[perl5.git] / cpan / Text-Balanced / README

NAME
    Text::Balanced - Extract delimited text sequences from strings.

SYNOPSIS
     use Text::Balanced qw (
                            extract_delimited
                            extract_bracketed
                            extract_quotelike
                            extract_codeblock
                            extract_variable
                            extract_tagged
                            extract_multiple
                            gen_delimited_pat
                            gen_extract_tagged
                           );

     # Extract the initial substring of $text that is delimited by
     # two (unescaped) instances of the first character in $delim.

            ($extracted, $remainder) = extract_delimited($text,$delim);


     # Extract the initial substring of $text that is bracketed
     # with a delimiter(s) specified by $delim (where the string
     # in $delim contains one or more of '(){}[]<>').

            ($extracted, $remainder) = extract_bracketed($text,$delim);


     # Extract the initial substring of $text that is bounded by
     # an XML tag.

            ($extracted, $remainder) = extract_tagged($text);


     # Extract the initial substring of $text that is bounded by
     # a C<BEGIN>...C<END> pair. Don't allow nested C<BEGIN> tags

            ($extracted, $remainder) =
                    extract_tagged($text,"BEGIN","END",undef,{bad=>["BEGIN"]});


     # Extract the initial substring of $text that represents a
     # Perl "quote or quote-like operation"

            ($extracted, $remainder) = extract_quotelike($text);


     # Extract the initial substring of $text that represents a block
     # of Perl code, bracketed by any of character(s) specified by $delim
     # (where the string $delim contains one or more of '(){}[]<>').

            ($extracted, $remainder) = extract_codeblock($text,$delim);


     # Extract the initial substrings of $text that would be extracted by
     # one or more sequential applications of the specified functions
     # or regular expressions

            @extracted = extract_multiple($text,
                                          [ \&extract_bracketed,
                                            \&extract_quotelike,
                                            \&some_other_extractor_sub,
                                            qr/[xyz]*/,
                                            'literal',
                                          ]);

    # Create a string representing an optimized pattern (a la Friedl) # that
    matches a substring delimited by any of the specified characters # (in
    this case: any type of quote or a slash)

            $patstring = gen_delimited_pat(q{'"`/});

    # Generate a reference to an anonymous sub that is just like
    extract_tagged # but pre-compiled and optimized for a specific pair of
    tags, and consequently # much faster (i.e. 3 times faster). It uses qr//
    for better performance on # repeated calls, so it only works under Perl
    5.005 or later.

            $extract_head = gen_extract_tagged('<HEAD>','</HEAD>');

            ($extracted, $remainder) = $extract_head->($text);

DESCRIPTION
    The various "extract_..." subroutines may be used to extract a delimited
    substring, possibly after skipping a specified prefix string. By
    default, that prefix is optional whitespace ("/\s*/"), but you can
    change it to whatever you wish (see below).

    The substring to be extracted must appear at the current "pos" location
    of the string's variable (or at index zero, if no "pos" position is
    defined). In other words, the "extract_..." subroutines *don't* extract
    the first occurrence of a substring anywhere in a string (like an
    unanchored regex would). Rather, they extract an occurrence of the
    substring appearing immediately at the current matching position in the
    string (like a "\G"-anchored regex would).

  General behaviour in list contexts
    In a list context, all the subroutines return a list, the first three
    elements of which are always:

    [0] The extracted string, including the specified delimiters. If the
        extraction fails "undef" is returned.

    [1] The remainder of the input string (i.e. the characters after the
        extracted string). On failure, the entire string is returned.

    [2] The skipped prefix (i.e. the characters before the extracted
        string). On failure, "undef" is returned.

    Note that in a list context, the contents of the original input text
    (the first argument) are not modified in any way.

    However, if the input text was passed in a variable, that variable's
    "pos" value is updated to point at the first character after the
    extracted text. That means that in a list context the various
    subroutines can be used much like regular expressions. For example:

            while ( $next = (extract_quotelike($text))[0] )
            {
                    # process next quote-like (in $next)
            }

  General behaviour in scalar and void contexts
    In a scalar context, the extracted string is returned, having first been
    removed from the input text. Thus, the following code also processes
    each quote-like operation, but actually removes them from $text:

            while ( $next = extract_quotelike($text) )
            {
                    # process next quote-like (in $next)
            }

    Note that if the input text is a read-only string (i.e. a literal), no
    attempt is made to remove the extracted text.

    In a void context the behaviour of the extraction subroutines is exactly
    the same as in a scalar context, except (of course) that the extracted
    substring is not returned.

