Dream 52/Amiga_Dream_52.iso/RiscOS/APP/DEVS/PERL/RPC106.ZIP/Rpc106/Docs/perlop

Text File | 1997-11-23 | 49KB | 1,317 lines

NAME
    perlop - Perl operators and precedence

SYNOPSIS
    Perl operators have the following associativity and precedence,
    listed from highest precedence to lowest. Note that all
    operators borrowed from C keep the same precedence relationship
    with each other, even where C's precedence is slightly screwy.
    (This makes learning Perl easier for C folks.) With very few
    exceptions, these all operate on scalar values only, not array
    values.

    left        terms and list operators (leftward)
    left        ->
    nonassoc    ++ --
    right        **
    right        ! ~ \ and unary + and -
    left        =~ !~
    left        * / % x
    left        + - .
    left        << >>
    nonassoc    named unary operators
    nonassoc    < > <= >= lt gt le ge
    nonassoc    == != <=> eq ne cmp
    left        &
    left        | ^
    left        &&
    left        ||
    nonassoc    ..    ...
    right        ?:
    right        = += -= *= etc.
    left        , =>
    nonassoc    list operators (rightward)
    right        not
    left        and
    left        or xor

    In the following sections, these operators are covered in
    precedence order.

DESCRIPTION
  Terms and List Operators (Leftward)

    A TERM has the highest precedence in Perl. They includes
    variables, quote and quote-like operators, any expression in
    parentheses, and any function whose arguments are parenthesized.
    Actually, there aren't really functions in this sense, just list
    operators and unary operators behaving as functions because you
    put parentheses around the arguments. These are all documented
    in the perlfunc manpage.

    If any list operator (print(), etc.) or any unary operator
    (chdir(), etc.) is followed by a left parenthesis as the next
    token, the operator and arguments within parentheses are taken
    to be of highest precedence, just like a normal function call.

    In the absence of parentheses, the precedence of list operators
    such as `print', `sort', or `chmod' is either very high or very
    low depending on whether you are looking at the left side or the
    right side of the operator. For example, in

    @ary = (1, 3, sort 4, 2);
    print @ary;        # prints 1324

    the commas on the right of the sort are evaluated before the
    sort, but the commas on the left are evaluated after. In other
    words, list operators tend to gobble up all the arguments that
    follow them, and then act like a simple TERM with regard to the
    preceding expression. Note that you have to be careful with
    parentheses:

    # These evaluate exit before doing the print:
    print($foo, exit);  # Obviously not what you want.
    print $foo, exit;   # Nor is this.

    # These do the print before evaluating exit:
    (print $foo), exit; # This is what you want.
    print($foo), exit;  # Or this.
    print ($foo), exit; # Or even this.

    Also note that

    print ($foo & 255) + 1, "\n";

    probably doesn't do what you expect at first glance. See the
    section on "Named Unary Operators" for more discussion of this.

    Also parsed as terms are the `do {}' and `eval {}' constructs,
    as well as subroutine and method calls, and the anonymous
    constructors `[]' and `{}'.

    See also the section on "Quote and Quote-like Operators" toward
    the end of this section, as well as the section on "I/O
    Operators".

  The Arrow Operator

    Just as in C and C++, "`->'" is an infix dereference operator.
    If the right side is either a `[...]' or `{...}' subscript, then
    the left side must be either a hard or symbolic reference to an
    array or hash (or a location capable of holding a hard
    reference, if it's an lvalue (assignable)). See the perlref
    manpage.

    Otherwise, the right side is a method name or a simple scalar
    variable containing the method name, and the left side must
    either be an object (a blessed reference) or a class name (that
    is, a package name). See the perlobj manpage.

  Auto-increment and Auto-decrement

    "++" and "--" work as in C. That is, if placed before a
    variable, they increment or decrement the variable before
    returning the value, and if placed after, increment or decrement
    the variable after returning the value.

    The auto-increment operator has a little extra builtin 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 been used in only 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:

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

    The auto-decrement operator is not magical.

  Exponentiation

    Binary "**" is the exponentiation operator. Note that it binds
    even more tightly than unary minus, so -2**4 is -(2**4), not (-
    2)**4. (This is implemented using C's pow(3) function, which
    actually works on doubles internally.)

  Symbolic Unary Operators

    Unary "!" performs logical negation, i.e., "not". See also `not'
    for a lower precedence version of this.

    Unary "-" performs arithmetic negation if the operand is
    numeric. If the operand is an identifier, a string consisting of
    a minus sign concatenated with the identifier is returned.
    Otherwise, if the string starts with a plus or minus, a string
    starting with the opposite sign is returned. One effect of these
    rules is that `-bareword' is equivalent to `"-bareword"'.

    Unary "~" performs bitwise negation, i.e., 1's complement. (See
    also the section on "Integer Arithmetic".)

    Unary "+" has no effect whatsoever, even on strings. It is
    useful syntactically for separating a function name from a
    parenthesized expression that would otherwise be interpreted as
    the complete list of function arguments. (See examples above
    under the section on "Terms and List Operators (Leftward)".)

