Text::Balanced
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 delim-
ited 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" loca-
tion 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
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 an empty string is returned.
[1] The remainder of the input string (i.e. the characters after the
extracted string). On failure, the entire string is returned.
[2] The skipped prefix (i.e. the characters before the extracted
string). On failure, the empty string is returned.
Note that in a list context, the contents of the original input text
(the first argument) are not modified in any way.
However, if the input text was passed in a variable, that variable’s
"pos" value is updated to point at the first character after the
extracted text. That means that in a list context the various subrou-
tines 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 pro-
cesses 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 argu-
ment specifies the escape character to be used for each delimiter.
All arguments are optional. If the escape characters are not speci-
fied, every delimiter is escaped with a backslash ("\"). If the pre-
fix 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 ele-
ments, the extracted substring (including the surrounding delimiters),
the remainder of the text, and the skipped prefix (if any). If a suit-
able 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 speci-
fied by giving either the left or right (or both!) versions of the
required bracket(s). Note that the order in which two or more delim-
iter 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 bal-
anced. (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 speci-
fied 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:
$text = ’<leftop: conj /and/ conj>’;
See also: "extract_quotelike" and "extract_codeblock".
"extract_variable"
"extract_variable" extracts any valid Perl variable or variable-
involved expression, including scalars, arrays, hashes, array
accesses, hash look-ups, method calls through objects, subroutine
calles through subroutine references, etc.
The subroutine takes up to two optional arguments:
1. A string to be processed ($_ 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 sub-
string that matched a variablish expression. "undef" is returned on
failure. In addition, the original input text has the returned sub-
string (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) speci-
fied 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 char-
acters 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 => $listref"
The list reference contains one or more strings specifying pat-
terns that must not appear within the tagged text.
For example, to extract an HTML link (which should not contain
nested links) use:
extract_tagged($text, ’<A>’, ’</A>’, undef, {reject => [’<A>’]} );
"ignore => $listref"
The list reference contains one or more strings specifying pat-
terns that are not be be treated as nested tags within the tagged
text (even if they would match the start tag pattern).
For example, to extract an arbitrary XML tag, but ignore "empty"
elements:
extract_tagged($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 rein-
stated.
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 sub-
string 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 subrou-
tines:
· 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 inter-
polation 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 con-
taining the current pattern delimiter will cause the regex to be imme-
diately 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 con-
tents of a quote, the regex of a match or substitution or the tar-
get 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 trans-
lation),
[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 mes-
sage.\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, concate-
nated),
[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_quote-
like" 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_code-
block" 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 $_ instead.
Omitting the second argument (delimiter brackets) indicates that only
’{’ is to be used. Omitting the third argument (prefix argument)
implies optional whitespace at the start. Omitting the fourth argu-
ment (outermost delimiter brackets) indicates that the value of the
second argument is to be used for the outermost delimiters.
Once the prefix an dthe outermost opening delimiter bracket have been
recognized, code blocks are extracted by stepping through the input
text and trying the following alternatives in sequence:
1. Try and match a closing delimiter bracket. If the bracket was the
same species as the last opening bracket, return the substring to
that point. If the bracket was mismatched, return an error.
2. Try to match a quote or quotelike operator. If found, call
"extract_quotelike" to eat it. If "extract_quotelike" fails,
return the error it returned. Otherwise go back to step 1.
3. Try to match an opening delimiter bracket. If found, call
"extract_codeblock" recursively to eat the embedded block. If the
recursive call fails, return an error. Otherwise, go back to step
1.
4. Unconditionally match a bareword or any other single character,
and then go back to step 1.
Examples:
# Find a while loop in the text
if ($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 $text, "(){}", ’[^(]*’;
The ability to specify a different outermost delimiter bracket is use-
ful in some circumstances. For example, in the Parse::RecDescent mod-
ule, parser actions which are to be performed only on a successful
parse are specified using a "<defer:...>" directive. For example:
sentence: subject verb object
<defer: {$::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 ($count>
because the "less than" operator is interpreted as a closing delim-
iter.