  A note about prefixes
    Prefix patterns are matched without any trailing modifiers ("/gimsox"
    etc.) This can bite you if you're expecting a prefix specification like
    '.*?(?=<H1>)' to skip everything up to the first <H1> tag. Such a prefix
    pattern will only succeed if the <H1> tag is on the current line, since
    . normally doesn't match newlines.

    To overcome this limitation, you need to turn on /s matching within the
    prefix pattern, using the "(?s)" directive: '(?s).*?(?=<H1>)'

  "extract_delimited"
    The "extract_delimited" function formalizes the common idiom of
    extracting a single-character-delimited substring from the start of a
    string. For example, to extract a single-quote delimited string, the
    following code is typically used:

            ($remainder = $text) =~ s/\A('(\\.|[^'])*')//s;
            $extracted = $1;

    but with "extract_delimited" it can be simplified to:

            ($extracted,$remainder) = extract_delimited($text, "'");

    "extract_delimited" takes up to four scalars (the input text, the
    delimiters, a prefix pattern to be skipped, and any escape characters)
    and extracts the initial substring of the text that is appropriately
    delimited. If the delimiter string has multiple characters, the first
    one encountered in the text is taken to delimit the substring. The third
    argument specifies a prefix pattern that is to be skipped (but must be
    present!) before the substring is extracted. The final argument
    specifies the escape character to be used for each delimiter.

    All arguments are optional. If the escape characters are not specified,
    every delimiter is escaped with a backslash ("\"). If the prefix is not
    specified, the pattern '\s*' - optional whitespace - is used. If the
    delimiter set is also not specified, the set "/["'`]/" is used. If the
    text to be processed is not specified either, $_ is used.

    In list context, "extract_delimited" returns a array of three elements,
    the extracted substring (*including the surrounding delimiters*), the
    remainder of the text, and the skipped prefix (if any). If a suitable
    delimited substring is not found, the first element of the array is the
    empty string, the second is the complete original text, and the prefix
    returned in the third element is an empty string.

    In a scalar context, just the extracted substring is returned. In a void
    context, the extracted substring (and any prefix) are simply removed
    from the beginning of the first argument.

    Examples:

            # Remove a single-quoted substring from the very beginning of $text:

                    $substring = extract_delimited($text, "'", '');

            # Remove a single-quoted Pascalish substring (i.e. one in which
            # doubling the quote character escapes it) from the very
            # beginning of $text:

                    $substring = extract_delimited($text, "'", '', "'");

            # Extract a single- or double- quoted substring from the
            # beginning of $text, optionally after some whitespace
            # (note the list context to protect $text from modification):

                    ($substring) = extract_delimited $text, q{"'};

            # Delete the substring delimited by the first '/' in $text:

                    $text = join '', (extract_delimited($text,'/','[^/]*')[2,1];

    Note that this last example is *not* the same as deleting the first
    quote-like pattern. For instance, if $text contained the string:

            "if ('./cmd' =~ m/$UNIXCMD/s) { $cmd = $1; }"

    then after the deletion it would contain:

            "if ('.$UNIXCMD/s) { $cmd = $1; }"

    not:

            "if ('./cmd' =~ ms) { $cmd = $1; }"

    See "extract_quotelike" for a (partial) solution to this problem.

  "extract_bracketed"
    Like "extract_delimited", the "extract_bracketed" function takes up to
    three optional scalar arguments: a string to extract from, a delimiter
    specifier, and a prefix pattern. As before, a missing prefix defaults to
    optional whitespace and a missing text defaults to $_. However, a
    missing delimiter specifier defaults to '{}()[]<>' (see below).

    "extract_bracketed" extracts a balanced-bracket-delimited substring
    (using any one (or more) of the user-specified delimiter brackets:
    '(..)', '{..}', '[..]', or '<..>'). Optionally it will also respect
    quoted unbalanced brackets (see below).

    A "delimiter bracket" is a bracket in list of delimiters passed as
    "extract_bracketed"'s second argument. Delimiter brackets are specified
    by giving either the left or right (or both!) versions of the required
    bracket(s). Note that the order in which two or more delimiter brackets
    are specified is not significant.

    A "balanced-bracket-delimited substring" is a substring bounded by
    matched brackets, such that any other (left or right) delimiter bracket
    *within* the substring is also matched by an opposite (right or left)
    delimiter bracket *at the same level of nesting*. Any type of bracket
    not in the delimiter list is treated as an ordinary character.

    In other words, each type of bracket specified as a delimiter must be
    balanced and correctly nested within the substring, and any other kind
    of ("non-delimiter") bracket in the substring is ignored.

