text::balanced.3

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.IX Subsection "extract_multiple"
The \f(CW\*(C`extract_multiple\*(C'\fR subroutine takes a string to be processed and a 
list of extractors (subroutines or regular expressions) to apply to that string.
.PP
In an array context \f(CW\*(C`extract_multiple\*(C'\fR returns an array of substrings
of the original string, as extracted by the specified extractors.
In a scalar context, \f(CW\*(C`extract_multiple\*(C'\fR 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
\&\f(CW\*(C`extract_multiple\*(C'\fR starts at the current \f(CW\*(C`pos\*(C'\fR of the string, and
sets that \f(CW\*(C`pos\*(C'\fR appropriately after it matches.
.PP
Hence, the aim of of a call to \f(CW\*(C`extract_multiple\*(C'\fR 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 \f(CW\*(C`extract_multiple\*(C'\fR is
a generalized form of Perl's \f(CW\*(C`split\*(C'\fR subroutine.
.PP
The subroutine takes up to four optional arguments:
.IP "1." 4
A string to be processed (\f(CW$_\fR if the string is omitted or \f(CW\*(C`undef\*(C'\fR)
.IP "2." 4
A reference to a list of subroutine references and/or qr// objects and/or
literal strings and/or hash references, specifying the extractors
to be used to split the string. If this argument is omitted (or
\&\f(CW\*(C`undef\*(C'\fR) the list:
.Sp
.Vb 5
\&        [
\&                sub { extract_variable($_[0], \*(Aq\*(Aq) },
\&                sub { extract_quotelike($_[0],\*(Aq\*(Aq) },
\&                sub { extract_codeblock($_[0],\*(Aq{}\*(Aq,\*(Aq\*(Aq) },
\&        ]
.Ve
.Sp
is used.
.IP "3." 4
An number specifying the maximum number of fields to return. If this
argument is omitted (or \f(CW\*(C`undef\*(C'\fR), split continues as long as possible.
.Sp
If the third argument is \fIN\fR, then extraction continues until \fIN\fR fields
have been successfully extracted, or until the string has been completely 
processed.
.Sp
Note that in scalar and void contexts the value of this argument is 
automatically reset to 1 (under \f(CW\*(C`\-w\*(C'\fR, a warning is issued if the argument 
has to be reset).
.IP "4." 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.
.PP
The extraction process works by applying each extractor in
sequence to the text string.
.PP
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
\&\f(CW\*(C`extract_multiple\*(C'\fR. 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).
.PP
If the extractor is a precompiled regular expression or a string,
it is matched against the text in a scalar context with a leading
\&'\eG' and the gc modifiers enabled. The extracted value is either
\&\f(CW$1\fR if that variable is defined after the match, or else the
complete match (i.e. $&).
.PP
If the extractor is a hash reference, it must contain exactly one element.
The value of that element is one of the
above extractor types (subroutine reference, regular expression, or string).
The key of that element is the name of a class into which the successful
return value of the extractor will be blessed.
.PP
If an extractor returns a defined value, that value is immediately
treated as the next extracted field and pushed onto the list of fields.
If the extractor was specified in a hash reference, the field is also
blessed into the appropriate class,
.PP
If the extractor fails to match (in the case of a regex extractor), or returns an empty list or an undefined value (in the case of a subroutine extractor), it is
assumed to have failed to extract.
If none of the extractor subroutines succeeds, then one
character is extracted from the start of the text and the extraction
subroutines reapplied. Characters which are thus removed are accumulated and
eventually become the next field (unless the fourth argument is true, in which
case they are discarded).
.PP
For example, the following extracts substrings that are valid Perl variables:
.PP
.Vb 3
\&        @fields = extract_multiple($text,
\&                                   [ sub { extract_variable($_[0]) } ],
\&                                   undef, 1);
.Ve
.PP
This example separates a text into fields which are quote delimited,
curly bracketed, and anything else. The delimited and bracketed
parts are also blessed to identify them (the \*(L"anything else\*(R" is unblessed):
.PP
.Vb 5
\&        @fields = extract_multiple($text,
\&                   [
\&                        { Delim => sub { extract_delimited($_[0],q{\*(Aq"}) } },
\&                        { Brack => sub { extract_bracketed($_[0],\*(Aq{}\*(Aq) } },
\&                   ]);
.Ve
.PP
This call extracts the next single substring that is a valid Perl quotelike
operator (and removes it from \f(CW$text\fR):
.PP
.Vb 4
\&        $quotelike = extract_multiple($text,
\&                                      [
\&                                        sub { extract_quotelike($_[0]) },
\&                                      ], undef, 1);
.Ve
.PP
Finally, here is yet another way to do comma-separated value parsing:
.PP
.Vb 6
\&        @fields = extract_multiple($csv_text,
\&                                  [
\&                                        sub { extract_delimited($_[0],q{\*(Aq"}) },
\&                                        qr/([^,]+)(.*)/,
\&                                  ],
\&                                  undef,1);
.Ve
.PP
The list in the second argument means:
\&\fI\*(L"Try and extract a ' or \*(R" delimited string, otherwise extract anything up to a comma..."\fR.
