NAME¶
Text::Balanced - Extract delimited text sequences from strings.
SYNOPSIS¶
use Text::Balanced qw (
extract_delimited
extract_bracketed
extract_quotelike
extract_codeblock
extract_variable
extract_tagged
extract_multiple
gen_delimited_pat
gen_extract_tagged
);
# Extract the initial substring of $text that is delimited by
# two (unescaped) instances of the first character in $delim.
($extracted, $remainder) = extract_delimited($text,$delim);
# Extract the initial substring of $text that is bracketed
# with a delimiter(s) specified by $delim (where the string
# in $delim contains one or more of '(){}[]<>').
($extracted, $remainder) = extract_bracketed($text,$delim);
# Extract the initial substring of $text that is bounded by
# an XML tag.
($extracted, $remainder) = extract_tagged($text);
# Extract the initial substring of $text that is bounded by
# a C<BEGIN>...C<END> pair. Don't allow nested C<BEGIN> tags
($extracted, $remainder) =
extract_tagged($text,"BEGIN","END",undef,{bad=>["BEGIN"]});
# Extract the initial substring of $text that represents a
# Perl "quote or quote-like operation"
($extracted, $remainder) = extract_quotelike($text);
# Extract the initial substring of $text that represents a block
# of Perl code, bracketed by any of character(s) specified by $delim
# (where the string $delim contains one or more of '(){}[]<>').
($extracted, $remainder) = extract_codeblock($text,$delim);
# Extract the initial substrings of $text that would be extracted by
# one or more sequential applications of the specified functions
# or regular expressions
@extracted = extract_multiple($text,
[ \&extract_bracketed,
\&extract_quotelike,
\&some_other_extractor_sub,
qr/[xyz]*/,
'literal',
]);
# Create a string representing an optimized pattern (a la Friedl) # that matches
a substring delimited by any of the specified characters # (in this case: any
type of quote or a slash)
$patstring = gen_delimited_pat(q{'"`/});
# Generate a reference to an anonymous sub that is just like extract_tagged #
but pre-compiled and optimized for a specific pair of tags, and consequently #
much faster (i.e. 3 times faster). It uses qr// for better performance on #
repeated calls, so it only works under Perl 5.005 or later.
$extract_head = gen_extract_tagged('<HEAD>','</HEAD>');
($extracted, $remainder) = $extract_head->($text);
DESCRIPTION¶
The various "extract_..." subroutines may be used to extract a
delimited substring, possibly after skipping a specified prefix string. By
default, that prefix is optional whitespace ("/\s*/"), but you can
change it to whatever you wish (see below).
The substring to be extracted must appear at the current "pos"
location of the string's variable (or at index zero, if no "pos"
position is defined). In other words, the "extract_..." subroutines
don't extract the first occurrence of a substring anywhere in a string
(like an unanchored regex would). Rather, they extract an occurrence of the
substring appearing immediately at the current matching position in the string
(like a "\G"-anchored regex would).
General behaviour in list contexts¶
In a list context, all the subroutines return a list, the first three elements
of which are always:
- [0]
- The extracted string, including the specified delimiters. If the
extraction fails "undef" is returned.
- [1]
- The remainder of the input string (i.e. the characters after the extracted
string). On failure, the entire string is returned.
- [2]
- The skipped prefix (i.e. the characters before the extracted string). On
failure, "undef" is returned.
Note that in a list context, the contents of the original input text (the first
argument) are not modified in any way.
However, if the input text was passed in a variable, that variable's
"pos" value is updated to point at the first character after the
extracted text. That means that in a list context the various subroutines can
be used much like regular expressions. For example:
while ( $next = (extract_quotelike($text))[0] )
{
# process next quote-like (in $next)
}
General behaviour in scalar and void contexts¶
In a scalar context, the extracted string is returned, having first been removed
from the input text. Thus, the following code also processes each quote-like
operation, but actually removes them from $text:
while ( $next = extract_quotelike($text) )
{
# process next quote-like (in $next)
}
Note that if the input text is a read-only string (i.e. a literal), no attempt
is made to remove the extracted text.
In a void context the behaviour of the extraction subroutines is exactly the
same as in a scalar context, except (of course) that the extracted substring
is not returned.