    For example, given the string:

            $text = "{ an '[irregularly :-(] {} parenthesized >:-)' string }";

    then a call to "extract_bracketed" in a list context:

            @result = extract_bracketed( $text, '{}' );

    would return:

            ( "{ an '[irregularly :-(] {} parenthesized >:-)' string }" , "" , "" )

    since both sets of '{..}' brackets are properly nested and evenly
    balanced. (In a scalar context just the first element of the array would
    be returned. In a void context, $text would be replaced by an empty
    string.)

    Likewise the call in:

            @result = extract_bracketed( $text, '{[' );

    would return the same result, since all sets of both types of specified
    delimiter brackets are correctly nested and balanced.

    However, the call in:

            @result = extract_bracketed( $text, '{([<' );

    would fail, returning:

            ( undef , "{ an '[irregularly :-(] {} parenthesized >:-)' string }"  );

    because the embedded pairs of '(..)'s and '[..]'s are "cross-nested" and
    the embedded '>' is unbalanced. (In a scalar context, this call would
    return an empty string. In a void context, $text would be unchanged.)

    Note that the embedded single-quotes in the string don't help in this
    case, since they have not been specified as acceptable delimiters and
    are therefore treated as non-delimiter characters (and ignored).

    However, if a particular species of quote character is included in the
    delimiter specification, then that type of quote will be correctly
    handled. for example, if $text is:

            $text = '<A HREF=">>>>">link</A>';

    then

            @result = extract_bracketed( $text, '<">' );

    returns:

            ( '<A HREF=">>>>">', 'link</A>', "" )

    as expected. Without the specification of """ as an embedded quoter:

            @result = extract_bracketed( $text, '<>' );

    the result would be:

            ( '<A HREF=">', '>>>">link</A>', "" )

    In addition to the quote delimiters "'", """, and "`", full Perl
    quote-like quoting (i.e. q{string}, qq{string}, etc) can be specified by
    including the letter 'q' as a delimiter. Hence:

            @result = extract_bracketed( $text, '<q>' );

    would correctly match something like this:

            $text = '<leftop: conj /and/ conj>';

    See also: "extract_quotelike" and "extract_codeblock".

  "extract_variable"
    "extract_variable" extracts any valid Perl variable or variable-involved
    expression, including scalars, arrays, hashes, array accesses, hash
    look-ups, method calls through objects, subroutine calls through
    subroutine references, etc.

    The subroutine takes up to two optional arguments:

    1.  A string to be processed ($_ if the string is omitted or "undef")

    2.  A string specifying a pattern to be matched as a prefix (which is to
        be skipped). If omitted, optional whitespace is skipped.

    On success in a list context, an array of 3 elements is returned. The
    elements are:

    [0] the extracted variable, or variablish expression

    [1] the remainder of the input text,

    [2] the prefix substring (if any),

    On failure, all of these values (except the remaining text) are "undef".

    In a scalar context, "extract_variable" returns just the complete
    substring that matched a variablish expression. "undef" is returned on
    failure. In addition, the original input text has the returned substring
    (and any prefix) removed from it.

    In a void context, the input text just has the matched substring (and
    any specified prefix) removed.

  "extract_tagged"
    "extract_tagged" extracts and segments text between (balanced) specified
    tags.

    The subroutine takes up to five optional arguments:

    1.  A string to be processed ($_ if the string is omitted or "undef")

    2.  A string specifying a pattern to be matched as the opening tag. If
        the pattern string is omitted (or "undef") then a pattern that
        matches any standard XML tag is used.

    3.  A string specifying a pattern to be matched at the closing tag. If
        the pattern string is omitted (or "undef") then the closing tag is
        constructed by inserting a "/" after any leading bracket characters
        in the actual opening tag that was matched (*not* the pattern that
        matched the tag). For example, if the opening tag pattern is
        specified as '{{\w+}}' and actually matched the opening tag
        "{{DATA}}", then the constructed closing tag would be "{{/DATA}}".

    4.  A string specifying a pattern to be matched as a prefix (which is to
        be skipped). If omitted, optional whitespace is skipped.

    5.  A hash reference containing various parsing options (see below)

    The various options that can be specified are:

    "reject => $listref"
        The list reference contains one or more strings specifying patterns
        that must *not* appear within the tagged text.

        For example, to extract an HTML link (which should not contain
        nested links) use:

                extract_tagged($text, '<A>', '</A>', undef, {reject => ['<A>']} );

    "ignore => $listref"
        The list reference contains one or more strings specifying patterns
        that are *not* be be treated as nested tags within the tagged text
        (even if they would match the start tag pattern).

        For example, to extract an arbitrary XML tag, but ignore "empty"
        elements:

                extract_tagged($text, undef, undef, undef, {ignore => ['<[^>]*/>']} );

        (also see "gen_delimited_pat" below).