    Unary "\" creates a reference to whatever follows it. See the
    perlref manpage. Do not confuse this behavior with the behavior
    of backslash within a string, although both forms do convey the
    notion of protecting the next thing from interpretation.

  Binding Operators

    Binary "=~" binds a scalar expression to a pattern match.
    Certain operations search or modify the string $_ 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 $_.
    The return value indicates the success of the operation. (If the
    right argument is an expression rather than a search pattern,
    substitution, or translation, it is interpreted as a search
    pattern at run time. This can be is less efficient than an
    explicit search, because the pattern must be compiled every time
    the expression is evaluated.

    Binary "!~" is just like "=~" except the return value is negated
    in the logical sense.

  Multiplicative Operators

    Binary "*" multiplies two numbers.

    Binary "/" divides two numbers.

    Binary "%" computes the modulus of two numbers. Given integer
    operands `$a' and `$b': If `$b' is positive, then `$a % $b' is
    `$a' minus the largest multiple of `$b' that is not greater than
    `$a'. If `$b' is negative, then `$a % $b' is `$a' minus the
    smallest multiple of `$b' that is not less than `$a' (i.e. the
    result will be less than or equal to zero).

    Binary "x" is the repetition operator. In a scalar context, it
    returns a string consisting of the left operand repeated the
    number of times specified by the right operand. In a list
    context, if the left operand is a list in parentheses, it
    repeats the list.

    print '-' x 80;         # print row of dashes

    print "\t" x ($tab/8), ' ' x ($tab%8);        # tab over

    @ones = (1) x 80;        # a list of 80 1's
    @ones = (5) x @ones;        # set all elements to 5

  Additive Operators

    Binary "+" returns the sum of two numbers.

    Binary "-" returns the difference of two numbers.

    Binary "." concatenates two strings.

  Shift Operators

    Binary "<<" returns the value of its left argument shifted left
    by the number of bits specified by the right argument. Arguments
    should be integers. (See also the section on "Integer
    Arithmetic".)

    Binary ">>" returns the value of its left argument shifted right
    by the number of bits specified by the right argument. Arguments
    should be integers. (See also the section on "Integer
    Arithmetic".)

  Named Unary Operators

    The various named unary operators are treated as functions with
    one argument, with optional parentheses. These include the
    filetest operators, like `-f', `-M', etc. See the perlfunc
    manpage.

    If any list operator (print(), etc.) or any unary operator
    (chdir(), etc.) is followed by a left parenthesis as the next
    token, the operator and arguments within parentheses are taken
    to be of highest precedence, just like a normal function call.
    Examples:

    chdir $foo    || die;        # (chdir $foo) || die
    chdir($foo)   || die;        # (chdir $foo) || die
    chdir ($foo)  || die;        # (chdir $foo) || die
    chdir +($foo) || die;        # (chdir $foo) || die

    but, because * is higher precedence than ||:

    chdir $foo * 20;    # chdir ($foo * 20)
    chdir($foo) * 20;   # (chdir $foo) * 20
    chdir ($foo) * 20;  # (chdir $foo) * 20
    chdir +($foo) * 20; # chdir ($foo * 20)

    rand 10 * 20;        # rand (10 * 20)
    rand(10) * 20;        # (rand 10) * 20
    rand (10) * 20;     # (rand 10) * 20
    rand +(10) * 20;    # rand (10 * 20)

    See also the section on "Terms and List Operators (Leftward)".

  Relational Operators

    Binary "<" returns true if the left argument is numerically less
    than the right argument.

    Binary ">" returns true if the left argument is numerically
    greater than the right argument.

    Binary "<=" returns true if the left argument is numerically
    less than or equal to the right argument.

    Binary ">=" returns true if the left argument is numerically
    greater than or equal to the right argument.

    Binary "lt" returns true if the left argument is stringwise less
    than the right argument.

    Binary "gt" returns true if the left argument is stringwise
    greater than the right argument.

    Binary "le" returns true if the left argument is stringwise less
    than or equal to the right argument.

    Binary "ge" returns true if the left argument is stringwise
    greater than or equal to the right argument.

  Equality Operators

    Binary "==" returns true if the left argument is numerically
    equal to the right argument.

    Binary "!=" returns true if the left argument is numerically not
    equal to the right argument.

    Binary "<=>" returns -1, 0, or 1 depending on whether the left
    argument is numerically less than, equal to, or greater than the
    right argument.

    Binary "eq" returns true if the left argument is stringwise
    equal to the right argument.

    Binary "ne" returns true if the left argument is stringwise not
    equal to the right argument.

    Binary "cmp" returns -1, 0, or 1 depending on whether the left
    argument is stringwise less than, equal to, or greater than the
    right argument.

    "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order
    specified by the current locale if `use locale' is in effect.
    See the perllocale manpage.

  Bitwise And

    Binary "&" returns its operators ANDed together bit by bit. (See
    also the section on "Integer Arithmetic".)

  Bitwise Or and Exclusive Or

    Binary "|" returns its operators ORed together bit by bit. (See
    also the section on "Integer Arithmetic".)