The undef third argument means:
\&\fI\*(L"...as many times as possible...\*(R"\fR,
and the true value in the fourth argument means
\&\fI\*(L"...discarding anything else that appears (i.e. the commas)\*(R"\fR.
.PP
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).
.ie n .Sh """gen_delimited_pat"""
.el .Sh "\f(CWgen_delimited_pat\fP"
.IX Subsection "gen_delimited_pat"
The \f(CW\*(C`gen_delimited_pat\*(C'\fR subroutine takes a single (string) argument and
.PP
.Vb 1
\&        gen_delimited_pat(q{\*(Aq"})
.Ve
.PP
returns the regex:
.PP
.Vb 1
\&        (?:\e"(?:\e\e\e"|(?!\e").)*\e"|\e\*(Aq(?:\e\e\e\*(Aq|(?!\e\*(Aq).)*\e\*(Aq)
.Ve
.PP
Note that the specified delimiters are automatically quotemeta'd.
.PP
A typical use of \f(CW\*(C`gen_delimited_pat\*(C'\fR would be to build special purpose tags
for \f(CW\*(C`extract_tagged\*(C'\fR. For example, to properly ignore \*(L"empty\*(R" \s-1XML\s0 elements
(which might contain quoted strings):
.PP
.Vb 1
\&        my $empty_tag = \*(Aq<(\*(Aq . gen_delimited_pat(q{\*(Aq"}) . \*(Aq|.)+/>\*(Aq;
\&
\&        extract_tagged($text, undef, undef, undef, {ignore => [$empty_tag]} );
.Ve
.PP
\&\f(CW\*(C`gen_delimited_pat\*(C'\fR may also be called with an optional second argument,
which specifies the \*(L"escape\*(R" character(s) to be used for each delimiter.
For example to match a Pascal-style string (where ' is the delimiter
and '' is a literal ' within the string):
.PP
.Vb 1
\&        gen_delimited_pat(q{\*(Aq},q{\*(Aq});
.Ve
.PP
Different escape characters can be specified for different delimiters.
For example, to specify that '/' is the escape for single quotes
and '%' is the escape for double quotes:
.PP
.Vb 1
\&        gen_delimited_pat(q{\*(Aq"},q{/%});
.Ve
.PP
If more delimiters than escape chars are specified, the last escape char
is used for the remaining delimiters.
If no escape char is specified for a given specified delimiter, '\e' is used.
.ie n .Sh """delimited_pat"""
.el .Sh "\f(CWdelimited_pat\fP"
.IX Subsection "delimited_pat"
Note that \f(CW\*(C`gen_delimited_pat\*(C'\fR was previously called \f(CW\*(C`delimited_pat\*(C'\fR.
That name may still be used, but is now deprecated.
.SH "DIAGNOSTICS"
.IX Header "DIAGNOSTICS"
In a list context, all the functions return \f(CW\*(C`(undef,$original_text)\*(C'\fR
on failure. In a scalar context, failure is indicated by returning \f(CW\*(C`undef\*(C'\fR
(in this case the input text is not modified in any way).
.PP
In addition, on failure in \fIany\fR context, the \f(CW$@\fR variable is set.
Accessing \f(CW\*(C`$@\->{error}\*(C'\fR returns one of the error diagnostics listed
below.
Accessing \f(CW\*(C`$@\->{pos}\*(C'\fR returns the offset into the original string at
which the error was detected (although not necessarily where it occurred!)
Printing \f(CW$@\fR directly produces the error message, with the offset appended.
On success, the \f(CW$@\fR variable is guaranteed to be \f(CW\*(C`undef\*(C'\fR.
.PP
The available diagnostics are:
.ie n .IP """Did not find a suitable bracket: ""%s""""" 4
.el .IP "\f(CWDid not find a suitable bracket: ``%s''\fR" 4
.IX Item "Did not find a suitable bracket: ""%s"""
The delimiter provided to \f(CW\*(C`extract_bracketed\*(C'\fR was not one of
\&\f(CW\*(Aq()[]<>{}\*(Aq\fR.