    "fail => $str"
        The "fail" option indicates the action to be taken if a matching end
        tag is not encountered (i.e. before the end of the string or some
        "reject" pattern matches). By default, a failure to match a closing
        tag causes "extract_tagged" to immediately fail.

        However, if the string value associated with <reject> is "MAX", then
        "extract_tagged" returns the complete text up to the point of
        failure. If the string is "PARA", "extract_tagged" returns only the
        first paragraph after the tag (up to the first line that is either
        empty or contains only whitespace characters). If the string is "",
        the the default behaviour (i.e. failure) is reinstated.

        For example, suppose the start tag "/para" introduces a paragraph,
        which then continues until the next "/endpara" tag or until another
        "/para" tag is encountered:

                $text = "/para line 1\n\nline 3\n/para line 4";

                extract_tagged($text, '/para', '/endpara', undef,
                                        {reject => '/para', fail => MAX );

                # EXTRACTED: "/para line 1\n\nline 3\n"

        Suppose instead, that if no matching "/endpara" tag is found, the
        "/para" tag refers only to the immediately following paragraph:

                $text = "/para line 1\n\nline 3\n/para line 4";

                extract_tagged($text, '/para', '/endpara', undef,
                                {reject => '/para', fail => MAX );

                # EXTRACTED: "/para line 1\n"

        Note that the specified "fail" behaviour applies to nested tags as
        well.

    On success in a list context, an array of 6 elements is returned. The
    elements are:

    [0] the extracted tagged substring (including the outermost tags),

    [1] the remainder of the input text,

    [2] the prefix substring (if any),

    [3] the opening tag

    [4] the text between the opening and closing tags

    [5] the closing tag (or "" if no closing tag was found)

    On failure, all of these values (except the remaining text) are "undef".

    In a scalar context, "extract_tagged" returns just the complete
    substring that matched a tagged text (including the start and end tags).
    "undef" is returned on failure. In addition, the original input text has
    the returned substring (and any prefix) removed from it.

    In a void context, the input text just has the matched substring (and
    any specified prefix) removed.

  "gen_extract_tagged"
    (Note: This subroutine is only available under Perl5.005)

    "gen_extract_tagged" generates a new anonymous subroutine which extracts
    text between (balanced) specified tags. In other words, it generates a
    function identical in function to "extract_tagged".

    The difference between "extract_tagged" and the anonymous subroutines
    generated by "gen_extract_tagged", is that those generated subroutines:

    *   do not have to reparse tag specification or parsing options every
        time they are called (whereas "extract_tagged" has to effectively
        rebuild its tag parser on every call);

    *   make use of the new qr// construct to pre-compile the regexes they
        use (whereas "extract_tagged" uses standard string variable
        interpolation to create tag-matching patterns).

    The subroutine takes up to four optional arguments (the same set as
    "extract_tagged" except for the string to be processed). It returns a
    reference to a subroutine which in turn takes a single argument (the
    text to be extracted from).

    In other words, the implementation of "extract_tagged" is exactly
    equivalent to:

            sub extract_tagged
            {
                    my $text = shift;
                    $extractor = gen_extract_tagged(@_);
                    return $extractor->($text);
            }

    (although "extract_tagged" is not currently implemented that way, in
    order to preserve pre-5.005 compatibility).

    Using "gen_extract_tagged" to create extraction functions for specific
    tags is a good idea if those functions are going to be called more than
    once, since their performance is typically twice as good as the more
    general-purpose "extract_tagged".

  "extract_quotelike"
    "extract_quotelike" attempts to recognize, extract, and segment any one
    of the various Perl quotes and quotelike operators (see perlop(3))
    Nested backslashed delimiters, embedded balanced bracket delimiters (for
    the quotelike operators), and trailing modifiers are all caught. For
    example, in:

            extract_quotelike 'q # an octothorpe: \# (not the end of the q!) #'
            
        extract_quotelike '  "You said, \"Use sed\"."  '

            extract_quotelike ' s{([A-Z]{1,8}\.[A-Z]{3})} /\L$1\E/; '

            extract_quotelike ' tr/\\\/\\\\/\\\//ds; '

    the full Perl quotelike operations are all extracted correctly.

    Note too that, when using the /x modifier on a regex, any comment
    containing the current pattern delimiter will cause the regex to be
    immediately terminated. In other words:

            'm /
                    (?i)            # CASE INSENSITIVE
                    [a-z_]          # LEADING ALPHABETIC/UNDERSCORE
                    [a-z0-9]*       # FOLLOWED BY ANY NUMBER OF ALPHANUMERICS
               /x'

    will be extracted as if it were:

            'm /
                    (?i)            # CASE INSENSITIVE
                    [a-z_]          # LEADING ALPHABETIC/'

    This behaviour is identical to that of the actual compiler.