A note about prefixes¶
Prefix patterns are matched without any trailing modifiers ("/gimsox"
etc.) This can bite you if you're expecting a prefix specification like
'.*?(?=<H1>)' to skip everything up to the first <H1> tag. Such a
prefix pattern will only succeed if the <H1> tag is on the current line,
since . normally doesn't match newlines.
To overcome this limitation, you need to turn on /s matching within the prefix
pattern, using the "(?s)" directive: '(?s).*?(?=<H1>)'
The "extract_delimited" function formalizes the common idiom of
extracting a single-character-delimited substring from the start of a string.
For example, to extract a single-quote delimited string, the following code is
typically used:
($remainder = $text) =~ s/\A('(\\.|[^'])*')//s;
$extracted = $1;
but with "extract_delimited" it can be simplified to:
($extracted,$remainder) = extract_delimited($text, "'");
"extract_delimited" takes up to four scalars (the input text, the
delimiters, a prefix pattern to be skipped, and any escape characters) and
extracts the initial substring of the text that is appropriately delimited. If
the delimiter string has multiple characters, the first one encountered in the
text is taken to delimit the substring. The third argument specifies a prefix
pattern that is to be skipped (but must be present!) before the substring is
extracted. The final argument specifies the escape character to be used for
each delimiter.
All arguments are optional. If the escape characters are not specified, every
delimiter is escaped with a backslash ("\"). If the prefix is not
specified, the pattern '\s*' - optional whitespace - is used. If the delimiter
set is also not specified, the set "/["'`]/" is used. If the
text to be processed is not specified either, $_ is used.
In list context, "extract_delimited" returns a array of three
elements, the extracted substring (
including the surrounding
delimiters), the remainder of the text, and the skipped prefix (if
any). If a suitable delimited substring is not found, the first element of the
array is the empty string, the second is the complete original text, and the
prefix returned in the third element is an empty string.
In a scalar context, just the extracted substring is returned. In a void
context, the extracted substring (and any prefix) are simply removed from the
beginning of the first argument.
Examples:
# Remove a single-quoted substring from the very beginning of $text:
$substring = extract_delimited($text, "'", '');
# Remove a single-quoted Pascalish substring (i.e. one in which
# doubling the quote character escapes it) from the very
# beginning of $text:
$substring = extract_delimited($text, "'", '', "'");
# Extract a single- or double- quoted substring from the
# beginning of $text, optionally after some whitespace
# (note the list context to protect $text from modification):
($substring) = extract_delimited $text, q{"'};
# Delete the substring delimited by the first '/' in $text:
$text = join '', (extract_delimited($text,'/','[^/]*')[2,1];
Note that this last example is
not the same as deleting the first
quote-like pattern. For instance, if $text contained the string:
"if ('./cmd' =~ m/$UNIXCMD/s) { $cmd = $1; }"
then after the deletion it would contain:
"if ('.$UNIXCMD/s) { $cmd = $1; }"
not:
"if ('./cmd' =~ ms) { $cmd = $1; }"
See "extract_quotelike" for a (partial) solution to this problem.
Like "extract_delimited", the "extract_bracketed" function
takes up to three optional scalar arguments: a string to extract from, a
delimiter specifier, and a prefix pattern. As before, a missing prefix
defaults to optional whitespace and a missing text defaults to $_. However, a
missing delimiter specifier defaults to '{}()[]<>' (see below).
"extract_bracketed" extracts a balanced-bracket-delimited substring
(using any one (or more) of the user-specified delimiter brackets: '(..)',
'{..}', '[..]', or '<..>'). Optionally it will also respect quoted
unbalanced brackets (see below).
A "delimiter bracket" is a bracket in list of delimiters passed as
"extract_bracketed"'s second argument. Delimiter brackets are
specified by giving either the left or right (or both!) versions of the
required bracket(s). Note that the order in which two or more delimiter
brackets are specified is not significant.
A "balanced-bracket-delimited substring" is a substring bounded by
matched brackets, such that any other (left or right) delimiter bracket
within the substring is also matched by an opposite (right or left)
delimiter bracket
at the same level of nesting. Any type of bracket not
in the delimiter list is treated as an ordinary character.
In other words, each type of bracket specified as a delimiter must be balanced
and correctly nested within the substring, and any other kind of
("non-delimiter") bracket in the substring is ignored.