But, by extracting the directive using "extract_code-
block($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 suc-
cessfully extracted from the original string. In both scalar and void
contexts the original string has the first successfully extracted sub-
string removed from it. In all contexts "extract_multiple" starts at
the current "pos" of the string, and sets that "pos" appropriately
after it matches.
Hence, the aim of 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 sub-
string 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.
$&).
If the extractor is a hash reference, it must contain exactly one ele-
ment. The value of that element is one of the above extractor types
(subroutine reference, regular expression, or string). The key of
that element is the name of a class into which the successful return
value of the extractor will be blessed.
If an extractor returns a defined value, that value is immediately
treated as the next extracted field and pushed onto the list of
fields. If the extractor was specified in a hash reference, the field
is also blessed into the appropriate class,
If the extractor fails to match (in the case of a regex extractor), or
returns an empty list or an undefined value (in the case of a subrou-
tine extractor), it is assumed to have failed to extract. If none of
the extractor subroutines succeeds, then one character is extracted
from the start of the text and the extraction subroutines reapplied.
Characters which are thus removed are accumulated and eventually
become the next field (unless the fourth argument is true, in which
case they are disgarded).
For example, the following extracts substrings that are valid Perl
variables:
@fields = extract_multiple($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($_[0]) },
], undef, 1);
Finally, here is yet another way to do comma-separated value parsing:
@fields = extract_multiple($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 => [$empty_tag]} );
"gen_delimited_pat" may also be called with an optional second argu-
ment, which specifies the "escape" character(s) to be used for each
delimiter. For example to match a Pascal-style string (where ’ is the
delimiter and ’’ is a literal ’ within the string):
gen_delimited_pat(q{’},q{’});
Different escape characters can be specified for different delimiters.
For example, to specify that ’/’ is the escape for single quotes and
’%’ is the escape for double quotes:
gen_delimited_pat(q{’"},q{/%});
If more delimiters than escape chars are specified, the last escape
char is used for the remaining delimiters. If no escape char is spec-
ified for a given specified delimiter, ’\’ is used.
Note that "gen_delimited_pat" was previously called "delimited_pat".
That name may still be used, but is now deprecated.
DIAGNOSTICS
In a list context, all the functions return "(undef,$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 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 $@ variable is guaranteed to be
"undef".
The available diagnostics are:
"Did not find a suitable bracket: "%s""
The delimiter provided to "extract_bracketed" was not one of
’()[]<>{}’.
"Did not find prefix: /%s/"
A non-optional prefix was specified but wasn’t found at the start
of the text.
"Did not find opening bracket after prefix: "%s""
"extract_bracketed" or "extract_codeblock" was expecting a partic-
ular kind of bracket at the start of the text, and didn’t find it.
"No quotelike operator found after prefix: "%s""
"extract_quotelike" didn’t find one of the quotelike operators
"q", "qq", "qw", "qx", "s", "tr" or "y" at the start of the sub-
string it was extracting.
"Unmatched closing bracket: "%c""
"extract_bracketed", "extract_quotelike" or "extract_codeblock"
encountered a closing bracket where none was expected.
"Unmatched opening bracket(s): "%s""
"extract_bracketed", "extract_quotelike" or "extract_codeblock"
ran out of characters in the text before closing one or more lev-
els of nested brackets.
"Unmatched embedded quote (%s)"
"extract_bracketed" attempted to match an embedded quoted sub-
string, but failed to find a closing quote to match it.
"Did not find closing delimiter to match ’%s’"
"extract_quotelike" was unable to find a closing delimiter to
match the one that opened the quote-like operation.
"Mismatched closing bracket: expected "%c" but found "%s""
"extract_bracketed", "extract_quotelike" or "extract_codeblock"
found a valid bracket delimiter, but it was the wrong species.
This usually indicates a nesting error, but may indicate incorrect
quoting or escaping.
"No block delimiter found after quotelike "%s""
"extract_quotelike" or "extract_codeblock" found one of the quote-
like operators "q", "qq", "qw", "qx", "s", "tr" or "y" without a
suitable block after it.
"Did not find leading dereferencer"
"extract_variable" was expecting one of ’$’, ’@’, 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 quote-
like 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.
perl v5.8.8 2001-09-21 Text::Balanced(3)