    "extract_quotelike" takes two arguments: the text to be processed and a
    prefix to be matched at the very beginning of the text. If no prefix is
    specified, optional whitespace is the default. If no text is given, $_
    is used.

    In a list context, an array of 11 elements is returned. The elements
    are:

    [0] the extracted quotelike substring (including trailing modifiers),

    [1] the remainder of the input text,

    [2] the prefix substring (if any),

    [3] the name of the quotelike operator (if any),

    [4] the left delimiter of the first block of the operation,

    [5] the text of the first block of the operation (that is, the contents
        of a quote, the regex of a match or substitution or the target list
        of a translation),

    [6] the right delimiter of the first block of the operation,

    [7] the left delimiter of the second block of the operation (that is, if
        it is a "s", "tr", or "y"),

    [8] the text of the second block of the operation (that is, the
        replacement of a substitution or the translation list of a
        translation),

    [9] the right delimiter of the second block of the operation (if any),

    [10]
        the trailing modifiers on the operation (if any).

    For each of the fields marked "(if any)" the default value on success is
    an empty string. On failure, all of these values (except the remaining
    text) are "undef".

    In a scalar context, "extract_quotelike" returns just the complete
    substring that matched a quotelike operation (or "undef" on failure). In
    a scalar or void context, the input text has the same substring (and any
    specified prefix) removed.

    Examples:

            # Remove the first quotelike literal that appears in text

                    $quotelike = extract_quotelike($text,'.*?');

            # Replace one or more leading whitespace-separated quotelike
            # literals in $_ with "<QLL>"

                    do { $_ = join '<QLL>', (extract_quotelike)[2,1] } until $@;


            # Isolate the search pattern in a quotelike operation from $text

                    ($op,$pat) = (extract_quotelike $text)[3,5];
                    if ($op =~ /[ms]/)
                    {
                            print "search pattern: $pat\n";
                    }
                    else
                    {
                            print "$op is not a pattern matching operation\n";
                    }

  "extract_quotelike" and "here documents"
    "extract_quotelike" can successfully extract "here documents" from an
    input string, but with an important caveat in list contexts.

    Unlike other types of quote-like literals, a here document is rarely a
    contiguous substring. For example, a typical piece of code using here
    document might look like this:

            <<'EOMSG' || die;
            This is the message.
            EOMSG
            exit;

    Given this as an input string in a scalar context, "extract_quotelike"
    would correctly return the string "<<'EOMSG'\nThis is the
    message.\nEOMSG", leaving the string " || die;\nexit;" in the original
    variable. In other words, the two separate pieces of the here document
    are successfully extracted and concatenated.

    In a list context, "extract_quotelike" would return the list

    [0] "<<'EOMSG'\nThis is the message.\nEOMSG\n" (i.e. the full extracted
        here document, including fore and aft delimiters),

    [1] " || die;\nexit;" (i.e. the remainder of the input text,
        concatenated),

    [2] "" (i.e. the prefix substring -- trivial in this case),

    [3] "<<" (i.e. the "name" of the quotelike operator)

    [4] "'EOMSG'" (i.e. the left delimiter of the here document, including
        any quotes),

    [5] "This is the message.\n" (i.e. the text of the here document),

    [6] "EOMSG" (i.e. the right delimiter of the here document),

    [7..10]
        "" (a here document has no second left delimiter, second text,
        second right delimiter, or trailing modifiers).

    However, the matching position of the input variable would be set to
    "exit;" (i.e. *after* the closing delimiter of the here document), which
    would cause the earlier " || die;\nexit;" to be skipped in any sequence
    of code fragment extractions.

    To avoid this problem, when it encounters a here document whilst
    extracting from a modifiable string, "extract_quotelike" silently
    rearranges the string to an equivalent piece of Perl:

            <<'EOMSG'
            This is the message.
            EOMSG
            || die;
            exit;

    in which the here document *is* contiguous. It still leaves the matching
    position after the here document, but now the rest of the line on which
    the here document starts is not skipped.

    To prevent <extract_quotelike> from mucking about with the input in this
    way (this is the only case where a list-context "extract_quotelike" does
    so), you can pass the input variable as an interpolated literal:

            $quotelike = extract_quotelike("$var");

  "extract_codeblock"
    "extract_codeblock" attempts to recognize and extract a balanced bracket
    delimited substring that may contain unbalanced brackets inside Perl
    quotes or quotelike operations. That is, "extract_codeblock" is like a
    combination of "extract_bracketed" and "extract_quotelike".