For example, given the string:
$text = "{ an '[irregularly :-(] {} parenthesized >:-)' string }";
then a call to "extract_bracketed" in a list context:
@result = extract_bracketed( $text, '{}' );
would return:
( "{ an '[irregularly :-(] {} parenthesized >:-)' string }" , "" , "" )
since both sets of '{..}' brackets are properly nested and evenly balanced. (In
a scalar context just the first element of the array would be returned. In a
void context, $text would be replaced by an empty string.)
Likewise the call in:
@result = extract_bracketed( $text, '{[' );
would return the same result, since all sets of both types of specified
delimiter brackets are correctly nested and balanced.
However, the call in:
@result = extract_bracketed( $text, '{([<' );
would fail, returning:
( undef , "{ an '[irregularly :-(] {} parenthesized >:-)' string }" );
because the embedded pairs of '(..)'s and '[..]'s are "cross-nested"
and the embedded '>' is unbalanced. (In a scalar context, this call would
return an empty string. In a void context, $text would be unchanged.)
Note that the embedded single-quotes in the string don't help in this case,
since they have not been specified as acceptable delimiters and are therefore
treated as non-delimiter characters (and ignored).
However, if a particular species of quote character is included in the delimiter
specification, then that type of quote will be correctly handled. for example,
if $text is:
$text = '<A HREF=">>>>">link</A>';
then
@result = extract_bracketed( $text, '<">' );
returns:
( '<A HREF=">>>>">', 'link</A>', "" )
as expected. Without the specification of """ as an embedded
quoter:
@result = extract_bracketed( $text, '<>' );
the result would be:
( '<A HREF=">', '>>>">link</A>', "" )
In addition to the quote delimiters "'", """, and
"`", full Perl quote-like quoting (i.e. q{string}, qq{string}, etc)
can be specified by including the letter 'q' as a delimiter. Hence:
@result = extract_bracketed( $text, '<q>' );
would correctly match something like this:
$text = '<leftop: conj /and/ conj>';
See also: "extract_quotelike" and "extract_codeblock".
"extract_variable" extracts any valid Perl variable or
variable-involved expression, including scalars, arrays, hashes, array
accesses, hash look-ups, method calls through objects, subroutine calls
through subroutine references, etc.
The subroutine takes up to two optional arguments:
- 1.
- A string to be processed ($_ if the string is omitted or
"undef")
- 2.
- A string specifying a pattern to be matched as a prefix (which is to be
skipped). If omitted, optional whitespace is skipped.
On success in a list context, an array of 3 elements is returned. The elements
are:
- [0]
- the extracted variable, or variablish expression
- [1]
- the remainder of the input text,
- [2]
- the prefix substring (if any),
On failure, all of these values (except the remaining text) are
"undef".
In a scalar context, "extract_variable" returns just the complete
substring that matched a variablish expression. "undef" is returned
on failure. In addition, the original input text has the returned substring
(and any prefix) removed from it.
In a void context, the input text just has the matched substring (and any
specified prefix) removed.
"extract_tagged" extracts and segments text between (balanced)
specified tags.
The subroutine takes up to five optional arguments:
- 1.
- A string to be processed ($_ if the string is omitted or
"undef")
- 2.
- A string specifying a pattern to be matched as the opening tag. If the
pattern string is omitted (or "undef") then a pattern that
matches any standard XML tag is used.
- 3.
- A string specifying a pattern to be matched at the closing tag. If the
pattern string is omitted (or "undef") then the closing tag is
constructed by inserting a "/" after any leading bracket
characters in the actual opening tag that was matched ( not the
pattern that matched the tag). For example, if the opening tag pattern is
specified as '{{\w+}}' and actually matched the opening tag
"{{DATA}}", then the constructed closing tag would be
"{{/DATA}}".
- 4.
- A string specifying a pattern to be matched as a prefix (which is to be
skipped). If omitted, optional whitespace is skipped.
- 5.
- A hash reference containing various parsing options (see below)
The various options that can be specified are:
- "reject => $listref"
- The list reference contains one or more strings specifying patterns that
must not appear within the tagged text.
