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

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:

       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	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.

perl v5.28.3			  2020-05-14		     Text::Balanced(3)

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