.ie n .IP """Did not find prefix: /%s/""" 4
.el .IP "\f(CWDid not find prefix: /%s/\fR" 4
.IX Item "Did not find prefix: /%s/"
A non-optional prefix was specified but wasn't found at the start of the text.
.ie n .IP """Did not find opening bracket after prefix: ""%s""""" 4
.el .IP "\f(CWDid not find opening bracket after prefix: ``%s''\fR" 4
.IX Item "Did not find opening bracket after prefix: ""%s"""
\&\f(CW\*(C`extract_bracketed\*(C'\fR or \f(CW\*(C`extract_codeblock\*(C'\fR was expecting a
particular kind of bracket at the start of the text, and didn't find it.
.ie n .IP """No quotelike operator found after prefix: ""%s""""" 4
.el .IP "\f(CWNo quotelike operator found after prefix: ``%s''\fR" 4
.IX Item "No quotelike operator found after prefix: ""%s"""
\&\f(CW\*(C`extract_quotelike\*(C'\fR didn't find one of the quotelike operators \f(CW\*(C`q\*(C'\fR,
\&\f(CW\*(C`qq\*(C'\fR, \f(CW\*(C`qw\*(C'\fR, \f(CW\*(C`qx\*(C'\fR, \f(CW\*(C`s\*(C'\fR, \f(CW\*(C`tr\*(C'\fR or \f(CW\*(C`y\*(C'\fR at the start of the substring
it was extracting.
.ie n .IP """Unmatched closing bracket: ""%c""""" 4
.el .IP "\f(CWUnmatched closing bracket: ``%c''\fR" 4
.IX Item "Unmatched closing bracket: ""%c"""
\&\f(CW\*(C`extract_bracketed\*(C'\fR, \f(CW\*(C`extract_quotelike\*(C'\fR or \f(CW\*(C`extract_codeblock\*(C'\fR encountered
a closing bracket where none was expected.
.ie n .IP """Unmatched opening bracket(s): ""%s""""" 4
.el .IP "\f(CWUnmatched opening bracket(s): ``%s''\fR" 4
.IX Item "Unmatched opening bracket(s): ""%s"""
\&\f(CW\*(C`extract_bracketed\*(C'\fR, \f(CW\*(C`extract_quotelike\*(C'\fR or \f(CW\*(C`extract_codeblock\*(C'\fR ran 
out of characters in the text before closing one or more levels of nested
brackets.
.ie n .IP """Unmatched embedded quote (%s)""" 4
.el .IP "\f(CWUnmatched embedded quote (%s)\fR" 4
.IX Item "Unmatched embedded quote (%s)"
\&\f(CW\*(C`extract_bracketed\*(C'\fR attempted to match an embedded quoted substring, but
failed to find a closing quote to match it.
.ie n .IP """Did not find closing delimiter to match \*(Aq%s\*(Aq""" 4
.el .IP "\f(CWDid not find closing delimiter to match \*(Aq%s\*(Aq\fR" 4
.IX Item "Did not find closing delimiter to match %s"
\&\f(CW\*(C`extract_quotelike\*(C'\fR was unable to find a closing delimiter to match the
one that opened the quote-like operation.
.ie n .IP """Mismatched closing bracket: expected ""%c"" but found ""%s""""" 4
.el .IP "\f(CWMismatched closing bracket: expected ``%c'' but found ``%s''\fR" 4
.IX Item "Mismatched closing bracket: expected ""%c"" but found ""%s"""
\&\f(CW\*(C`extract_bracketed\*(C'\fR, \f(CW\*(C`extract_quotelike\*(C'\fR or \f(CW\*(C`extract_codeblock\*(C'\fR found
a valid bracket delimiter, but it was the wrong species. This usually
indicates a nesting error, but may indicate incorrect quoting or escaping.
.ie n .IP """No block delimiter found after quotelike ""%s""""" 4
.el .IP "\f(CWNo block delimiter found after quotelike ``%s''\fR" 4
.IX Item "No block delimiter found after quotelike ""%s"""
\&\f(CW\*(C`extract_quotelike\*(C'\fR or \f(CW\*(C`extract_codeblock\*(C'\fR found one of the
quotelike operators \f(CW\*(C`q\*(C'\fR, \f(CW\*(C`qq\*(C'\fR, \f(CW\*(C`qw\*(C'\fR, \f(CW\*(C`qx\*(C'\fR, \f(CW\*(C`s\*(C'\fR, \f(CW\*(C`tr\*(C'\fR or \f(CW\*(C`y\*(C'\fR
without a suitable block after it.