For example, to extract an HTML link (which should not contain nested links)
use:
extract_tagged($text, '<A>', '</A>', undef, {reject => ['<A>']} );
- "ignore => $listref"
- The list reference contains one or more strings specifying patterns that
are not be be treated as nested tags within the tagged text (even
if they would match the start tag pattern).
For example, to extract an arbitrary XML tag, but ignore "empty"
elements:
extract_tagged($text, undef, undef, undef, {ignore => ['<[^>]*/>']} );
(also see "gen_delimited_pat" below).
- "fail => $str"
- The "fail" option indicates the action to be taken if a matching
end tag is not encountered (i.e. before the end of the string or some
"reject" pattern matches). By default, a failure to match a
closing tag causes "extract_tagged" to immediately fail.
However, if the string value associated with <reject> is
"MAX", then "extract_tagged" returns the complete text
up to the point of failure. If the string is "PARA",
"extract_tagged" returns only the first paragraph after the tag
(up to the first line that is either empty or contains only whitespace
characters). If the string is "", the the default behaviour
(i.e. failure) is reinstated.
For example, suppose the start tag "/para" introduces a paragraph,
which then continues until the next "/endpara" tag or until
another "/para" tag is encountered:
$text = "/para line 1\n\nline 3\n/para line 4";
extract_tagged($text, '/para', '/endpara', undef,
{reject => '/para', fail => MAX );
# EXTRACTED: "/para line 1\n\nline 3\n"
Suppose instead, that if no matching "/endpara" tag is found, the
"/para" tag refers only to the immediately following paragraph:
$text = "/para line 1\n\nline 3\n/para line 4";
extract_tagged($text, '/para', '/endpara', undef,
{reject => '/para', fail => MAX );
# EXTRACTED: "/para line 1\n"
Note that the specified "fail" behaviour applies to nested tags as
well.
On success in a list context, an array of 6 elements is returned. The elements
are:
- [0]
- the extracted tagged substring (including the outermost tags),
- [1]
- the remainder of the input text,
- [2]
- the prefix substring (if any),
- [3]
- the opening tag
- [4]
- the text between the opening and closing tags
- [5]
- the closing tag (or "" if no closing tag was found)
On failure, all of these values (except the remaining text) are
"undef".
In a scalar context, "extract_tagged" returns just the complete
substring that matched a tagged text (including the start and end tags).
"undef" is returned on failure. In addition, the original input text
has the returned substring (and any prefix) removed from it.
In a void context, the input text just has the matched substring (and any
specified prefix) removed.
(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 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" attempts to recognize, extract, and segment any
one of the various Perl quotes and quotelike operators (see
perlop(3))
Nested backslashed delimiters, embedded balanced bracket delimiters (for the
quotelike operators), and trailing modifiers are all caught. For example, in:
extract_quotelike 'q # an octothorpe: \# (not the end of the q!) #'
extract_quotelike ' "You said, \"Use sed\"." '
extract_quotelike ' s{([A-Z]{1,8}\.[A-Z]{3})} /\L$1\E/; '
extract_quotelike ' tr/\\\/\\\\/\\\//ds; '
the full Perl quotelike operations are all extracted correctly.
Note too that, when using the /x modifier on a regex, any comment containing the
current pattern delimiter will cause the regex to be immediately terminated.
In other words:
'm /
(?i) # CASE INSENSITIVE
[a-z_] # LEADING ALPHABETIC/UNDERSCORE
[a-z0-9]* # FOLLOWED BY ANY NUMBER OF ALPHANUMERICS
/x'
will be extracted as if it were:
'm /
(?i) # CASE INSENSITIVE
[a-z_] # LEADING ALPHABETIC/'
This behaviour is identical to that of the actual compiler.
"extract_quotelike" takes two arguments: the text to be processed and
a prefix to be matched at the very beginning of the text. If no prefix is
specified, optional whitespace is the default. If no text is given, $_ is
used.
In a list context, an array of 11 elements is returned. The elements are:
- [0]
- the extracted quotelike substring (including trailing modifiers),
- [1]
- the remainder of the input text,
- [2]
- the prefix substring (if any),
- [3]
- the name of the quotelike operator (if any),
- [4]
- the left delimiter of the first block of the operation,
- [5]
- the text of the first block of the operation (that is, the contents of a
quote, the regex of a match or substitution or the target list of a
translation),
- [6]
- the right delimiter of the first block of the operation,
- [7]
- the left delimiter of the second block of the operation (that is, if it is
a "s", "tr", or "y"),
- [8]
- the text of the second block of the operation (that is, the replacement of
a substitution or the translation list of a translation),
- [9]
- the right delimiter of the second block of the operation (if any),
- [10]
- the trailing modifiers on the operation (if any).