    Binary "^" returns its operators XORed together bit by bit. (See
    also the section on "Integer Arithmetic".)

  C-style Logical And

    Binary "&&" performs a short-circuit logical AND operation. That
    is, if the left operand is false, the right operand is not even
    evaluated. Scalar or list context propagates down to the right
    operand if it is evaluated.

  C-style Logical Or

    Binary "||" performs a short-circuit logical OR operation. That
    is, if the left operand is true, the right operand is not even
    evaluated. Scalar or list context propagates down to the right
    operand if it is evaluated.

    The `||' and `&&' operators differ from C's in that, rather than
    returning 0 or 1, they return the last value evaluated. Thus, a
    reasonably portable way to find out the home directory (assuming
    it's not "0") might be:

    $home = $ENV{'HOME'} || $ENV{'LOGDIR'} ||
        (getpwuid($<))[7] || die "You're homeless!\n";

    As more readable alternatives to `&&' and `||', Perl provides
    "and" and "or" operators (see below). The short-circuit behavior
    is identical. The precedence of "and" and "or" is much lower,
    however, so that you can safely use them after a list operator
    without the need for parentheses:

    unlink "alpha", "beta", "gamma"
        or gripe(), next LINE;

    With the C-style operators that would have been written like
    this:

    unlink("alpha", "beta", "gamma")
        || (gripe(), next LINE);

  Range Operator

    Binary ".." is the range operator, which is really two different
    operators depending on the context. In a list context, it
    returns an array of values counting (by ones) from the left
    value to the right value. This is useful for writing `for
    (1..10)' loops and for doing slice operations on arrays. Be
    aware that under the current implementation, a temporary array
    is created, so you'll burn a lot of memory if you write
    something like this:

    for (1 .. 1_000_000) {
        # code
    }

    In a scalar context, ".." returns a boolean value. The operator
    is bistable, like a flip-flop, and emulates the line-range
    (comma) operator of sed, awk, and various editors. 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 test the right operand and become false on the
    same evaluation it became true (as in awk), but it still returns
    true once. If you don't want it to test the right operand till
    the next evaluation (as in sed), use three dots ("...") instead
    of two.) The right operand is not evaluated while the operator
    is in the "false" state, and the left operand is not evaluated
    while the operator is in the "true" state. 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 a numeric literal, that operand is implicitly
    compared to the `$.' variable, the current line number.
    Examples:

    As a scalar operator:

    if (101 .. 200) { print; }  # print 2nd hundred lines
    next line if (1 .. /^$/);   # skip header lines
    s/^/> / if (/^$/ .. eof()); # quote body

    As a list operator:

    for (101 .. 200) { print; } # print $_ 100 times
    @foo = @foo[0 .. $#foo];    # an expensive no-op
    @foo = @foo[$#foo-4 .. $#foo];        # slice last 5 items

    The range operator (in a list context) makes use of the magical
    auto-increment algorithm if the operands are strings. You can
    say

    @alphabet = ('A' .. 'Z');

    to get all the letters of the alphabet, or

    $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];

    to get a hexadecimal digit, or

    @z2 = ('01' .. '31');  print $z2[$mday];

    to get dates with leading zeros. If the final value specified is
    not in the sequence that the magical increment would produce,
    the sequence goes until the next value would be longer than the
    final value specified.

  Conditional Operator

    Ternary "?:" is the conditional operator, just as in C. It works
    much like an if-then-else. If the argument before the ? is true,
    the argument before the : is returned, otherwise the argument
    after the : is returned. For example:

    printf "I have %d dog%s.\n", $n,
        ($n == 1) ? '' : "s";

    Scalar or list context propagates downward into the 2nd or 3rd
    argument, whichever is selected.

    $a = $ok ? $b : $c;  # get a scalar
    @a = $ok ? @b : @c;  # get an array
    $a = $ok ? @b : @c;  # oops, that's just a count!

    The operator may be assigned to if both the 2nd and 3rd
    arguments are legal lvalues (meaning that you can assign to
    them):

    ($a_or_b ? $a : $b) = $c;

    This is not necessarily guaranteed to contribute to the
    readability of your program.

  Assignment Operators

    "=" is the ordinary assignment operator.

    Assignment operators work as in C. That is,

    $a += 2;

    is equivalent to

    $a = $a + 2;

    although without duplicating any side effects that dereferencing
    the lvalue might trigger, such as from tie(). Other assignment
    operators work similarly. The following are recognized:

    **=    +=    *=    &=     <<=    &&=
           -=    /=    |=     >>=    ||=
           .=    %=    ^=
             x=

    Note that while these are grouped by family, they all have the
    precedence of assignment.

    Unlike in C, the assignment operator produces a valid lvalue.
    Modifying an assignment is equivalent to doing the assignment
    and then modifying the variable that was assigned to. This is
    useful for modifying a copy of something, like this:

    ($tmp = $global) =~ tr [A-Z] [a-z];

    Likewise,

    ($a += 2) *= 3;

    is equivalent to

    $a += 2;
    $a *= 3;

  Comma Operator

    Binary "," is the comma operator. In a scalar context it
    evaluates its left argument, throws that value away, then
    evaluates its right argument and returns that value. This is
    just like C's comma operator.