.ie n .IP """Did not find leading dereferencer""" 4
.el .IP "\f(CWDid not find leading dereferencer\fR" 4
.IX Item "Did not find leading dereferencer"
\&\f(CW\*(C`extract_variable\*(C'\fR was expecting one of '$', '@', or '%' at the start of
a variable, but didn't find any of them.
.ie n .IP """Bad identifier after dereferencer""" 4
.el .IP "\f(CWBad identifier after dereferencer\fR" 4
.IX Item "Bad identifier after dereferencer"
\&\f(CW\*(C`extract_variable\*(C'\fR found a '$', '@', or '%' indicating a variable, but that
character was not followed by a legal Perl identifier.
.ie n .IP """Did not find expected opening bracket at %s""" 4
.el .IP "\f(CWDid not find expected opening bracket at %s\fR" 4
.IX Item "Did not find expected opening bracket at %s"
\&\f(CW\*(C`extract_codeblock\*(C'\fR failed to find any of the outermost opening brackets
that were specified.
.ie n .IP """Improperly nested codeblock at %s""" 4
.el .IP "\f(CWImproperly nested codeblock at %s\fR" 4
.IX Item "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.
.ie n .IP """Missing second block for quotelike ""%s""""" 4
.el .IP "\f(CWMissing second block for quotelike ``%s''\fR" 4
.IX Item "Missing second block for quotelike ""%s"""
\&\f(CW\*(C`extract_codeblock\*(C'\fR or \f(CW\*(C`extract_quotelike\*(C'\fR found one of the
quotelike operators \f(CW\*(C`s\*(C'\fR, \f(CW\*(C`tr\*(C'\fR or \f(CW\*(C`y\*(C'\fR followed by only one block.
.ie n .IP """No match found for opening bracket""" 4
.el .IP "\f(CWNo match found for opening bracket\fR" 4
.IX Item "No match found for opening bracket"
\&\f(CW\*(C`extract_codeblock\*(C'\fR failed to find a closing bracket to match the outermost
opening bracket.
.ie n .IP """Did not find opening tag: /%s/""" 4
.el .IP "\f(CWDid not find opening tag: /%s/\fR" 4
.IX Item "Did not find opening tag: /%s/"
\&\f(CW\*(C`extract_tagged\*(C'\fR did not find a suitable opening tag (after any specified
prefix was removed).
.ie n .IP """Unable to construct closing tag to match: /%s/""" 4
.el .IP "\f(CWUnable to construct closing tag to match: /%s/\fR" 4
.IX Item "Unable to construct closing tag to match: /%s/"
\&\f(CW\*(C`extract_tagged\*(C'\fR 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.
.ie n .IP """Found invalid nested tag: %s""" 4
.el .IP "\f(CWFound invalid nested tag: %s\fR" 4
.IX Item "Found invalid nested tag: %s"
\&\f(CW\*(C`extract_tagged\*(C'\fR found a nested tag that appeared in the \*(L"reject\*(R" list
(and the failure mode was not \*(L"\s-1MAX\s0\*(R" or \*(L"\s-1PARA\s0\*(R").
.ie n .IP """Found unbalanced nested tag: %s""" 4
.el .IP "\f(CWFound unbalanced nested tag: %s\fR" 4
.IX Item "Found unbalanced nested tag: %s"
\&\f(CW\*(C`extract_tagged\*(C'\fR found a nested opening tag that was not matched by a
corresponding nested closing tag (and the failure mode was not \*(L"\s-1MAX\s0\*(R" or \*(L"\s-1PARA\s0\*(R").
.ie n .IP """Did not find closing tag""" 4
.el .IP "\f(CWDid not find closing tag\fR" 4
.IX Item "Did not find closing tag"
\&\f(CW\*(C`extract_tagged\*(C'\fR reached the end of the text without finding a closing tag
to match the original opening tag (and the failure mode was not
\&\*(L"\s-1MAX\s0\*(R" or \*(L"\s-1PARA\s0\*(R").
.SH "AUTHOR"
.IX Header "AUTHOR"
Damian Conway ([e-mail])
.SH "BUGS AND IRRITATIONS"
.IX Header "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.
.PP
Bug reports and other feedback are most welcome.
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
.Vb 3
\& 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.
.Ve