For each of the fields marked "(if any)" the default value on success
is an empty string. On failure, all of these values (except the remaining
text) are "undef".
In a scalar context, "extract_quotelike" returns just the complete
substring that matched a quotelike operation (or "undef" on
failure). In a scalar or void context, the input text has the same substring
(and any specified prefix) removed.
Examples:
# Remove the first quotelike literal that appears in text
$quotelike = extract_quotelike($text,'.*?');
# Replace one or more leading whitespace-separated quotelike
# literals in $_ with "<QLL>"
do { $_ = join '<QLL>', (extract_quotelike)[2,1] } until $@;
# Isolate the search pattern in a quotelike operation from $text
($op,$pat) = (extract_quotelike $text)[3,5];
if ($op =~ /[ms]/)
{
print "search pattern: $pat\n";
}
else
{
print "$op is not a pattern matching operation\n";
}
"extract_quotelike" and "here documents"¶
"extract_quotelike" can successfully extract "here
documents" from an input string, but with an important caveat in list
contexts.
Unlike other types of quote-like literals, a here document is rarely a
contiguous substring. For example, a typical piece of code using here document
might look like this:
<<'EOMSG' || die;
This is the message.
EOMSG
exit;
Given this as an input string in a scalar context, "extract_quotelike"
would correctly return the string "<<'EOMSG'\nThis is the
message.\nEOMSG", leaving the string " || die;\nexit;" in the
original variable. In other words, the two separate pieces of the here
document are successfully extracted and concatenated.
In a list context, "extract_quotelike" would return the list
- [0]
- "<<'EOMSG'\nThis is the message.\nEOMSG\n" (i.e. the full
extracted here document, including fore and aft delimiters),
- [1]
- " || die;\nexit;" (i.e. the remainder of the input text,
concatenated),
- [2]
- "" (i.e. the prefix substring -- trivial in this case),
- [3]
- "<<" (i.e. the "name" of the quotelike
operator)
- [4]
- "'EOMSG'" (i.e. the left delimiter of the here document,
including any quotes),
- [5]
- "This is the message.\n" (i.e. the text of the here
document),
- [6]
- "EOMSG" (i.e. the right delimiter of the here document),
- [7..10]
- "" (a here document has no second left delimiter, second text,
second right delimiter, or trailing modifiers).
However, the matching position of the input variable would be set to
"exit;" (i.e.
after the closing delimiter of the here
document), which would cause the earlier " || die;\nexit;" to be
skipped in any sequence of code fragment extractions.
To avoid this problem, when it encounters a here document whilst extracting from
a modifiable string, "extract_quotelike" silently rearranges the
string to an equivalent piece of Perl:
<<'EOMSG'
This is the message.
EOMSG
|| die;
exit;
in which the here document
is contiguous. It still leaves the matching
position after the here document, but now the rest of the line on which the
here document starts is not skipped.
To prevent <extract_quotelike> from mucking about with the input in this
way (this is the only case where a list-context "extract_quotelike"
does so), you can pass the input variable as an interpolated literal:
$quotelike = extract_quotelike("$var");
"extract_codeblock" attempts to recognize and extract a balanced
bracket delimited substring that may contain unbalanced brackets inside Perl
quotes or quotelike operations. That is, "extract_codeblock" is like
a combination of "extract_bracketed" and
"extract_quotelike".
"extract_codeblock" takes the same initial three parameters as
"extract_bracketed": a text to process, a set of delimiter brackets
to look for, and a prefix to match first. It also takes an optional fourth
parameter, which allows the outermost delimiter brackets to be specified
separately (see below).
Omitting the first argument (input text) means process $_ instead. Omitting the
second argument (delimiter brackets) indicates that only '{' is to be used.
Omitting the third argument (prefix argument) implies optional whitespace at
the start. Omitting the fourth argument (outermost delimiter brackets)
indicates that the value of the second argument is to be used for the
outermost delimiters.