    In a list context, it's just the list argument separator, and
    inserts both its arguments into the list.

    The => digraph is mostly just a synonym for the comma operator.
    It's useful for documenting arguments that come in pairs. As of
    release 5.001, it also forces any word to the left of it to be
    interpreted as a string.

  List Operators (Rightward)

    On the right side of a list operator, it has very low
    precedence, such that it controls all comma-separated
    expressions found there. The only operators with lower
    precedence are the logical operators "and", "or", and "not",
    which may be used to evaluate calls to list operators without
    the need for extra parentheses:

    open HANDLE, "filename"
        or die "Can't open: $!\n";

    See also discussion of list operators in the section on "Terms
    and List Operators (Leftward)".

  Logical Not

    Unary "not" returns the logical negation of the expression to
    its right. It's the equivalent of "!" except for the very low
    precedence.

  Logical And

    Binary "and" returns the logical conjunction of the two
    surrounding expressions. It's equivalent to && except for the
    very low precedence. This means that it short-circuits: i.e.,
    the right expression is evaluated only if the left expression is
    true.

  Logical or and Exclusive Or

    Binary "or" returns the logical disjunction of the two
    surrounding expressions. It's equivalent to || except for the
    very low precedence. This means that it short-circuits: i.e.,
    the right expression is evaluated only if the left expression is
    false.

    Binary "xor" returns the exclusive-OR of the two surrounding
    expressions. It cannot short circuit, of course.

  C Operators Missing From Perl

    Here is what C has that Perl doesn't:

    unary & Address-of operator. (But see the "\" operator for taking a
        reference.)

    unary * Dereference-address operator. (Perl's prefix dereferencing
        operators are typed: $, @, %, and &.)

    (TYPE)  Type casting operator.

  Quote and Quote-like Operators

    While we usually think of quotes as literal values, in Perl they
    function as operators, providing various kinds of interpolating
    and pattern matching capabilities. Perl provides customary quote
    characters for these behaviors, but also provides a way for you
    to choose your quote character for any of them. In the following
    table, a `{}' represents any pair of delimiters you choose. Non-
    bracketing delimiters use the same character fore and aft, but
    the 4 sorts of brackets (round, angle, square, curly) will all
    nest.

    Customary  Generic     Meaning      Interpolates
        ''         q{}       Literal           no
        ""        qq{}       Literal           yes
        ``        qx{}       Command           yes
            qw{}      Word list        no
        //         m{}    Pattern match      yes
             s{}{}   Substitution      yes
            tr{}{}   Translation       no

    Note that there can be whitespace between the operator and the
    quoting characters, except when `#' is being used as the quoting
    character. `q#foo#' is parsed as being the string `foo', which
    `q #foo#' is the operator `q' followed by a comment. Its
    argument will be taken from the next line. This allows you to
    write:

    s {foo}  # Replace foo
      {bar}  # with bar.

    For constructs that do interpolation, variables beginning with
    "`$'" or "`@'" are interpolated, as are the following sequences:

    \t        tab         (HT, TAB)
    \n        newline        (LF, NL)
    \r        return        (CR)
    \f        form feed        (FF)
    \b        backspace        (BS)
    \a        alarm (bell)    (BEL)
    \e        escape        (ESC)
    \033        octal char
    \x1b        hex char
    \c[        control char
    \l        lowercase next char
    \u        uppercase next char
    \L        lowercase till \E
    \U        uppercase till \E
    \E        end case modification
    \Q        quote regexp metacharacters till \E

    If `use locale' is in effect, the case map used by `\l', `\L',
    `\u' and <\U> is taken from the current locale. See the
    perllocale manpage.

    Patterns are subject to an additional level of interpretation as
    a regular expression. This is done as a second pass, after
    variables are interpolated, so that regular expressions may be
    incorporated into the pattern from the variables. If this is not
    what you want, use `\Q' to interpolate a variable literally.

    Apart from the above, there are no multiple levels of
    interpolation. In particular, contrary to the expectations of
    shell programmers, back-quotes do *NOT* interpolate within
    double quotes, nor do single quotes impede evaluation of
    variables when used within double quotes.

  Regexp Quote-Like Operators

    Here are the quote-like operators that apply to pattern matching
    and related activities.

    ?PATTERN?
        This is just like the `/pattern/' search, except that it
        matches only once between calls to the reset() operator.
        This is a useful optimization when you want to see only
        the first occurrence of something in each file of a set
        of files, for instance. Only `??' patterns local to the
        current package are reset.

        This usage is vaguely deprecated, and may be removed in
        some future version of Perl.

    m/PATTERN/cgimosx
    /PATTERN/cgimosx
        Searches a string for a pattern match, and in a scalar
        context 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 perlre manpage. See the perllocale manpage
        for discussion of additional considerations which apply
        when `use locale' is in effect.

