MirOS Manual: Text::Balanced(3p)


Text::Balanced(3pPerl Programmers Reference GuiText::Balanced(3p)

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);

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      # 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 whi-
     tespace ("/\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 occurance
     of a substring anywhere in a string (like an unanchored
     regex would). Rather, they extract an occurance of the sub-
     string appearing immediately at the current matching posi-
     tion 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 delim-
         iters. If the extraction fails an empty string is

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         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 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, hav-
     ing first been removed from the input text. Thus, the fol-
     lowing 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 subrou-
     tines 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 . nor-
     mally doesn't match newlines.

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     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 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 a array of
     three elements, the extracted substring (including the sur-
     rounding 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:

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             # 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 delim-
     iter 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).

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     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 speci-
     fied 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 delim-
     iter must be balanced and correctly nested within the sub-
     string, 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 }"  );

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     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 calles through subroutine references,

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     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 whi-
         tespace 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 sub-
     string (and any specified prefix) removed.

     "extract_tagged"

     "extract_tagged" extracts and segments text between (bal-
     anced) 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 open-
         ing 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 clos-
         ing 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 open-
         ing tag that was matched (not the pattern that matched

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         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 whi-
         tespace 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 specify-
         ing 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 specify-
         ing 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 default behaviour (i.e. failure) is reinstated.

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

                 $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 com-
     plete 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.

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

     "gen_extract_tagged"

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

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

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

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

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

     The subroutine takes up to four optional arguments (the same
     set as "extract_tagged" except for the string to be pro-
     cessed). 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".

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     "extract_quotelike"

     "extract_quotelike" attempts to recognize, extract, and seg-
     ment any one of the various Perl quotes and quotelike opera-
     tors (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 pro-
     cessed 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),

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     [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 substi-
         tution 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

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                     ($op,$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 docu-
     ments" 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 success-
     fully 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),

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     [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 bal-
     anced bracket delimited substring that may contain unbal-
     anced 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 delim-
     iter 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

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     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

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     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 '>' char-
     acter 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 pro-
     cessed 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 refer-
         ences, specifying the extractors to be used to split the
         string. If this argument is omitted (or "undef") the
         list:

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                 [
                         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 con-
     text 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 facil-
     itate 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 expres-
     sion, or string). The key of that element is the name of a
     class into which the successful return value of the

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     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 appropri-
     ate 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. Charac-
     ters which are thus removed are accumulated and eventually
     become the next field (unless the fourth argument is true,
     in which case they are disgarded).

     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:

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             @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 unde-
     fined (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 spe-
     cial 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):

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             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.

     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 diag-
     nostics 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.

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     "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 delim-
         iter to match the one that opened the quote-like opera-
         tion.

     "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 nest-
         ing error, but may indicate incorrect quoting or escap-
         ing.

     "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 delim-
         iter 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

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         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 (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.

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