Once the prefix an dthe outermost opening delimiter bracket have been
recognized, code blocks are extracted by stepping through the input text and
trying the following alternatives in sequence:
- 1.
- Try and match a closing delimiter bracket. If the bracket was the same
species as the last opening bracket, return the substring to that point.
If the bracket was mismatched, return an error.
- 2.
- Try to match a quote or quotelike operator. If found, call
"extract_quotelike" to eat it. If "extract_quotelike"
fails, return the error it returned. Otherwise go back to step 1.
- 3.
- Try to match an opening delimiter bracket. If found, call
"extract_codeblock" recursively to eat the embedded block. If
the recursive call fails, return an error. Otherwise, go back to step
1.
- 4.
- Unconditionally match a bareword or any other single character, and then
go back to step 1.
Examples:
# Find a while loop in the text
if ($text =~ s/.*?while\s*\{/{/)
{
$loop = "while " . extract_codeblock($text);
}
# Remove the first round-bracketed list (which may include
# round- or curly-bracketed code blocks or quotelike operators)
extract_codeblock $text, "(){}", '[^(]*';
The ability to specify a different outermost delimiter bracket is useful in some
circumstances. For example, in the Parse::RecDescent module, parser actions
which are to be performed only on a successful parse are specified using a
"<defer:...>" directive. For example:
sentence: subject verb object
<defer: {$::theVerb = $item{verb}} >
Parse::RecDescent uses "extract_codeblock($text, '{}<>')" to
extract the code within the "<defer:...>" directive, but
there's a problem.
A deferred action like this:
<defer: {if ($count>10) {$count--}} >
will be incorrectly parsed as:
<defer: {if ($count>
because the "less than" operator is interpreted as a closing
delimiter.
But, by extracting the directive using
"extract_codeblock($text, '{}', undef, '<>')"
the '>' character is only treated as a delimited at the outermost level of
the code block, so the directive is parsed correctly.
The "extract_multiple" subroutine takes a string to be processed and a
list of extractors (subroutines or regular expressions) to apply to that
string.
In an array context "extract_multiple" returns an array of substrings
of the original string, as extracted by the specified extractors. In a scalar
context, "extract_multiple" returns the first substring successfully
extracted from the original string. In both scalar and void contexts the
original string has the first successfully extracted substring removed from
it. In all contexts "extract_multiple" starts at the current
"pos" of the string, and sets that "pos" appropriately
after it matches.
Hence, the aim of of a call to "extract_multiple" in a list context is
to split the processed string into as many non-overlapping fields as possible,
by repeatedly applying each of the specified extractors to the remainder of
the string. Thus "extract_multiple" is a generalized form of Perl's
"split" subroutine.
The subroutine takes up to four optional arguments:
- 1.
- A string to be processed ($_ if the string is omitted or
"undef")
- 2.
- A reference to a list of subroutine references and/or qr// objects and/or
literal strings and/or hash references, specifying the extractors to be
used to split the string. If this argument is omitted (or
"undef") the list:
[
sub { extract_variable($_[0], '') },
sub { extract_quotelike($_[0],'') },
sub { extract_codeblock($_[0],'{}','') },
]
is used.
- 3.
- An number specifying the maximum number of fields to return. If this
argument is omitted (or "undef"), split continues as long as
possible.
If the third argument is N, then extraction continues until N
fields have been successfully extracted, or until the string has been
completely processed.
Note that in scalar and void contexts the value of this argument is
automatically reset to 1 (under "-w", a warning is issued if the
argument has to be reset).
- 4.
- A value indicating whether unmatched substrings (see below) within the
text should be skipped or returned as fields. If the value is true, such
substrings are skipped. Otherwise, they are returned.
The extraction process works by applying each extractor in sequence to the text
string.
If the extractor is a subroutine it is called in a list context and is expected
to return a list of a single element, namely the extracted text. It may
optionally also return two further arguments: a string representing the text
left after extraction (like $' for a pattern match), and a string representing
any prefix skipped before the extraction (like $` in a pattern match). Note
that this is designed to facilitate the use of other Text::Balanced
subroutines with "extract_multiple". Note too that the value
returned by an extractor subroutine need not bear any relationship to the
corresponding substring of the original text (see examples below).