        Options are:

        c   Do not reset search position on a failed match when /g is in effect.
        g   Match globally, i.e., find all occurrences.
        i   Do case-insensitive pattern matching.
        m   Treat string as multiple lines.
        o   Compile pattern only once.
        s   Treat string as single line.
        x   Use extended regular expressions.

        If "/" is the delimiter then the initial `m' is
        optional. With the `m' you can use any pair of non-
        alphanumeric, non-whitespace characters as delimiters.
        This is particularly useful for matching Unix path names
        that contain "/", to avoid LTS (leaning toothpick
        syndrome). If "?" is the delimiter, then the match-only-
        once rule of `?PATTERN?' applies.

        PATTERN may contain variables, which will be
        interpolated (and the pattern recompiled) every time the
        pattern search is evaluated. (Note that `$)' and `$|'
        might not be interpolated because they look like end-of-
        string tests.) If you want such a pattern to be compiled
        only once, add a `/o' after the trailing delimiter. This
        avoids expensive run-time recompilations, and is useful
        when the value you are interpolating won't change over
        the life of the script. However, mentioning `/o'
        constitutes a promise that you won't change the
        variables in the pattern. If you change them, Perl won't
        even notice.

        If the PATTERN evaluates to a null string, the last
        successfully executed regular expression is used
        instead.

        If used in a context that requires a list value, a
        pattern match returns a list consisting of the
        subexpressions matched by the parentheses in the
        pattern, i.e., (`$1', $2, $3...). (Note that here $1
        etc. are also set, and that this differs from Perl 4's
        behavior.) If the match fails, a null array is returned.
        If the match succeeds, but there were no parentheses, a
        list value of (1) is returned.

        Examples:

        open(TTY, '/dev/tty');
        <TTY> =~ /^y/i && foo();    # do foo if desired

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

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

        # poor man's grep
        $arg = shift;
        while (<>) {
            print if /$arg/o;        # compile only once
        }

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

        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.

        The `/g' modifier specifies global pattern matching--
        that is, matching as many times as possible within the
        string. How it behaves depends on the context. In a list
        context, it returns a list of all the substrings matched
        by all the parentheses in the regular expression. If
        there are no parentheses, it returns a list of all the
        matched strings, as if there were parentheses around the
        whole pattern.

        In a scalar context, `m//g' iterates through the string,
        returning TRUE each time it matches, and FALSE when it
        eventually runs out of matches. (In other words, it
        remembers where it left off last time and restarts the
        search at that point. You can actually find the current
        match position of a string or set it using the pos()
        function; see the "pos" entry in the perlfunc manpage.)
        A failed match normally resets the search position to
        the beginning of the string, but you can avoid that by
        adding the `/c' modifier (e.g. `m//gc'). Modifying the
        target string also resets the search position.

        You can intermix `m//g' matches with `m/\G.../g', where
        `\G' is a zero-width assertion that matches the exact
        position where the previous `m//g', if any, left off.
        The `\G' assertion is not supported without the `/g'
        modifier; currently, without `/g', `\G' behaves just
        like `\A', but that's accidental and may change in the
        future.

        Examples:

        # list context
        ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);

        # scalar context
        $/ = ""; $* = 1;  # $* deprecated in modern perls
        while (defined($paragraph = <>)) {
            while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
            $sentences++;
            }
        }
        print "$sentences\n";

        # using m//gc with \G
        $_ = "ppooqppqq";
        while ($i++ < 2) {
            print "1: '";
            print $1 while /(o)/gc; print "', pos=", pos, "\n";
            print "2: '";
            print $1 if /\G(q)/gc;  print "', pos=", pos, "\n";
            print "3: '";
            print $1 while /(p)/gc; print "', pos=", pos, "\n";
        }

        The last example should print:

        1: 'oo', pos=4
        2: 'q', pos=5
        3: 'pp', pos=7
        1: '', pos=7
        2: 'q', pos=8
        3: '', pos=8

        A useful idiom for `lex'-like scanners is `/\G.../gc'.
        You can combine several regexps like this to process a
        string part-by-part, doing different actions depending
        on which regexp matched. Each regexp tries to match
        where the previous one leaves off.

         $_ = <<'EOL';
          $url = new URI::URL "http://www/";   die if $url eq "xXx";
         EOL
         LOOP:
        {
          print(" digits"),        redo LOOP if /\G\d+\b[,.;]?\s*/gc;
          print(" lowercase"),        redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
          print(" UPPERCASE"),        redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
          print(" Capitalized"),    redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
          print(" MiXeD"),        redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
          print(" alphanumeric"),   redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
          print(" line-noise"),     redo LOOP if /\G[^A-Za-z0-9]+/gc;
          print ". That's all!\n";
        }

        Here is the output (split into several lines):

         line-noise lowercase line-noise lowercase UPPERCASE line-noise
         UPPERCASE line-noise lowercase line-noise lowercase line-noise
         lowercase lowercase line-noise lowercase lowercase line-noise
         MiXeD line-noise. That's all!

    q/STRING/
    `'STRING''
        A single-quoted, literal string. A backslash represents
        a backslash unless followed by the delimiter or another
        backslash, in which case the delimiter or backslash is
        interpolated.