    "extract_codeblock" takes the same initial three parameters as
    "extract_bracketed": a text to process, a set of delimiter brackets to
    look for, and a prefix to match first. It also takes an optional fourth
    parameter, which allows the outermost delimiter brackets to be specified
    separately (see below).

    Omitting the first argument (input text) means process $_ instead.
    Omitting the second argument (delimiter brackets) indicates that only
    '{' is to be used. Omitting the third argument (prefix argument) implies
    optional whitespace at the start. Omitting the fourth argument
    (outermost delimiter brackets) indicates that the value of the second
    argument is to be used for the outermost delimiters.

    Once the prefix an dthe outermost opening delimiter bracket have been
    recognized, code blocks are extracted by stepping through the input text
    and trying the following alternatives in sequence:

    1.  Try and match a closing delimiter bracket. If the bracket was the
        same species as the last opening bracket, return the substring to
        that point. If the bracket was mismatched, return an error.

    2.  Try to match a quote or quotelike operator. If found, call
        "extract_quotelike" to eat it. If "extract_quotelike" fails, return
        the error it returned. Otherwise go back to step 1.

    3.  Try to match an opening delimiter bracket. If found, call
        "extract_codeblock" recursively to eat the embedded block. If the
        recursive call fails, return an error. Otherwise, go back to step 1.

    4.  Unconditionally match a bareword or any other single character, and
        then go back to step 1.

    Examples:

            # Find a while loop in the text

                    if ($text =~ s/.*?while\s*\{/{/)
                    {
                            $loop = "while " . extract_codeblock($text);
                    }

            # Remove the first round-bracketed list (which may include
            # round- or curly-bracketed code blocks or quotelike operators)

                    extract_codeblock $text, "(){}", '[^(]*';

    The ability to specify a different outermost delimiter bracket is useful
    in some circumstances. For example, in the Parse::RecDescent module,
    parser actions which are to be performed only on a successful parse are
    specified using a "<defer:...>" directive. For example:

            sentence: subject verb object
                            <defer: {$::theVerb = $item{verb}} >

    Parse::RecDescent uses "extract_codeblock($text, '{}<>')" to extract the
    code within the "<defer:...>" directive, but there's a problem.

    A deferred action like this:

                            <defer: {if ($count>10) {$count--}} >

    will be incorrectly parsed as:

                            <defer: {if ($count>

    because the "less than" operator is interpreted as a closing delimiter.

    But, by extracting the directive using
    "extract_codeblock($text, '{}', undef, '<>')" the '>' character is only
    treated as a delimited at the outermost level of the code block, so the
    directive is parsed correctly.

  "extract_multiple"
    The "extract_multiple" subroutine takes a string to be processed and a
    list of extractors (subroutines or regular expressions) to apply to that
    string.

    In an array context "extract_multiple" returns an array of substrings of
    the original string, as extracted by the specified extractors. In a
    scalar context, "extract_multiple" returns the first substring
    successfully extracted from the original string. In both scalar and void
    contexts the original string has the first successfully extracted
    substring removed from it. In all contexts "extract_multiple" starts at
    the current "pos" of the string, and sets that "pos" appropriately after
    it matches.

    Hence, the aim of of a call to "extract_multiple" in a list context is
    to split the processed string into as many non-overlapping fields as
    possible, by repeatedly applying each of the specified extractors to the
    remainder of the string. Thus "extract_multiple" is a generalized form
    of Perl's "split" subroutine.

    The subroutine takes up to four optional arguments:

    1.  A string to be processed ($_ if the string is omitted or "undef")

    2.  A reference to a list of subroutine references and/or qr// objects
        and/or literal strings and/or hash references, specifying the
        extractors to be used to split the string. If this argument is
        omitted (or "undef") the list:

                [
                        sub { extract_variable($_[0], '') },
                        sub { extract_quotelike($_[0],'') },
                        sub { extract_codeblock($_[0],'{}','') },
                ]

        is used.

    3.  An number specifying the maximum number of fields to return. If this
        argument is omitted (or "undef"), split continues as long as
        possible.

        If the third argument is *N*, then extraction continues until *N*
        fields have been successfully extracted, or until the string has
        been completely processed.

        Note that in scalar and void contexts the value of this argument is
        automatically reset to 1 (under "-w", a warning is issued if the
        argument has to be reset).

    4.  A value indicating whether unmatched substrings (see below) within
        the text should be skipped or returned as fields. If the value is
        true, such substrings are skipped. Otherwise, they are returned.

    The extraction process works by applying each extractor in sequence to
    the text string.