If the extractor is a precompiled regular expression or a string, it is matched
against the text in a scalar context with a leading '\G' and the gc modifiers
enabled. The extracted value is either $1 if that variable is defined after
the match, or else the complete match (i.e. $&).
If the extractor is a hash reference, it must contain exactly one element. The
value of that element is one of the above extractor types (subroutine
reference, regular expression, or string). The key of that element is the name
of a class into which the successful return value of the extractor will be
blessed.
If an extractor returns a defined value, that value is immediately treated as
the next extracted field and pushed onto the list of fields. If the extractor
was specified in a hash reference, the field is also blessed into the
appropriate class,
If the extractor fails to match (in the case of a regex extractor), or returns
an empty list or an undefined value (in the case of a subroutine extractor),
it is assumed to have failed to extract. If none of the extractor subroutines
succeeds, then one character is extracted from the start of the text and the
extraction subroutines reapplied. Characters which are thus removed are
accumulated and eventually become the next field (unless the fourth argument
is true, in which case they are discarded).
For example, the following extracts substrings that are valid Perl variables:
@fields = extract_multiple($text,
[ sub { extract_variable($_[0]) } ],
undef, 1);
This example separates a text into fields which are quote delimited, curly
bracketed, and anything else. The delimited and bracketed parts are also
blessed to identify them (the "anything else" is unblessed):
@fields = extract_multiple($text,
[
{ Delim => sub { extract_delimited($_[0],q{'"}) } },
{ Brack => sub { extract_bracketed($_[0],'{}') } },
]);
This call extracts the next single substring that is a valid Perl quotelike
operator (and removes it from $text):
$quotelike = extract_multiple($text,
[
sub { extract_quotelike($_[0]) },
], undef, 1);
Finally, here is yet another way to do comma-separated value parsing:
@fields = extract_multiple($csv_text,
[
sub { extract_delimited($_[0],q{'"}) },
qr/([^,]+)(.*)/,
],
undef,1);
The list in the second argument means:
"Try and extract a ' or "
delimited string, otherwise extract anything up to a comma...". The
undef third argument means:
"...as many times as
possible...", and the true value in the fourth argument means
"...discarding anything else that appears (i.e. the commas)".
If you wanted the commas preserved as separate fields (i.e. like split does if
your split pattern has capturing parentheses), you would just make the last
parameter undefined (or remove it).
"gen_delimited_pat"¶
The "gen_delimited_pat" subroutine takes a single (string) argument
and
> builds a Friedl-style optimized regex that matches a string delimited by
any one of the characters in the single argument. For example:
gen_delimited_pat(q{'"})
returns the regex:
(?:\"(?:\\\"|(?!\").)*\"|\'(?:\\\'|(?!\').)*\')
Note that the specified delimiters are automatically quotemeta'd.
A typical use of "gen_delimited_pat" would be to build special purpose
tags for "extract_tagged". For example, to properly ignore
"empty" XML elements (which might contain quoted strings):
my $empty_tag = '<(' . gen_delimited_pat(q{'"}) . '|.)+/>';
extract_tagged($text, undef, undef, undef, {ignore => [$empty_tag]} );
"gen_delimited_pat" may also be called with an optional second
argument, which specifies the "escape" character(s) to be used for
each delimiter. For example to match a Pascal-style string (where ' is the
delimiter and '' is a literal ' within the string):
gen_delimited_pat(q{'},q{'});
Different escape characters can be specified for different delimiters. For
example, to specify that '/' is the escape for single quotes and '%' is the
escape for double quotes:
gen_delimited_pat(q{'"},q{/%});
If more delimiters than escape chars are specified, the last escape char is used
for the remaining delimiters. If no escape char is specified for a given
specified delimiter, '\' is used.
"delimited_pat"¶
Note that "gen_delimited_pat" was previously called
"delimited_pat". That name may still be used, but is now deprecated.
DIAGNOSTICS¶
In a list context, all the functions return "(undef,$original_text)"
on failure. In a scalar context, failure is indicated by returning
"undef" (in this case the input text is not modified in any way).
In addition, on failure in
any context, the $@ variable is set. Accessing
"$@->{error}" returns one of the error diagnostics listed below.
Accessing "$@->{pos}" returns the offset into the original string
at which the error was detected (although not necessarily where it occurred!)