        $foo = q!I said, "You said, 'She said it.'"!;
        $bar = q('This is it.');
        $baz = '\n';            # a two-character string

    qq/STRING/
    "STRING"
        A double-quoted, interpolated string.

        $_ .= qq
         (*** The previous line contains the naughty word "$1".\n)
                if /(tcl|rexx|python)/;     # :-)
        $baz = "\n";            # a one-character string

    qx/STRING/
    `STRING`
        A string which is interpolated and then executed as a
        system command. The collected standard output of the
        command is returned. In scalar context, it comes back as
        a single (potentially multi-line) string. In list
        context, returns a list of lines (however you've defined
        lines with $/ or $INPUT_RECORD_SEPARATOR).

        $today = qx{ date };

        Note that how the string gets evaluated is entirely
        subject to the command interpreter on your system. On
        most platforms, you will have to protect shell
        metacharacters if you want them treated literally. On
        some platforms (notably DOS-like ones), the shell may
        not be capable of dealing with multiline commands, so
        putting newlines in the string may not get you what you
        want. You may be able to evaluate multiple commands in a
        single line by separating them with the command
        separator character, if your shell supports that (e.g.
        `;' on many Unix shells; `&' on the Windows NT `cmd'
        shell).

        Beware that some command shells may place restrictions
        on the length of the command line. You must ensure your
        strings don't exceed this limit after any necessary
        interpolations. See the platform-specific release notes
        for more details about your particular environment.

        Also realize that using this operator frequently leads
        to unportable programs.

        See the section on "I/O Operators" for more discussion.

    qw/STRING/
        Returns a list of the words extracted out of STRING,
        using embedded whitespace as the word delimiters. It is
        exactly equivalent to

        split(' ', q/STRING/);

        Some frequently seen examples:

        use POSIX qw( setlocale localeconv )
        @EXPORT = qw( foo bar baz );

        A common mistake is to try to separate the words with
        comma or to put comments into a multi-line qw-string.
        For this reason the `-w' switch produce warnings if the
        STRING contains the "," or the "#" character.

    s/PATTERN/REPLACEMENT/egimosx
        Searches a string for a pattern, and if found, replaces
        that pattern with the replacement text and returns the
        number of substitutions made. Otherwise it returns false
        (specifically, the empty string).

        If no string is specified via the `=~' or `!~' operator,
        the `$_' variable is searched and modified. (The string
        specified with `=~' must be a scalar variable, an array
        element, a hash element, or an assignment to one of
        those, i.e., an lvalue.)

        If the delimiter chosen is single quote, no variable
        interpolation is done on either the PATTERN or the
        REPLACEMENT. Otherwise, if the PATTERN contains a $ that
        looks like a variable rather than an end-of-string test,
        the variable will be interpolated into the pattern at
        run-time. If you want the pattern compiled only once the
        first time the variable is interpolated, use the `/o'
        option. If the pattern evaluates to a null string, the
        last successfully executed regular expression is used
        instead. See the perlre manpage for further explanation
        on these. See the perllocale manpage for discussion of
        additional considerations which apply when `use locale'
        is in effect.

        Options are:

        e   Evaluate the right side as an expression.
        g   Replace globally, i.e., all occurrences.
        i   Do case-insensitive pattern matching.
        m   Treat string as multiple lines.
        o   Compile pattern only once.
        s   Treat string as single line.
        x   Use extended regular expressions.

        Any non-alphanumeric, non-whitespace delimiter may
        replace the slashes. If single quotes are used, no
        interpretation is done on the replacement string (the
        `/e' modifier overrides this, however). Unlike Perl 4,
        Perl 5 treats backticks as normal delimiters; the
        replacement text is not evaluated as a command. If the
        PATTERN is delimited by bracketing quotes, the
        REPLACEMENT has its own pair of quotes, which may or may
        not be bracketing quotes, e.g., `s(foo)(bar)' or
        `s<foo>/bar/'. A `/e' will cause the replacement portion
        to be interpreter as a full-fledged Perl expression and
        eval()ed right then and there. It is, however, syntax
        checked at compile-time.

        Examples:

        s/\bgreen\b/mauve/g;            # don't change wintergreen

        $path =~ s|/usr/bin|/usr/local/bin|;

        s/Login: $foo/Login: $bar/; # run-time pattern

        ($foo = $bar) =~ s/this/that/;

        $count = ($paragraph =~ s/Mister\b/Mr./g);

        $_ = 'abc123xyz';
        s/\d+/$&*2/e;            # yields 'abc246xyz'
        s/\d+/sprintf("%5d",$&)/e;  # yields 'abc  246xyz'
        s/\w/$& x 2/eg;         # yields 'aabbcc  224466xxyyzz'

        s/%(.)/$percent{$1}/g;        # change percent escapes; no /e
        s/%(.)/$percent{$1} || $&/ge;        # expr now, so /e
        s/^=(\w+)/&pod($1)/ge;        # use function call

        # /e's can even nest;  this will expand
        # simple embedded variables in $_
        s/(\$\w+)/$1/eeg;

        # Delete C comments.
        $program =~ s {
            /\*     # Match the opening delimiter.
            .*?     # Match a minimal number of characters.
            \*/     # Match the closing delimiter.
        } []gsx;

        s/^\s*(.*?)\s*$/$1/;        # trim white space

        s/([^ ]*) *([^ ]*)/$2 $1/;  # reverse 1st two fields

        Note the use of $ instead of \ in the last example.
        Unlike sed, we use the \<*digit*> form in only the left
        hand side. Anywhere else it's $<*digit*>.