    If the extractor is a subroutine it is called in a list context and is
    expected to return a list of a single element, namely the extracted
    text. It may optionally also return two further arguments: a string
    representing the text left after extraction (like $' for a pattern
    match), and a string representing any prefix skipped before the
    extraction (like $` in a pattern match). Note that this is designed to
    facilitate the use of other Text::Balanced subroutines with
    "extract_multiple". Note too that the value returned by an extractor
    subroutine need not bear any relationship to the corresponding substring
    of the original text (see examples below).

    If the extractor is a precompiled regular expression or a string, it is
    matched against the text in a scalar context with a leading '\G' and the
    gc modifiers enabled. The extracted value is either $1 if that variable
    is defined after the match, or else the complete match (i.e. $&).

    If the extractor is a hash reference, it must contain exactly one
    element. The value of that element is one of the above extractor types
    (subroutine reference, regular expression, or string). The key of that
    element is the name of a class into which the successful return value of
    the extractor will be blessed.

    If an extractor returns a defined value, that value is immediately
    treated as the next extracted field and pushed onto the list of fields.
    If the extractor was specified in a hash reference, the field is also
    blessed into the appropriate class,

    If the extractor fails to match (in the case of a regex extractor), or
    returns an empty list or an undefined value (in the case of a subroutine
    extractor), it is assumed to have failed to extract. If none of the
    extractor subroutines succeeds, then one character is extracted from the
    start of the text and the extraction subroutines reapplied. Characters
    which are thus removed are accumulated and eventually become the next
    field (unless the fourth argument is true, in which case they are
    discarded).

    For example, the following extracts substrings that are valid Perl
    variables:

            @fields = extract_multiple($text,
                                       [ sub { extract_variable($_[0]) } ],
                                       undef, 1);

    This example separates a text into fields which are quote delimited,
    curly bracketed, and anything else. The delimited and bracketed parts
    are also blessed to identify them (the "anything else" is unblessed):

            @fields = extract_multiple($text,
                       [
                            { Delim => sub { extract_delimited($_[0],q{'"}) } },
                            { Brack => sub { extract_bracketed($_[0],'{}') } },
                       ]);

    This call extracts the next single substring that is a valid Perl
    quotelike operator (and removes it from $text):

            $quotelike = extract_multiple($text,
                                          [
                                            sub { extract_quotelike($_[0]) },
                                          ], undef, 1);

    Finally, here is yet another way to do comma-separated value parsing:

            @fields = extract_multiple($csv_text,
                                      [
                                            sub { extract_delimited($_[0],q{'"}) },
                                            qr/([^,]+)(.*)/,
                                      ],
                                      undef,1);

    The list in the second argument means: *"Try and extract a ' or "
    delimited string, otherwise extract anything up to a comma..."*. The
    undef third argument means: *"...as many times as possible..."*, and the
    true value in the fourth argument means *"...discarding anything else
    that appears (i.e. the commas)"*.

    If you wanted the commas preserved as separate fields (i.e. like split
    does if your split pattern has capturing parentheses), you would just
    make the last parameter undefined (or remove it).

  "gen_delimited_pat"
    The "gen_delimited_pat" subroutine takes a single (string) argument and
    > builds a Friedl-style optimized regex that matches a string delimited
    by any one of the characters in the single argument. For example:

            gen_delimited_pat(q{'"})

    returns the regex:

            (?:\"(?:\\\"|(?!\").)*\"|\'(?:\\\'|(?!\').)*\')

    Note that the specified delimiters are automatically quotemeta'd.

    A typical use of "gen_delimited_pat" would be to build special purpose
    tags for "extract_tagged". For example, to properly ignore "empty" XML
    elements (which might contain quoted strings):

            my $empty_tag = '<(' . gen_delimited_pat(q{'"}) . '|.)+/>';

            extract_tagged($text, undef, undef, undef, {ignore => [$empty_tag]} );

    "gen_delimited_pat" may also be called with an optional second argument,
    which specifies the "escape" character(s) to be used for each delimiter.
    For example to match a Pascal-style string (where ' is the delimiter and
    '' is a literal ' within the string):

            gen_delimited_pat(q{'},q{'});

    Different escape characters can be specified for different delimiters.
    For example, to specify that '/' is the escape for single quotes and '%'
    is the escape for double quotes:

            gen_delimited_pat(q{'"},q{/%});

    If more delimiters than escape chars are specified, the last escape char
    is used for the remaining delimiters. If no escape char is specified for
    a given specified delimiter, '\' is used.

  "delimited_pat"
    Note that "gen_delimited_pat" was previously called "delimited_pat".
    That name may still be used, but is now deprecated.

DIAGNOSTICS
    In a list context, all the functions return "(undef,$original_text)" on
    failure. In a scalar context, failure is indicated by returning "undef"
    (in this case the input text is not modified in any way).

