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RedHat 9 (Linux i386) - man page for text::balanced (redhat section 3pm)

Text::Balanced(3pm)		 Perl Programmers Reference Guide	      Text::Balanced(3pm)

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

	use Text::Balanced qw (


	# 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 HTML/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 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,

       # Create a string representing an optimized pattern (a la Friedl) # that matches a sub-
       string 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-com-
       piled 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);

       The various "extract_..." subroutines may be used to extract a delimited string (possibly
       after skipping a specified prefix string).  The search for the string always begins at the
       current "pos" location of the string's variable (or at index zero, if no "pos" position is

       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 an
	   empty string 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, the
	   empty string 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 exam-

	       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 actu-
       ally 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 cur-
       rent 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>)'


       The "extract_delimited" function formalizes the common idiom of extracting a single-char-
       acter-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 argu-
       ment 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 an 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


	       # 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; }"


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

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


       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 cor-
       rectly 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-delim-
       iter characters (and ignored).

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

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


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


	       ( '<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.

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

       would correctly match something like this:

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

       See also: "extract_quotelike" and "extract_codeblock".


       "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 HTML/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 clos-
	   ing 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

	   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 default behaviour (i.e. failure) is reinstated.

	   For example, suppose the start tag "/para" introduces a paragraph, which then contin-
	   ues 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 addi-
       tion, 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 pre-
       fix) removed.


       (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:

       o   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);

       o   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

       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 typi-
       cally twice as good as the more general-purpose "extract_tagged".


       "extract_quotelike" attempts to recognize, extract, and segment any one of the various
       Perl quotes and quotelike operators (see perlop(3)) Nested backslashed delimiters, embed-
       ded 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

       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 an "s",
	   "tr", or "y"),

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

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

	   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.


	       # 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: $pat\n";
			       print "$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 sub-
       string. For example, a typical piece of code using here document might look like this:

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

       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),

	   "" (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 while extracting from a modifi-
       able string, "extract_quotelike" silently rearranges the string to an equivalent piece of

	       This is the message.
	       || die;

       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

       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 vari-
       able as an interpolated literal:

	       $quotelike = extract_quotelike("$var");


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

       "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 the outermost opening delimiter bracket have been recognized, code
       blocks are extracted by stepping through the input text and trying the following alterna-
       tives 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 mis-
	   matched, 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" recur-
	   sively to eat the embedded block. If the recursive call fails, return an error. Other-
	   wise, go back to step 1.

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


	       # 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 circum-
       stances. For example, in the Parse::RecDescent module, parser actions which are to be per-
       formed only on a successful parse are specified using a "<defer:...>" directive. For exam-

	       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.


       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 sub-
       string 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 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 general-
       ized 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.  A number specifying the maximum number of fields to return. If this argument is omit-
	   ted (or "undef"), split continues as long as possible.

	   If the third argument is N, then extraction continues until N fields have been suc-
	   cessfully 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. Oth-
	   erwise, 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 com-
       plete 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 expres-
       sion, or string).  The key of that element is the name of a class into which the success-
       ful 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{'"}) },

       The list in the second argument means: "Try and extract a ' or " delimited string, other-
       wise 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 any-
       thing 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).


       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:


       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 con-
       tain 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):


       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:


       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.

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

       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 off-
       set 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

       "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

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

       "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 gen-
	   erate 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 fail-
	   ure 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").

       Damian Conway (damian@conway.org)

       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

       Bug reports and other feedback are most welcome.

	Copyright (c) 1997-2001, Damian Conway. All Rights Reserved.
	This module is free software. It may be used, redistributed
	    and/or modified under the same terms as Perl itself.

perl v5.8.0				    2002-06-01			      Text::Balanced(3pm)

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