program, only
in a different order:
.nf

.ne 3
        while (($key,$value) = each(ENV)) {
                print "$key=$value\en";
        }

.fi
See also keys() and values().
.Ip "eof(FILEHANDLE)" 8 8
.Ip "eof" 8
Returns 1 if the next read on FILEHANDLE will return end of file, or if
FILEHANDLE is not open.
FILEHANDLE may be an expression whose value gives the real filehandle name.
An eof without an argument returns the eof status for the last file read.
Empty parentheses () may be used to indicate the pseudo file formed of the
files listed on the command line, i.e. eof() is reasonable to use inside
a while (<>) loop to detect the end of only the last file.
Use eof(ARGV) or eof without the parens to test EACH file in a while (<>) loop.
Examples:
.nf

.ne 7
        # insert dashes just before last line of last file
        while (<>) {
                if (eof()) {
                        print "--------------\en";
                }
                print;
        }

.ne 7
        # reset line numbering on each input file
        while (<>) {
                print "$.\et$_";
                if (eof) {      # Not eof().
                        close(ARGV);
                }
        }

.fi
.Ip "eval EXPR" 8 6
EXPR is parsed and executed as if it were a little perl program.
It is executed in the context of the current perl program, so that
any variable settings, subroutine or format definitions remain afterwards.
The value returned is the value of the last expression evaluated, just
as with subroutines.
If there is a syntax error or runtime error, a null string is returned by
eval, and $@ is set to the error message.
If there was no error, $@ is null.
If EXPR is omitted, evaluates $_.
.Ip "exec LIST" 8 6
If there is more than one argument in LIST,
calls execvp() with the arguments in LIST.
If there is only one argument, the argument is checked for shell metacharacters.
If there are any, the entire argument is passed to /bin/sh -c for parsing.
If there are none, the argument is split into words and passed directly to
execvp(), which is more efficient.
Note: exec (and system) do not flush your output buffer, so you may need to
set $| to avoid lost output.
Examples:
.nf

        exec '/bin/echo', 'Your arguments are: ', @ARGV;
        exec "sort $outfile | uniq";

.fi
.Ip "exit EXPR" 8 6
Evaluates EXPR and exits immediately with that value.
Example:
.nf

.ne 2
        $ans = <stdin>;
        exit 0 \|if \|$ans \|=~ \|/\|^[Xx]\|/\|;

.fi
See also
.IR die .
.Ip "exp(EXPR)" 8 3
Returns e to the power of EXPR.
.Ip "fork" 8 4
Does a fork() call.
Returns the child pid to the parent process and 0 to the child process.
Note: unflushed buffers remain unflushed in both processes, which means
you may need to set $| to avoid duplicate output.
.Ip "gmtime(EXPR)" 8 4
Converts a time as returned by the time function to a 9-element array with
the time analyzed for the Greenwich timezone.
Typically used as follows:
.nf

.ne 3
    ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst)
       = gmtime(time);

.fi
All array elements are numeric, and come straight out of a struct tm.
In particular this means that $mon has the range 0..11 and $wday has the
range 0..6.
''' End of part 1