    In addition, on failure in *any* context, the $@ variable is set.
    Accessing "$@->{error}" returns one of the error diagnostics listed
    below. Accessing "$@->{pos}" returns the offset into the original string
    at which the error was detected (although not necessarily where it
    occurred!) Printing $@ directly produces the error message, with the
    offset appended. On success, the $@ variable is guaranteed to be
    "undef".

    The available diagnostics are:

    "Did not find a suitable bracket: "%s""
        The delimiter provided to "extract_bracketed" was not one of
        '()[]<>{}'.

    "Did not find prefix: /%s/"
        A non-optional prefix was specified but wasn't found at the start of
        the text.

    "Did not find opening bracket after prefix: "%s""
        "extract_bracketed" or "extract_codeblock" was expecting a
        particular kind of bracket at the start of the text, and didn't find
        it.

    "No quotelike operator found after prefix: "%s""
        "extract_quotelike" didn't find one of the quotelike operators "q",
        "qq", "qw", "qx", "s", "tr" or "y" at the start of the substring it
        was extracting.

    "Unmatched closing bracket: "%c""
        "extract_bracketed", "extract_quotelike" or "extract_codeblock"
        encountered a closing bracket where none was expected.

    "Unmatched opening bracket(s): "%s""
        "extract_bracketed", "extract_quotelike" or "extract_codeblock" ran
        out of characters in the text before closing one or more levels of
        nested brackets.

    "Unmatched embedded quote (%s)"
        "extract_bracketed" attempted to match an embedded quoted substring,
        but failed to find a closing quote to match it.

    "Did not find closing delimiter to match '%s'"
        "extract_quotelike" was unable to find a closing delimiter to match
        the one that opened the quote-like operation.

    "Mismatched closing bracket: expected "%c" but found "%s""
        "extract_bracketed", "extract_quotelike" or "extract_codeblock"
        found a valid bracket delimiter, but it was the wrong species. This
        usually indicates a nesting error, but may indicate incorrect
        quoting or escaping.

    "No block delimiter found after quotelike "%s""
        "extract_quotelike" or "extract_codeblock" found one of the
        quotelike operators "q", "qq", "qw", "qx", "s", "tr" or "y" without
        a suitable block after it.

    "Did not find leading dereferencer"
        "extract_variable" was expecting one of '$', '@', or '%' at the
        start of a variable, but didn't find any of them.

    "Bad identifier after dereferencer"
        "extract_variable" found a '$', '@', or '%' indicating a variable,
        but that character was not followed by a legal Perl identifier.

    "Did not find expected opening bracket at %s"
        "extract_codeblock" failed to find any of the outermost opening
        brackets that were specified.

    "Improperly nested codeblock at %s"
        A nested code block was found that started with a delimiter that was
        specified as being only to be used as an outermost bracket.

    "Missing second block for quotelike "%s""
        "extract_codeblock" or "extract_quotelike" found one of the
        quotelike operators "s", "tr" or "y" followed by only one block.

    "No match found for opening bracket"
        "extract_codeblock" failed to find a closing bracket to match the
        outermost opening bracket.

    "Did not find opening tag: /%s/"
        "extract_tagged" did not find a suitable opening tag (after any
        specified prefix was removed).

    "Unable to construct closing tag to match: /%s/"
        "extract_tagged" matched the specified opening tag and tried to
        modify the matched text to produce a matching closing tag (because
        none was specified). It failed to generate the closing tag, almost
        certainly because the opening tag did not start with a bracket of
        some kind.

    "Found invalid nested tag: %s"
        "extract_tagged" found a nested tag that appeared in the "reject"
        list (and the failure mode was not "MAX" or "PARA").

    "Found unbalanced nested tag: %s"
        "extract_tagged" found a nested opening tag that was not matched by
        a corresponding nested closing tag (and the failure mode was not
        "MAX" or "PARA").

    "Did not find closing tag"
        "extract_tagged" reached the end of the text without finding a
        closing tag to match the original opening tag (and the failure mode
        was not "MAX" or "PARA").

AUTHOR
    Damian Conway (damian@conway.org)

BUGS AND IRRITATIONS
    There are undoubtedly serious bugs lurking somewhere in this code, if
    only because parts of it give the impression of understanding a great
    deal more about Perl than they really do.

    Bug reports and other feedback are most welcome.

COPYRIGHT
    Copyright 1997 - 2001 Damian Conway. All Rights Reserved.

    Some (minor) parts copyright 2009 Adam Kennedy.

    This module is free software. It may be used, redistributed and/or
    modified under the same terms as Perl itself.