        Occasionally, you can't use just a `/g' to get all the
        changes to occur. Here are two common cases:

        # put commas in the right places in an integer
        1 while s/(.*\d)(\d\d\d)/$1,$2/g;      # perl4
        1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;  # perl5

        # expand tabs to 8-column spacing
        1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;

    tr/SEARCHLIST/REPLACEMENTLIST/cds
    y/SEARCHLIST/REPLACEMENTLIST/cds
        Translates all occurrences of the characters found in
        the search list with the corresponding character in the
        replacement list. It returns the number of characters
        replaced or deleted. If no string is specified via the
        =~ or !~ operator, the $_ string is translated. (The
        string specified with =~ must be a scalar variable, an
        array element, a hash element, or an assignment to one
        of those, i.e., an lvalue.) For sed devotees, `y' is
        provided as a synonym for `tr'. If the SEARCHLIST is
        delimited by bracketing quotes, the REPLACEMENTLIST has
        its own pair of quotes, which may or may not be
        bracketing quotes, e.g., `tr[A-Z][a-z]' or `tr(+-
        */)/ABCD/'.

        Options:

        c   Complement the SEARCHLIST.
        d   Delete found but unreplaced characters.
        s   Squash duplicate replaced characters.

        If the `/c' modifier is specified, the SEARCHLIST
        character set is complemented. If the `/d' modifier is
        specified, any characters specified by SEARCHLIST not
        found in REPLACEMENTLIST are deleted. (Note that this is
        slightly more flexible than the behavior of some tr
        programs, which delete anything they find in the
        SEARCHLIST, period.) If the `/s' modifier is specified,
        sequences of characters that were translated to the same
        character are squashed down to a single instance of the
        character.

        If the `/d' modifier is used, the REPLACEMENTLIST is
        always interpreted exactly as specified. Otherwise, if
        the REPLACEMENTLIST is shorter than the SEARCHLIST, the
        final character is replicated till it is long enough. If
        the REPLACEMENTLIST is null, the SEARCHLIST is
        replicated. This latter is useful for counting
        characters in a class or for squashing character
        sequences in a class.

        Examples:

        $ARGV[1] =~ tr/A-Z/a-z/;    # canonicalize to lower case

        $cnt = tr/*/*/;         # count the stars in $_

        $cnt = $sky =~ tr/*/*/;     # count the stars in $sky

        $cnt = tr/0-9//;        # count the digits in $_

        tr/a-zA-Z//s;            # bookkeeper -> bokeper

        ($HOST = $host) =~ tr/a-z/A-Z/;

        tr/a-zA-Z/ /cs;         # change non-alphas to single space

        tr [\200-\377]
           [\000-\177];         # delete 8th bit

        If multiple translations are given for a character, only
        the first one is used:

        tr/AAA/XYZ/

        will translate any A to X.

        Note that because the translation table is built at
        compile time, neither the SEARCHLIST nor the
        REPLACEMENTLIST are subjected to double quote
        interpolation. That means that if you want to use
        variables, you must use an eval():

        eval "tr/$oldlist/$newlist/";
        die $@ if $@;

        eval "tr/$oldlist/$newlist/, 1" or die $@;

  I/O Operators

    There are several I/O operators you should know about. A string
    is enclosed by backticks (grave accents) 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. In a scalar
    context, a single string consisting of all the output is
    returned. In a list context, a list of values is returned, one
    for each line of output. (You can set `$/' to use a different
    line terminator.) The command is executed each time the pseudo-
    literal is evaluated. The status value of the command is
    returned in `$?' (see the perlvar manpage for the interpretation
    of `$?'). Unlike in csh, 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. The generalized form of backticks is
    `qx//'. (Because backticks always undergo shell expansion as
    well, see the perlsec manpage for security concerns.)

    Evaluating a filehandle in angle brackets yields the next line
    from that file (newline, if any, included), or `undef' at end of
    file. Ordinarily you must assign that value to a variable, but
    there is one situation where an automatic assignment happens.
    *If and ONLY if* the input symbol is the only thing inside the
    conditional of a `while' or `for(;;)' loop, the value is
    automatically assigned to the variable `$_'. The assigned value
    is then tested to see if it is defined. (This may seem like an
    odd thing to you, but you'll use the construct in almost every
    Perl script you write.) Anyway, the following lines are
    equivalent to each other:

    while (defined($_ = <STDIN>)) { print; }
    while (<STDIN>) { print; }
    for (;<STDIN>;) { print; }
    print while defined($_ = <STDIN>);
    print while <STDIN>;

    The filehandles STDIN, STDOUT, and STDERR are predefined. (The
    filehandles `stdin', `stdout', and `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 open() function. See the "open()" entry in the
    perlfunc manpage for details on this.