Printing $@ directly produces the error message, with the offset appended. On
success, the $@ variable is guaranteed to be "undef".
The available diagnostics are:
- "Did not find a suitable bracket: "%s""
- The delimiter provided to "extract_bracketed" was not one of
'()[]<>{}'.
- "Did not find prefix: /%s/"
- A non-optional prefix was specified but wasn't found at the start of the
text.
- "Did not find opening bracket after prefix: "%s""
- "extract_bracketed" or "extract_codeblock" was
expecting a particular kind of bracket at the start of the text, and
didn't find it.
- "No quotelike operator found after prefix: "%s""
- "extract_quotelike" didn't find one of the quotelike operators
"q", "qq", "qw", "qx",
"s", "tr" or "y" at the start of the
substring it was extracting.
- "Unmatched closing bracket: "%c""
- "extract_bracketed", "extract_quotelike" or
"extract_codeblock" encountered a closing bracket where none was
expected.
- "Unmatched opening bracket(s): "%s""
- "extract_bracketed", "extract_quotelike" or
"extract_codeblock" ran out of characters in the text before
closing one or more levels of nested brackets.
- "Unmatched embedded quote (%s)"
- "extract_bracketed" attempted to match an embedded quoted
substring, but failed to find a closing quote to match it.
- "Did not find closing delimiter to match '%s'"
- "extract_quotelike" was unable to find a closing delimiter to
match the one that opened the quote-like operation.
- "Mismatched closing bracket: expected "%c" but found
"%s""
- "extract_bracketed", "extract_quotelike" or
"extract_codeblock" found a valid bracket delimiter, but it was
the wrong species. This usually indicates a nesting error, but may
indicate incorrect quoting or escaping.
- "No block delimiter found after quotelike "%s""
- "extract_quotelike" or "extract_codeblock" found one
of the quotelike operators "q", "qq", "qw",
"qx", "s", "tr" or "y" without a
suitable block after it.
- "Did not find leading dereferencer"
- "extract_variable" was expecting one of '$', '@', or '%' at the
start of a variable, but didn't find any of them.
- "Bad identifier after dereferencer"
- "extract_variable" found a '$', '@', or '%' indicating a
variable, but that character was not followed by a legal Perl
identifier.
- "Did not find expected opening bracket at %s"
- "extract_codeblock" failed to find any of the outermost opening
brackets that were specified.
- "Improperly nested codeblock at %s"
- A nested code block was found that started with a delimiter that was
specified as being only to be used as an outermost bracket.
- "Missing second block for quotelike "%s""
- "extract_codeblock" or "extract_quotelike" found one
of the quotelike operators "s", "tr" or "y"
followed by only one block.
- "No match found for opening bracket"
- "extract_codeblock" failed to find a closing bracket to match
the outermost opening bracket.
- "Did not find opening tag: /%s/"
- "extract_tagged" did not find a suitable opening tag (after any
specified prefix was removed).
- "Unable to construct closing tag to match: /%s/"
- "extract_tagged" matched the specified opening tag and tried to
modify the matched text to produce a matching closing tag (because none
was specified). It failed to generate the closing tag, almost certainly
because the opening tag did not start with a bracket of some kind.
- "Found invalid nested tag: %s"
- "extract_tagged" found a nested tag that appeared in the
"reject" list (and the failure mode was not "MAX" or
"PARA").
- "Found unbalanced nested tag: %s"
- "extract_tagged" found a nested opening tag that was not matched
by a corresponding nested closing tag (and the failure mode was not
"MAX" or "PARA").
- "Did not find closing tag"
- "extract_tagged" reached the end of the text without finding a
closing tag to match the original opening tag (and the failure mode was
not "MAX" or "PARA").
AUTHOR¶
Damian Conway (damian@conway.org)
BUGS AND IRRITATIONS¶
There are undoubtedly serious bugs lurking somewhere in this code, if only
because parts of it give the impression of understanding a great deal more
about Perl than they really do.
Bug reports and other feedback are most welcome.
COPYRIGHT¶
Copyright 1997 - 2001 Damian Conway. All Rights Reserved.
Some (minor) parts copyright 2009 Adam Kennedy.
This module is free software. It may be used, redistributed and/or modified
under the same terms as Perl itself.