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