    If a <FILEHANDLE> is used in a context that is looking for a
    list, a list consisting of all the input lines is returned, one
    line per list element. It's easy to make a *LARGE* data space
    this way, so use with care.

    The null filehandle <> is special and can be used to emulate the
    behavior of sed and awk. 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

    while (<>) {
        ...             # code for each line
    }

    is equivalent to the following Perl-like pseudo code:

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

    except that it isn't so cumbersome to say, and will actually
    work. It really does shift array @ARGV and put the current
    filename into variable $ARGV. It also uses filehandle *ARGV*
    internally--<> is just a synonym for <ARGV>, which is magical.
    (The pseudo code above doesn't work because it treats <ARGV> as
    non-magical.)

    You can modify @ARGV before the first <> as long as the array
    ends up containing the list of filenames you really want. Line
    numbers (`$.') continue as if the input were one big happy file.
    (But see example under eof() for how to reset line numbers on
    each file.)

    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
    use one of the Getopts modules or put a loop on the front like
    this:

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

    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.

    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, or a reference to the
    same. For example:

    $fh = \*STDIN;
    $line = <$fh>;

    If the string inside angle brackets is not a filehandle or a
    scalar variable containing a filehandle name or reference, then
    it is interpreted as a filename pattern to be globbed, and
    either a list 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 `<$foo>' because that's an
    indirect filehandle as explained in the previous paragraph. (In
    older versions of Perl, programmers would insert curly brackets
    to force interpretation as a filename glob: `<${foo}>'. These
    days, it's considered cleaner to call the internal function
    directly as `glob($foo)', which is probably the right way to
    have done it in the first place.) Example:

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

    is equivalent to

    open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
    while (<FOO>) {
        chop;
        chmod 0644, $_;
    }

    In fact, it's currently implemented that way. (Which means it
    will not work on filenames with spaces in them unless you have
    csh(1) on your machine.) Of course, the shortest way to do the
    above is:

    chmod 0644, <*.c>;

    Because globbing invokes a shell, it's often faster to call
    readdir() yourself and do your own grep() on the filenames.
    Furthermore, due to its current implementation of using a shell,
    the glob() routine may get "Arg list too long" errors (unless
    you've installed tcsh(1L) as /bin/csh).

    A glob evaluates its (embedded) argument only when it is
    starting a new list. All values must be read before it will
    start over. In a list context this isn't important, because you
    automatically get them all anyway. In a scalar context, however,
    the operator returns the next value each time it is called, or a
    FALSE value if you've just run out. Again, FALSE is returned
    only once. So if you're expecting a single value from a glob, it
    is much better to say

    ($file) = <blurch*>;

    than

    $file = <blurch*>;

    because the latter will alternate between returning a filename
    and returning FALSE.

    It you're trying to do variable interpolation, it's definitely
    better to use the glob() function, because the older notation
    can cause people to become confused with the indirect filehandle
    notation.

    @files = glob("$dir/*.[ch]");
    @files = glob($files[$i]);

  Constant Folding

    Like C, 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

    'Now is the time for all' . "\n" .
        'good men to come to.'

    and this all reduces to one string internally. Likewise, if you
    say

    foreach $file (@filenames) {
        if (-s $file > 5 + 100 * 2**16) { ... }
    }

    the compiler will precompute the number that expression
    represents so that the interpreter won't have to.

  Integer Arithmetic

    By default Perl assumes that it must do most of its arithmetic
    in floating point. But by saying

    use integer;

    you may tell the compiler that it's okay to use integer
    operations from here to the end of the enclosing BLOCK. An inner
    BLOCK may countermand this by saying

    no integer;

    which lasts until the end of that BLOCK.

    The bitwise operators ("&", "|", "^", "~", "<<", and ">>")
    always produce integral results. However, `use integer' still
    has meaning for them. By default, their results are interpreted
    as unsigned integers. However, if `use integer' is in effect,
    their results are interpreted as signed integers. For example,
    `~0' usually evaluates to a large integral value. However, `use
    integer; ~0' is -1.

  Floating-point Arithmetic

    While `use integer' provides integer-only arithmetic, there is
    no similar ways to provide rounding or truncation at a certain
    number of decimal places. For rounding to a certain number of
    digits, sprintf() or printf() is usually the easiest route.

    The POSIX module (part of the standard perl distribution)
    implements ceil(), floor(), and a number of other mathematical
    and trigonometric functions. The Math::Complex module (part of
    the standard perl distribution) defines a number of mathematical
    functions that can also work on real numbers. Math::Complex not
    as efficient as POSIX, but POSIX can't work with complex
    numbers.

    Rounding in financial applications can have serious
    implications, and the rounding method used should be specified
    precisely. In these cases, it probably pays not to trust
    whichever system rounding is being used by Perl, but to instead
    implement the rounding function you need yourself.